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Albedo

Albedo

The albedo is a measure of reflectivity of a surface or body. It is the ratio of electromagnetic radiation (EM radiation) reflected to the amount incident upon it. The fraction, usually expressed as a percentage from 0% to 100%, is an important concept in climatology and astronomy. This ratio depends on the frequency of the radiation considered: unqualified, it refers to an average across the spectrum of visible light. It also depends on the angle of incidence of the radiation: unqualified, normal incidence. Fresh snow albedos are high: up to 90%. The ocean surface has a low albedo. Earth has an average albedo of 31% whereas the albedo of the Moon is about 12%. In astronomy, the albedo of satellites and asteroids can be used to infer surface composition, most notably ice content. Enceladus, a moon of Saturn, has the highest known albedo of any body in the solar system, with 99% of EM radiation reflected. Human activities have changed the albedo (via forest clearance and farming, for example) of various areas around the globe. However, quantification of this effect is difficult on the global scale: it is not clear whether the changes have tended to increase or decrease global warming. The "classical" example of albedo effect is the snow-temperature feedback. If a snow covered area warms and the snow melts, the albedo decreases, more sunlight is absorbed, and the temperature tends to increase. The converse is true: if snow forms, a cooling cycle happens. The intensity of the albedo effect depends on the size of the change in albedo and the amount of insolation; for this reason it can be potentially very large in the tropics.

Some examples of albedo effects

Fairbanks, Alaska

According to the National Climatic Data Center's GHCN 2 data, which is composed of 30-year smoothed climatic means for thousands of weather stations across the world, the college weather station at Fairbanks, Alaska, is about 3 °C (5 °F) warmer than the airport at Fairbanks, partly because of drainage patterns but also largely because of the lower albedo at the college resulting from a higher concentration of pine trees and therefore less open snowy ground to reflect the heat back into space. Neunke and Kukla have shown that this difference is especially marked during the late winter months, when solar radiation is greater.

The tropics

Although the albedo-temperature effect is most famous in colder regions of Earth, because more snow falls there, it is actually much stronger in tropical regions because in the tropics there is consistently more sunlight. When Brazilian ranchers cut down dark, tropical rainforest trees to replace them with even darker soil in order to grow crops, the average temperature of the area appears to increase by an average of about 3 °C (5 °F) year-round.

Small scale effects

Albedo works on a smaller scale, too. People who wear dark clothes in the summertime put themselves at a greater risk of heatstroke than those who wear white clothes.

Pine forests

The albedo of a pine forest at 45°N in the winter in which the trees cover the land surface completely is only about 9%, among the lowest of any naturally occurring land environment. This is partly due to the color of the pines, and partly due to multiple scattering of sunlight within the trees which lowers the overall reflected light level. Due to light penetration, the ocean's albedo is even lower at about 3.5%, though this depends strongly on the angle of the incident radiation. Dense swampland averages between 9% and 14%. Deciduous trees average about 13%. A grassy field usually comes in at about 20%. A barren field will depend on the color of the soil, and can be as low as 5% or as high as 40%, with 15% being about the average for farmland. A desert or large beach usually averages around 25% but varies depending on the color of the sand. [Reference: Edward Walker's study in the Great Plains in the winter around 45°N].

Urban areas

Urban areas in particular have very unnatural values for albedo because of the many human-built structures which absorb light before the light can reach the surface. In the northern part of the world, cities are relatively dark, and Walker has shown that their average albedo is about 7%, with only a slight increase during the summer. In most tropical countries, cities average around 12%. This is similar to the values found in northern suburban transitional zones. Part of the reason for this is the different natural environment of cities in tropical regions, e.g., there are more very dark trees around; another reason is that portions of the tropics are very poor, and city buildings must be built with different materials. Warmer regions may also choose lighter colored building materials so the structures will remain cooler.

Trees

Because trees tend to have a low albedo, removing forests would tend to (increase albedo and thereby) cool (?) the planet. Cloud feedbacks further complicate the issue. In seasonally snow-covered zones, winter albedos of treeless areas are 10% to 50% higher than nearby forested areas because snow does not cover the trees as readily. Studies by the Hadley Centre have investigated the relative (generally warming) effect of albedo change and (cooling) effect of carbon sequestration on planting forests. They found that new forests in tropical and midlatitude areas tended to cool; new forests in high latitudes (e.g. Siberia) were neutral or perhaps warming [http://66.102.11.104/search?q=cache:o7LD-owSkNgJ:www.ulapland.fi/home/arktinen/feed_pdf/Betts_revised.pdf+hadley+albedo+forest&hl=en].

Snow

Snow albedos can be as high as 90%. This is for the ideal example, however: fresh deep snow over a featureless landscape. Over Antarctica they average a little more than 80%. If a marginally snow-covered area warms, snow tends to melt, lowering the albedo, and hence leading to more snowmelt (the ice-albedo feedback). This is the basis for predictions of enhanced warming in the polar and seasonally snow covered regions as a result of global warming.

Clouds

Clouds are another source of albedo that play into the global warming equation. Different types of clouds have different albedo values, theoretically ranging from a minimum of near 0% to a maximum in the high 70s. Climate models have shown that if the whole Earth were to be suddenly covered by white clouds, the surface temperatures would drop to a value of about -150 °C (-240 °F). This model, though it is far from perfect, also predicts that to offset a 5 °C (9 °F) temperature change due to an increase in the magnitude of the greenhouse effect, "all" we would need to do is increase the Earth's overall albedo by about 12% by adding more white clouds. Albedo and climate in some areas are already affected by artificial clouds, such as those created by the contrails of heavy commercial airliner traffic. A study following the September 11 attacks, after which all major airlines in the U.S. shut down for three days, showed a local 1 °C increase in the diurnal temperature range (the difference of day and night temperatures) (see: contrail).

Aerosol effects

Aerosol (very fine particles/droplets in the atmosphere) has two effects, direct and indirect. The direct (albedo) effect is generally to cool the planet; the indirect effect (the particles act as CCNs and thereby change cloud properties) is less certain [http://www.grida.no/climate/ipcc_tar/wg1/231.htm#671].

Black carbon

Another albedo-related effect on the climate is from black carbon particles. The size of this effect is difficult to quantify: the IPCC say that their "estimate of the global mean radiative forcing for BC aerosols from fossil fuels is ... +0.2 W m-2 (from +0.1 W m-2 in the SAR)) with a range +0.1 to +0.4 W m-2". [http://www.grida.no/climate/ipcc_tar/wg1/233.htm]. Category:Electromagnetic radiation Category:Climatology Category:Climate forcing Category:Astrophysics ko:반사율 ja:アルベド

Reflectivity

In optics, reflectivity is the reflectance (the ratio of reflected power to incident power, generally expressed in decibels or percentage) at the surface of a material so thick that the reflectance does not change with increasing thickness; i.e., the intrinsic reflectance of the surface, irrespective of other parameters such as the reflectance of the rear surface. The concept is of some importance in telecommunications. Surface reflectance may be subdivided into diffuse or Lambertian reflectance and specular reflectance. The apparent reflectance for an ideal Lambertian surface is independent of the observer's angle of view. This contrasts with a shiny (specular) surface, where the apparent brightness is highest when the observing angle is equal and opposite to the source angle. Most real objects have some mixture of diffuse and specular qualities. light can be reflected by black board. Source: from Federal Standard 1037C Note: In climatology, reflectivity is called albedo. Category:OpticsCategory:Computer vision

Climatology

Climatology is the study of climate, and is a branch of the atmospheric sciences. In contrast to meteorology, which studies short term weather systems lasting up to a few weeks, climatology studies the frequency with which these weather systems occur. It does not study precise instances of atmospheric phenomena (for example cloud formation, rainfall and thunder), but rather their average occurrence over years to millennia, as well as changes in long-term average weather patterns, in relation to atmospheric conditions. Climatologists, those who practice climatology, study both the nature of climates - local, regional or global - and the natural or human-induced factors that cause climates to change. Climatology considers both past and potential future climate change. Phenomena of climatological interest include the atmospheric boundary layer, circulation patterns, heat transfer (radiative, convective and latent), interactions between the atmosphere and the oceans and land surface (particularly vegetation, land use and topography), and the chemical and physical composition of the atmosphere. Related disciplines include chemistry, ecology, geology, geophysics, glaciology, hydrology, oceanography, and volcanology. Climatology is approached in a variety of ways. Paleoclimatology seeks to reconstruct past climates by examining records such as ice cores and tree rings. The study of contemporary climates incorporates meteorological data accumulated over many years, such as records of rainfall, temperature and atmospheric composition. Knowledge of the atmosphere and its dynamics is also embodied in models, either statistical or mathematical, which help by integrating different observations and testing how they fit together. Modeling is used for understanding past, present and potential future climates. Climate research is made difficult by the large scale, long time periods, and complex processes which govern climate. It is generally accepted that climate is governed by differential equations based on physical laws, but what, exactly, are these equations, and what can be concluded from them, is still subject to debate. Climate is sometimes modeled as a stochastic process but this is generally accepted as a approximation to processes that are otherwise too complicated to analyze.

History of climatology

Early climate researchers include Edmund Halley, who published a map of the trade winds in 1686, after a voyage to the southern hemisphere. The weather map as we know it today was first published by Francis Galton in 1863. Galton also invented the term anticyclone.

Famous climatologists


- Francis Galton
- Edmund Halley
- Wladimir Köppen
- Milutin Milankovic
- Hubert Lamb Category:Atmospheric sciences Category:Physical geography

Astronomy

:This article is about the science branch. For information about the magazine, see Astronomy (magazine). Astronomy (magazine) as they circled the Moon in 1969. Located near the center of the far side of Earth's Moon, its diameter is about 58 miles (93 km).]] Astronomy (Greek: αστρονομία = άστρον + νόμος, astronomia = astron + nomos, literally, "law of the stars") is the science of celestial objects and phenomena that originate outside the Earth's atmosphere, such as stars, planets, comets, galaxies, and the cosmic background radiation. It is concerned with the formation and development of the universe, the evolution and physical and chemical properties of celestial objects and the calculation of their motions. Astronomical observations are not only relevant for astronomy as such, but provide essential information for the verification of fundamental theories in physics, such as general relativity theory. Complementary to observational astronomy, theoretical astrophysics seeks to explain astronomical phenomena. Astronomy is one of the oldest sciences, with a scientific methodology existing at the time of Ancient Greece and advanced observation techniques possibly much earlier (see archaeoastronomy). Historically, amateurs have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena. Astronomy is not to be confused with astrology, which assumes that people's destiny and human affairs in general are correlated to the apparent positions of astronomical objects in the sky -- although the two fields share a common origin, they are quite different; astronomers embrace the scientific method, while astrologers do not. In other words, there is no proof that the stars predict our future, but there is proof that our planet is 93 million miles from the sun.

Divisions

In ancient Greece and other early civilizations, astronomy consisted largely of astrometry, measuring positions of stars and planets in the sky. Later, the work of Kepler and Newton, whose work led to the development of celestial mechanics, mathematically predicting the motions of celestial bodies interacting under gravity, and solar system objects in particular. Much of the effort in these two areas, once done largely by hand, is highly automated nowadays, to the extent that they are rarely considered as independent disciplines anymore. Motions and positions of objects are now more easily determined, and modern astronomy is more concerned with observing and understanding the actual physical nature of celestial objects. Since the twentieth century, the field of professional astronomy has split into observational astronomy and theoretical astrophysics. Although most astronomers incorporate elements of both into their research, because of the different skills involved, most professional astronomers tend to specialize in one or the other. Observational astronomy is concerned mostly with acquiring data, which involves building and maintaining instruments and processing the results; this branch is at times referred to as "astrometry" or simply as "astronomy". Theoretical astrophysics is concerned mainly with ascertaining the observational implications of different models, and involves working with computer or analytic models. The fields of study can also be categorized in other ways. Categorization by the region of space under study (for example, Galactic astronomy, Planetary Sciences); by subject, such as star formation or cosmology; or by the method used for obtaining information.

By subject or problem addressed

theoretical astrophysics. Photographed by Mars Global Surveyor, the long dark streak is formed by a moving swirling column of Martian atmosphere (with similarities to a terrestrial tornado). The dust devil itself (the black spot) is climbing the crater wall. The streaks on the right are sand dunes on the crater floor.]]
- Astrometry: the study of the position of objects in the sky and their changes of position. Defines the system of coordinates used and the kinematics of objects in our galaxy.
- Astrophysics: the study of physics of the universe, including the physical properties (luminosity, density, temperature, chemical composition) of astronomical objects.
- Cosmology: the study of the origin of the universe and its evolution. The study of cosmology is theoretical astrophysics at its largest scale.
- Galaxy formation and evolution: the study of the formation of the galaxies, and their evolution.
- Galactic astronomy: the study of the structure and components of our galaxy and of other galaxies.
- Extragalactic astronomy: the study of objects (mainly galaxies) outside our galaxy.
- Stellar astronomy: the study of the stars.
- Stellar evolution: the study of the evolution of stars from their formation to their end as a stellar remnant.
- Star formation: the study of the condition and processes that led to the formation of stars in the interior of gas clouds, and the process of formation itself.
- Planetary Sciences: the study of the planets of the Solar System.
- Astrobiology: the study of the advent and evolution of biological systems in the Universe. Other disciplines that may be considered part of astronomy:
- Archaeoastronomy
- Astrochemistry
- Astrosociobiology
- Astrophilosophy See the list of astronomical topics for a more exhaustive list of astronomy-related pages.

Ways of obtaining information

list of astronomical topics :Main article: Observational astronomy. In astronomy, information is mainly received from the detection and analysis of light and other forms of electromagnetic radiation. Other cosmic rays are also observed, and several experiments are designed to detect gravitational waves in the near future. A traditional division of astronomy is given by the region of the electromagnetic spectrum observed:
- Optical astronomy is the part of astronomy that uses optical components (mirrors, lenses, CCD detectors and photographic films) to observe light from near infrared to near ultraviolet wavelengths. Visible light astronomy (using wavelengths that can be detected with the eyes, about 400 - 700 nm) falls in the middle of this range. The most common tool is the telescope, with electronic imagers and spectrographs.
- Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). The most common tool is the telescope but using a detector which is sensitive to the infrared. Space telescopes are also used to avoid atmospheric thermal emission, atmospheric opacity, and the effects of astronomical seeing at infrared and other wavelengths.
- Radio astronomy detects radiation of millimetre to dekametre wavelength. The receivers are similar to those used in radio broadcast transmission but much more sensitive. See also Radio telescopes.
- High-energy astronomy includes X-ray astronomy, gamma-ray astronomy, and extreme UV (ultraviolet) astronomy, as well as studies of neutrinos and cosmic rays. Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is transparent at the wavelengths being detected. Infrared light is heavily absorbed by water vapor, so infrared observatories have to be located in high, dry places or in space. The atmosphere is opaque at the wavelengths of X-ray astronomy, gamma-ray astronomy, UV astronomy and (except for a few wavelength "windows") Far infrared astronomy, so observations must be carried out mostly from balloons or space observatories. Powerful gamma rays can, however be detected by the large air showers they produce, and the study of cosmic rays can also be regarded as a branch of astronomy.

History of astronomy

cosmic ray :Main article: History of astronomy. In early times, astronomy only comprised the observation and predictions of the motions of the naked-eye objects. Aristotle said that the Earth was the center of the Universe and everything rotated around it in orbits that were perfect circles. Aristotle had to be right because people thought that Earth had to be in the center with everything rotating around it because the wind would not scatter leaves, and birds would only fly in one direction. For a long time, people thought that Aristotle was right, but it is probable that Aristotle accidentally did more to hinder our knowledge than help it. The Rigveda refers to the 27 constellations associated with the motions of the sun and also the 12 zodiacal divisions of the sky. The ancient Greeks made important contributions to astronomy, among them the definition of the magnitude system. The Bible contains a number of statements on the position of the earth in the universe and the nature of the stars and planets, most of which are poetic rather than literal; see Biblical cosmology. In 500 AD, Aryabhata presented a mathematical system that described the earth as spinning on its axis and considered the motions of the planets with respect to the sun. Observational astronomy was mostly stagnant in medieval Europe, but flourished in the Iranian world and other parts of Islamic realm. The late 9th century Persian astronomer al-Farghani wrote extensively on the motion of celestial bodies. His work was translated into Latin in the 12th century. In the late 10th century, a huge observatory was built near Tehran, Persia (now Iran), by the Persian astronomer al-Khujandi, who observed a series of meridian transits of the Sun, which allowed him to calculate the obliquity of the ecliptic. Also in Persia, Omar Khayyám performed a reformation of the calendar that was more accurate than the Julian and came close to the Gregorian. Abraham Zacuto was responsible in the 15th century for the adaptations of astronomical theory for the practical needs of Portuguese caravel expeditions. During the Renaissance, Copernicus proposed a heliocentric model of the Solar System. His work was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileo added the innovation of using telescopes to enhance his observations. Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was left to Newton's invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope. Stars were found to be faraway objects. With the advent of spectroscopy it was proved that they were similar to our own sun, but with a wide range of temperatures, masses, and sizes. The existence of our galaxy, the Milky Way, as a separate group of stars was only proven in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe, seen in the recession of most galaxies from us. Modern astronomy has also discovered many exotic objects such as quasars, pulsars, blazars and radio galaxies, and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as black holes and neutron stars. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's Law, and cosmological abundances of elements.

Timelines in astronomy

cosmological abundances of elements
- Artificial satellites and space probes
- Astronomical maps, catalogs, and surveys
- Big Bang
- Black hole physics
- Cosmic microwave background astronomy
- Cosmology
- Galaxies, clusters of galaxies, and large scale structure
- Interstellar medium and intergalactic medium
- Natural satellites
- Other background radiation fields
- Solar astronomy
- Solar system astronomy
- Stellar astronomy
- Telescopes, observatories, and observing technology
- White dwarfs, neutron stars, and supernovae

See also


- List of astronomical topics
- Astronomers and Astrophysicists
- Astronomical cycles
- Astronomical naming conventions
- Astronomical object
- Astronomical observatories
- Astronomy organizations
- Astronomical symbols
- Space science
- Celestial navigation

Astronomy tools


- Binoculars
- Telescope
- Computers
- Calculator
- Observatory
- Space observatory
- Maksutov telescope

External Links


- [http://www.space.com/ Space.com]
- [http://www.Astronomy.com/ Astronomy.com]
- [http://www.AbsoluteAstronomy.com/ AbsoluteAstronomy.com]
- [http://www.badastronomy.com/ Bad Astronomy]
- [http://www.nasa.gov/ Nasa]
- [http://www.run4space.com Run4Space Forum]
- [http://antwrp.gsfc.nasa.gov/apod/astropix.html/ Astronomy Picture of the Day] ko:천문학 ms:Astronomi ja:天文学 simple:Astronomy th:ดาราศาสตร์

Visible light

The visible spectrum is the portion of the optical spectrum (light or electromagnetic spectrum) that is visible to the human eye. There are no exact bounds to the optical spectrum, but there are to the visible spectrum. A typical human eye will respond to wavelengths from 400 to 700 nm, although some people may be able to perceive wavelengths from 380 to 780 nm. A light-adapted eye typically has its maximum sensitivity at around 555 nm, in the green region of the optical spectrum. Wavelengths visible to the eye also pass through the "visible window", the region of the electromagnetic spectrum which passes largely unattenuated through the Earth's atmosphere (although blue light is scattered more than red light, which is the reason the sky is blue). The response of the human eye is defined by subjective testing (see CIE), but the atmospheric windows are defined by physical measurement. The "visible window" is so called because it overlaps the human visible response spectrum; the near infrared (NIR) windows lie just out of human response window, and the Medium Wavelength IR (MWIR) and Long Wavelength or Far Infrared (FIR or LWIR) are far beyond the human response region. The eyes of many species perceive wavelengths different than the spectrum visible to the human eye. For example, many insects, such as bees, can see light in the ultraviolet, which is useful for finding nectar in flowers. flower into the colors of the optical spectrum.]]

Historical use of the term

Sir Isaac Newton first used the word spectrum (Latin for "appearance" or "apparition") in print in 1671 in describing his experiments in optics. Newton observed that, when a narrow beam of white sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into and through the glass, emerging as different colored bands. Newton hypothesized that light was made up of "corpuscles" (particles) of different colors, and that the different colors of light moved at different speeds in transparent matter, with red light moving more quickly in glass than violet light. The result is that red light was bent (refracted) less sharply that violet light as it passed through the prism, creating a spectrum of colors. It is now known light is composed of photons (which display some of the properties of a wave and some of the properties of a particle), and that all light travels at the same speed (the speed of light) in a vacuum. The speed of light within a material is lower than the speed of light in a vacuum, and the ratio of speeds is known as the refractive index of the material. In some materials, known as non-dispersive, the speed of different frequencies (corresponding to the different colors) does not vary, and so the refractive index is a constant. However, in other (dispersive) materials, the refractive index (and thus the speed) depends on frequency in accorance with a dispersion relation: glass is one such material, which enables glass prisms to create an optical spectrum from white light.

Spectroscopy

dispersion relation transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light.]] The scientific study of objects based on the spectrum of the light they emit is called spectroscopy. One particularly important application of spectroscopy is in astronomy, where spectroscopy is essential for analysing the properties of distant objects. Typically, astronomical spectroscopy utilises high-dispersion diffraction gratings to observe spectra at very high spectral resolutions. Helium was first detected through an analysis of the spectrum of the Sun; chemical elements can be detected in astronomical objects by emission lines and absorption lines; and the shifting of spectral lines can be used to measure the redshift or blueshift of distant or fast-moving objects. The first exoplanets to be discovered were found by analysing the doppler shift of stars at such high resolution that variations in their radial velocity as small as a few metres per second could be detected: the presence of planets was revealed by their gravitational influence on the motion of the stars analysed.

See also


- Frequency
- Rydberg formula
- Wavelength Category:Color Category:Electromagnetic spectrum
- Category:Vision ko:가시광선 ja:可視光線



Moon

:For other moons in the solar system see natural satellite. For the astrological meaning of the Moon, see Solar system in astrology. For other uses see Moon (disambiguation). The Moon is the planet Earth's only natural satellite. It has no formal name other than "The Moon", although it is occasionally called Luna (Latin for moon), or Selene, to distinguish it from the generic "moon" (natural satellites of other planets are also called moons). Its symbol is a crescent (Unicode: ☾). The terms lunar, selene/seleno-, and cynthion (from the Lunar deities Selene and Cynthia) refer to the Moon (aposelene, selenocentric, pericynthion, etc.). The average distance from the Moon to the Earth is 384,403 kilometers (238,857 miles). The Moon's diameter is 3,476 kilometers (2,160 miles). The first manmade object to land on the Moon was Luna 2 in 1959, the first photographs of the otherwise occluded far side of the Moon were made by Luna 3 that same year, and the first people to land on the Moon came aboard Apollo 11 in 1969.

The two sides

The far side is sometimes called the "dark side". In this case "dark" means "unknown and hidden" and not "lacking light" as percieved by the name; in fact the far side receives (on average) as much sunlight as the near side, but at opposite times. Spacecraft are cut off from direct radio communication with the Earth when on the far side of the Moon. One distinguishing feature of the far side is its almost complete lack of maria (singular: mare), which are the dark albedo features.

Orbit

The Moon makes a complete orbit about once every 28 days. Each hour the Moon moves relative to the stars by an amount roughly equal to its angular diameter, or by about 0.5°. The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic and not in the Earth's equatorial plane. Several ways to consider a complete orbit are detailed in the table below, but the two most familiar are: the sidereal month being the time it takes to make a complete orbit with respect to the stars, about 27.3 days; and the synodic month being the time it takes to reach the same phase, about 29.5 days. These differ because in the meantime the Earth and Moon have both orbited some distance around the Sun. The gravitational attraction that the Moon exerts on Earth is the cause of tides in the sea. The tidal flow period, but not the phase, is synchronized to the Moon's orbit around Earth. The tidal bulges on Earth, caused by the Moon's gravity, are carried ahead of the apparent position of the Moon by the Earth's rotation, in part because of the friction of the water as it slides over the ocean bottom and into or out of bays and estuaries. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, resulting in the Moon slowly receding from Earth at the rate of approximately 38 mm per year. At the same time the Earth's rotation is gradually slowing, the Earth's day thus lengthens by about 15 µs every year. A more detailed discussion follows in the section titled Earth & Moon. The Moon is in synchronous rotation, meaning that it keeps the same face turned to the Earth at all times. This synchronous rotation is only true on average because the Moon's orbit has definite eccentricity. When the Moon is at its perigee, its rotation is slower than its orbital motion, and this allows us to see up to an extra eight degrees of longitude of its East (right) side. Conversely, when the Moon reaches its apogee, its rotation is faster than its orbital motion and reveals another eight degrees of longitude of its West (left) side. This is called longitudinal libration. Because the lunar orbit is also inclined to the Earth's equator, the Moon seems to oscillate up and down (as a person's head does when nodding) as it moves in celestial latitude (declination). This is called latitudinal libration and reveals the Moon's polar zones over about seven degrees of latitude. Finally, because the Moon is only at about 60 Earth radii distance, an observer at the equator who observes the Moon throughout the night moves by an Earth diameter sideways. This is diurnal libration and reveals about one degree's worth of lunar longitude. Earth and Moon orbit about their barycenter, or common center of mass, which lies about 4700 km from Earth's center (about 3/4 of the way to the surface). Since the barycenter is located below the Earth's surface, Earth's motion is more commonly described as a "wobble". When viewed from Earth's North pole, Earth and Moon rotate counter-clockwise about their axes; the Moon orbits Earth counter-clockwise and Earth orbits the Sun counter-clockwise. It may seem curious that the inclination of the lunar orbit and the tilt of the Moon's axis of rotation are listed as varying considerably. One must be reminded here that the orbital inclination is measured with respect to the primary's equatorial plane (in this case the Earth's), and that the axis of rotation's tilt is measured with respect to the normal to the satellite's orbital plane (the Moon's). For most planetary satellites, but not for the Moon, these conventions model physical reality and the values are therefore stable. The plane of the lunar orbit maintains an inclination of 5.145 396° with respect to the ecliptic (the orbital plane of the Earth), and the lunar axis of rotation maintains an inclination of 1.5424° with respect to the normal to that same plane. The lunar orbital plane precesses quickly (i.e. its intersection with the ecliptic rotates clockwise), in 6793.5 days (18.5996 years), mostly because of the gravitational perturbation induced by the Sun. During that period, the lunar orbital plane thus sees its inclination with respect to the Earth's equator (itself inclined 23.45° to the ecliptic) vary between 23.45° + 5.15° = 28.60° and 23.45° - 5.15° = 18.30°. Simultaneously, the axis of lunar rotation sees its tilt with respect to the Moon's orbital plane vary between 5.15° + 1.54° = 6.69° and 5.15° - 1.54° = 3.60°. Note that the Earth's tilt reacts to this process and itself varies by 0.002 56° on either side of its mean value; this is called nutation. The points where the Moon's orbit crosses the ecliptic are called the "lunar nodes": the North (or ascending) node is where the Moon crosses to the North of the ecliptic; the South (or descending) node where it crosses to the South. Solar eclipses occur when a node coincides with the new Moon; lunar eclipses when a node coincides with the full Moon.

Earth & Moon

The tides on Earth are generated by the Moon's gravitation (see tide and tidal force for a more detailed discussion). There are two tidal bulges, one in the direction of the Moon, and one in the opposite direction (figure 1). The buildup of these bulges and their movement around the earth causes an energy loss due to friction. The energy loss decreases the rotational energy of the Earth. Since the Earth spins faster than the Moon moves around it, the tidal bulges are dragged along with the Earth's surface faster than the Moon moves, and move "in front of the Moon" (figure 2). Because of this, the Earth's gravitational pull on the Moon has a component in the Moon's "forward" direction with respect to its orbit. This component of the gravitational forces between the two bodies acts like a torque on the Earth's rotation, and transfers angular momentum and rotational energy from the Earth's spin to the Moon's orbital movement. angular momentum Because the Moon is accelerated in forward direction, it moves to a higher orbit. As a result, the distance between the Earth and Moon increases, and the Earth's spin slows down (figure 3). Measurements reveal that the Moon's distance to the Earth increases by 38 mm per year (lunar laser ranging experiments with laser reflectors are used to determine this). Atomic clocks also show that the Earth's day lengthens by about 15 µs every year. However, the formation of tidal bulges on Earth is irregular and not directly related to the frictional energy loss which accompanies the tides. For example, continents on Earth may cause an increase in frictional energy losses and hamper the buildup of tidal bulges (figure 4). The energy loss of the Earth's spin (loss of rotational energy of the Earth) is related to both the energy transfer to the Moon, which depends on the geometry of the mass distributions on Earth (causing a gravity component which pulls the Moon forward), and also to frictional losses, which depends on the properties of the material moving around within tides. The transfer of angular momentum to the Moon's orbit, in contrast, depends only on the geometry of the mass distribution. In general, the angular momentum transferred to the Moon will not correspond to an equivalent energy transfer. There will be a surplus or a deficit in the transfer of angular momentum to the Moon, compared to the energy transfer (figure 5). Since both angular momentum and energy are conserved, there must be a mechanism on earth to store a surplus or a deficit of angular momentum. Candidates for this mechanism are the Earth's magnetic field and internal material currents of the Earth (figure 6). The lunar surface is also subjected to tides from earth, and rises and falls by around 10 cm over 27 days. The lunar tides comprise a mobile component, due to the Sun, and a selenographically fixed one, due to Earth (the Moon keeps the same face turned to the Earth, but not to the Sun). The vertical motion of the Earth-induced component comes entirely from the Moon's orbital eccentricity; if the Moon's orbit were perfectly circular, there would be solar tides only. The magnitude of the Moon's tides corresponds to a Love number of 0.0266, and supports the idea of a partially melted zone around its core. Moonquake waves lose energy below 1000 km depth, and this may also show that the deep material is at least partially melted. The Earth’s Love number is 0.3, corresponding to a movement of 0.5 metres per day; for Venus the Love number is also 0.3. (Source: Patrick Moore, The Data Book of Astronomy - June 2003 Updates)

Origin and history

magnetic field The inclination of the Moon's orbit makes it implausible that the Moon formed along with the Earth or was captured later; its origin is the subject of some scientific debate. Early speculation proposed that the Moon broke off from the Earth's crust due to centrifugal force, leaving an ocean basin (presumed to be the Pacific) behind as a scar. This concept requires too great an initial spin of the Earth. Others speculated the Moon formed elsewhere and was captured into its orbit. Two of the other theories include the coformation or condensation theory and the impact theory, which speculates that the Moon formed from the debris that resulted from a collision between the early Earth and a planetesimal. The Coformation or Condensation hypothesis posits that the Earth and the Moon formed together at about the same time from the primordial accretion disk, the Moon forming from material surrounding the coalescing proto-Earth, similar to the way the planets formed around the Sun. Some suggest that this hypothesis fails to adequately explain the depletion of iron in the Moon. Recently, the Giant Impact theory has been considered a more viable scientific theory for the moon's origin than the coformation or condensation theory. The Giant Impact theory holds that the Moon formed from the ejecta resulting from a collision between a semi-molten Earth and a planet-like object the size of Mars, which has been referred to as Theia. The geological epochs of the Moon are defined based on the dating of various significant impact events in the Moon's history. Analysis of craters and Moon rocks show that there was a late heavy bombardment by asteroids around the period 4000 to 3800 million years ago. Tidal forces deformed the once molten Moon into an ellipsoid, with the major axis pointed towards Earth.

Physical characteristics

Composition

More than 4.5 billion years ago, the surface of the Moon was a liquid magma ocean. Scientists think that one component of lunar rocks, KREEP (K-potassium, Rare Earth Elements, and P-phosphorus), represents the last chemical remnant of that magma ocean. KREEP is actually a composite of what scientists term "incompatible elements": those which cannot fit into a crystal structure and thus were left behind, floating to the surface of the magma. For researchers, KREEP is a convenient tracer, useful for reporting the story of the volcanic history of the lunar crust and chronicling the frequency of impacts by comets and other celestial bodies. The lunar crust is composed of a variety of primary elements, including uranium, thorium, potassium, oxygen, silicon, magnesium, iron, titanium, calcium, aluminium and hydrogen. When bombarded by cosmic rays, each element bounces back into space its own radiation, in the form of gamma rays. Some elements, such as uranium, thorium and potassium, are radioactive and emit gamma rays on their own. However, regardless of what causes them, gamma rays for each element are all different from one another — each produces a unique spectral "signature", detectable by a spectrometer. A complete global mapping of the Moon for the abundance of these elements has never been performed. However, some spacecraft have done so for portions of the Moon; Galileo did so when it flew by the Moon in 1992. [http://photojournal.jpl.nasa.gov/catalog/PIA00131] The overall composition of the Moon is believed to be similar to that of the Earth other than a depletion of volatile elements and of iron.

Selenography

1992 photo.]] When observed with earth based telescopes, the moon can be seen to have some 30,000 craters having a diameter of at least 1 kilometers, but close up observation from lunar orbit reveals a multitude of ever smaller craters. Most are hundreds of millions or billions of years old; the lack of atmosphere or weather or recent geological processes ensures that most of them remain permanently preserved. In the lunar terrae, it is indeed impossible to add a crater of any size without obliterating another; this is termed saturation. The largest crater on the Moon, and indeed the largest known crater within the solar system, forms the South Pole-Aitken basin. This crater is located on the far side, near the south pole, and is some 2,240 km in diameter, and 13 km in depth. The dark and relatively featureless lunar plains are called maria, Latin for seas, since they were believed by ancient astronomers to be water-filled seas. They are actually vast ancient basaltic lava flows that filled the basins of large impact craters. The lighter-colored highlands are called terrae. Maria are found almost exclusively on the Lunar nearside, with the Lunar farside having only a few scattered patches. Scientists think that this asymmetry of lunar features was caused by the synchronization between the Moon's rotation and orbit about the Earth. This synchronization exposes the far side of the Moon to more asteroid and meteor impacts than the near, thereby allowing the maria on the near side to remain relatively undisturbed for many hundreds of millennia. Blanketed atop the Moon's crust is a dusty outer rock layer called regolith. Both the crust and regolith are unevenly distributed over the entire Moon. The crust ranges from 60 km (38 mi) on the near side to 100 km (63 mi) on the far side. The regolith varies from 3 to 5 m (10 to 16 ft) in the maria to 10 to 20 m (33 to 66 ft) in the highlands. In 2004, a team led by Dr. Ben Bussey of Johns Hopkins University using images taken by the Clementine mission determined that four mountainous regions on the rim of the 73 km wide Peary crater at the Moon's north pole appeared to remain illuminated for the entire Lunar day. These unnamed "mountains of eternal light" are possible due to the Moon's extremely small axial tilt, which also gives rise to permanent shadow at the bottoms of many polar craters. No similar regions of eternal light exist at the less-mountainous south pole, although the rim of Shackleton crater is illuminated for 80% of the lunar day. Clementine's images were taken during the northern Lunar hemisphere's summer season, and it remains unknown whether these four mountains are shaded at any point during their local winter season.

Presence of water

Over time, comets and meteorites continuously bombard the Moon. Many of these objects are water-rich. Energy from sunlight splits much of this water into its constituent elements hydrogen and oxygen, both of which usually fly off into space immediately. However, it has been hypothesized that significant traces of water remain on the Moon, either on the surface, or embedded within the crust. The results of the Clementine mission suggested that small, frozen pockets of water ice (remnants of water-rich comet impacts) may be embedded unmelted in the permanently shadowed regions of the lunar crust. Although the pockets are thought to be small, the overall amount of water was suggested to be quite significant — 1 km³. Some water molecules, however, may have literally hopped along the surface and gotten trapped inside craters at the lunar poles. Due to the very slight "tilt" of the Moon's axis, only 1.5°, some of these deep craters never receive any light from the Sun — they are permanently shadowed. Clementine has mapped ([http://www.lpi.usra.edu/research/clemen/clemen.html]) craters at the lunar south pole ([http://www.lpi.usra.edu/research/clemen/2polar.gif]) which are shadowed in this way. It is in such craters that scientists expect to find frozen water if it is there at all. If found, water ice could be mined and then split into hydrogen and oxygen by solar panel-equipped electric power stations or a nuclear generator. The presence of usable quantities of water on the Moon would be an important factor in rendering lunar habitation cost-effective, since transporting water (or hydrogen and oxygen) from Earth would be prohibitively expensive. Clementine twisting the shadow due to the fact that cosmic rays are charged particles.]] The equatorial Moon rock collected by Apollo astronauts contained no traces of water. Neither the Lunar Prospector nor more recent surveys, such as those of the Smithsonian Institution, have found direct evidence of lunar water, ice, or water vapor. Lunar Prospector results, however, indicate the presence of hydrogen in the permanently shadowed regions, which could be in the form of water ice.

Magnetic field

Compared to that of Earth, the Moon has a very weak magnetic field. While some of the Moon's magnetism is thought to be intrinsic (such as a strip of the lunar crust called the Rima Sirsalis), collision with other celestial bodies might have imparted some of the Moon's magnetic properties. Indeed, a long-standing question in planetary science is whether an airless solar system body, such as the Moon, can obtain magnetism from impact processes such as comets and asteroids. Magnetic measurements can also supply information about the size and electrical conductivity of the lunar core — evidence that will help scientists better understand the Moon's origins. For instance, if the core contains more magnetic elements (such as iron) than Earth, then the impact theory loses some credibility (although there are alternate explanations for why the lunar core might contain less iron).

Atmosphere

The Moon has a relatively insignificant and tenuous atmosphere. One source of this atmosphere is outgassing — the release of gases, for instance radon, which originate deep within the Moon's interior. Another important source of gases is the solar wind, which is briefly captured by the Moon's gravity.

Eclipses

The angular diameters of the Moon and the Sun as seen from Earth overlap in their variation, so that both total and annular solar eclipses are possible. In a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye. Since the distance between the Moon and the Earth is very slightly increasing over time, the angular diameter of the Moon is decreasing. This means that several million years ago the Moon always completely covered the Sun on solar eclipses so that no annular eclipses occurred. Likewise, in several million years the Moon will no longer cover the Sun completely and no total eclipses will occur. Eclipses happen only if Sun, Earth and Moon are lined up. Solar eclipses can only occur at new moon; lunar eclipses can only occur at full moon. See also Solar eclipse and Lunar Eclipse.

Observation of the Moon

Lunar Eclipse During the brightest full moons, the Moon can have an apparent magnitude of about −12.6. For comparison, the Sun has an apparent magnitude of −26.8. The Moon appears larger when close to the horizon. This is a purely psychological effect (see Moon illusion). The angular diameter of the Moon from Earth is about one half of one degree. Various lighter and darker colored areas (primarily maria) create the patterns seen by different cultures as the Man in the Moon, the rabbit and the buffalo, amongst others. Craters and mountain chains are also prominent lunar features. From any location on Earth, the highest altitude of the Moon on a day varies between the same limits as the Sun, and depends on season and lunar phase. For example, in winter the Moon is highest in the sky when it is full, and the full moon is highest in winter. The orientation of the Moon's crescent side also depends on the latitude of the observing site. Close to the equator an observer can see a boat Moon. [http://curious.astro.cornell.edu/question.php?number=393] Like the Sun, the Moon can also give rise to an optical effect known as a halo. For more information on how the Moon appears in Earth's sky, see Lunar phase.

Exploration of the Moon

Lunar phase prepares to descend towards the surface of the Moon. NASA photo.]] NASA standing next to boulder at Taurus-Littrow during third EVA (extravehicular activity). NASA photo.]] The first leap in Lunar observation was caused by the invention of the telescope. Especially Galileo Galilei made good use of this new instrument and observed mountains and craters on the Moon's surface. The Cold War-inspired space race between the Soviet Union and the United States of America led to an acceleration. What was the next big step is politically laden. In the US (and the West in general) the landing of the first humans on the moon in 1969 is seen as a culmination, indeed of the space race in general. But from a scientific point of view the first photographs of the until then unseen far side of the moon in 1959 constituted the second leap in Lunar observation. 1959 and Luna missions]] The first man-made object to reach the Moon was the unmanned Soviet probe Luna 2, which made a hard landing on September 14, 1959, at 21:02:24 Z. The far side of the Moon was first photographed on October 7, 1959 by the Soviet probe Luna 3. Luna 9 was the first probe to soft land on the Moon and transmit pictures from the Lunar surface on February 3, 1966. It was proven that a lunar lander would not sink into a thick layer of dust, as had been feared. The first artificial satellite of the Moon was the Soviet probe Luna 10 (launched March 31, 1966). The first robot lunar rover to land on the Moon was the Soviet vessel Lunokhod 1 on November 17 1970 as part of the Lunokhod program. On December 24, 1968 the crew of Apollo 8, Frank Borman, James Lovell, and William Anders became the first human beings to see the far side of the Moon with their own eyes (as opposed to seeing it on a photograph). Humans first landed on the Moon on July 20, 1969. The first man to walk on the lunar surface was Neil Armstrong, commander of the American mission Apollo 11. The last man to stand on the Moon was Eugene Cernan, who as part of the mission Apollo 17 walked on the Moon in December 1972. See also: A full list of lunar astronauts. Moon samples have been brought back to Earth by three Luna missions (nrs. 16, 20, and 24) and the Apollo missions 11 through 17 (minus Apollo 13, which almost ended in a disaster). On January 14 2004, US President George W. Bush called for a plan to return manned missions to the Moon by 2020. NASA's [http://www.nasa.gov/missions/solarsystem/cev.html plan] to accomplish that goal was announced on March 19 2005, and was promptly dubbed Apollo 2.0 by critics. The European Space Agency has plans to launch probes to explore the Moon in the near future, too. European spacecraft Smart 1 was launched September 27, 2003 and entered lunar orbit on November 15 2004. It will survey the lunar environment and create an X-ray map of the Moon. [http://news.bbc.co.uk/2/hi/science/nature/2818551.stm] [http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=36091] The People's Republic of China has expressed ambitious plans for exploring the Moon and is investigating the prospect of lunar mining, specifically looking for the isotope Helium-3 for use as an energy source on Earth [http://space.com/missionlaunches/china_moon_030304.html]. Japan has two planned lunar missions, LUNAR-A and Selene; even a manned lunar base is planned by the Japanese Space Agency (JAXA). India will also try an unmanned orbiting satellite, called Chandrayan. From the mid-1960's to the mid-1970's there were 65 moon landings (with 10 in 1971 alone), but after Luna 24 in 1976 it suddenly stopped. The Soviet Union started focusing on Venus and space stations and the US on Mars and beyond. In 1990 Japan visited the moon with the Hiten spacecraft, becoming the third country to orbit the moon. The spacecraft released the Hagormo probe into lunar orbit, but the transmitter failed rendering the mission scientifically useless.

Human understanding of the Moon

Myth and folk culture

The Moon as muse

The Moon has been the subject of many works of art and literature and the inspiration for countless others.

Astrology

Scientific understanding

A 5,000 year old rock carving at Knowth, Ireland may represent the Moon, which would be the earliest depiction discovered. In many prehistoric and ancient cultures, the Moon was thought to be a deity or other supernatural phenomenon. Among the first in the Western world to offer a scientific explanation for the Moon was the Greek philosopher Anaxagoras, who reasoned that the Sun and Moon were both giant spherical rocks, and that the latter reflected the light of the former. His atheistic view of the heavens was one cause for his imprisonment and eventual exile. By the Middle Ages, before the invention of the telescope, more and more people began to recognize the Moon as a sphere, though they believed that it was "perfectly smooth". sphere In 1609, Galileo Galilei drew one of the first telescopic drawings of the Moon in his book Sidereus Nuncius and noted that it was not smooth but had craters. Later in the 17th century, Giovanni Battista Riccioli and Francesco Maria Grimaldi drew a map of the Moon and gave many craters the names they still have today. Francesco Maria Grimaldi. Surprisingly, the Moon is actually brighter than the Sun at gamma ray wavelengths.]] On maps, the dark parts of the Moon's surface were called maria (singular mare) or "seas", and the light parts were called terrae or continents. The possibility that the Moon could contain vegetation and be inhabited by "selenites" was seriously considered by some major astronomers even into the first decades of the 19th century. In 1835, the Great Moon Hoax fooled some people into thinking that there were exotic animals living on the Moon. Almost at the same time however (during 1834–1836), Wilhelm Beer and Johann Heinrich Mädler were publishing their four-volume Mappa Selenographica and the book Der Mond in 1837, which firmly established the conclusion that the Moon has no bodies of water nor any appreciable atmosphere. There remained some controversy over whether features on the Moon could undergo changes. Some observers claimed that some small craters had appeared or disappeared, but in the 20th century it was determined that these claims were illusory, due to observing under different lighting conditions or due to the inadequacy of earlier drawings. It is however known that the phenomenon of outgassing occasionally occurs. During the Nazi era in Germany, the Welteislehre theory, which claimed the Moon was made of solid ice, was promoted by Nazi leaders. The far side of the Moon remained completely unknown until the Luna 3 probe was launched in 1959, and was extensively mapped by the Lunar Orbiter program in the 1960s. From the 1950s through the 1990s, NASA aerodynamicist Dean Chapman and others advanced the "lunar origin" theory of tektites. Chapman used complex orbital computer models and extensive wind tunnel tests to support the theory that the so-called Australasian tektites originated from the Rosse ejecta ray of the large crater Tycho on the Moon's nearside. Until the Rosse ray is sampled, a lunar origin for these tektites cannot be ruled out. In 1997 the asteroid 3753 Cruithne was found to have an unusual Earth-associated orbit, and has been dubbed by some to be a second "moon" of Earth. It is not considered a moon by astronomers, however, and its orbit is not stable in the long term.

Legal status

Though several flags of the United States have been symbolically planted on the moon, the U.S. government makes no claim to any part of the Moon's surface. The U.S. is party to the Outer Space Treaty, which places the Moon under the same jurisdiction as international waters (res communis). This treaty also restricts use of the Moon to peaceful purposes, explicitly banning weapons of mass destruction (including nuclear weapons) and military installations of any kind. A second treaty, the Moon Treaty, was proposed to restrict the exploitation of the Moon's resources by any single nation, but it has not been signed by any of the space-faring nations. Several individuals have made claims to the Moon in whole or in part, though none of these claims are generally considered credible (see Moon for sale).

Satellites


- Clementine mission - Observation and research satellite
- Smart 1 (or SMART-1) - a European Space Agency research satellite

Surface installations

Multiple scientific instruments were installed during the Apollo missions, some of them still function today. Among those were seismic detectors and reflecting mirrors for laser ranging. laser ranging laser ranging

See also


- Apollo moon landing hoax accusations
- Blue moon
- Chang'e (mythology), Chinese moon goddess
- Crescent
- Colonization of the Moon
- Detailed image of an almost full Moon
- Earthshine
- Lunar effect
- Lunar geologic timescale
- Lunar mare
- Lunar meteorite
- Lunar phase
- Moon landing
- Selene, Greek moon goddess
- Transient lunar phenomenon

Lunar location listings


- List of artificial objects on the Moon
- List of craters on the Moon
- List of features on the Moon
- List of maria on the Moon
- List of mountains on the Moon
- List of valleys on the Moon

References


- Ben Bussey and Paul Spudis, The Clementine Atlas of the Moon, Cambridge University Press, 2004, ISBN 0521815282.
- Patrick Moore, On the Moon, Sterling Publishing Co., 2001 edition, ISBN 0304354694.
- Paul D. Spudis, The Once and Future Moon, Smithsonian Institution Press, 1996, ISBN 1-56098-634-4.

External links

Moon phases


- [http://tycho.usno.navy.mil/vphase.html US Naval Observatory: phase of the Moon for any date and time 1800-2199 A.D.]
- [http://www.moonphaseinfo.com/ Current Moon Phase]
- [http://www.bapuli.co.nr/moon.htm Display current moon phase as wallpaper in Windows]

Space missions


- [http://www.lpi.usra.edu/research/lunar_orbiter/ Digital Lunar Orbiter Photographic Atlas of the Moon]
- [http://www.apolloarchive.com/apollo_archive.html The Project Apollo Archive]
- [http://www.cmf.nrl.navy.mil/clementine/clib/ Clementine Lunar Image Browser]

Scientific


- [http://www.solarviews.com/eng/moon.htm The Moon - by Rosanna and Calvin Hamilton]
- [http://seds.lpl.arizona.edu/nineplanets/nineplanets/luna.html The Moon - by Bill Arnett]
- [http://www.inconstantmoon.com Inconstant Moon - by Kevin Clarke]
- [http://www.moonsociety.org The Moon Society (non-profit educational site)]
- [http://cps.earth.northwestern.edu/GHM/ Geologic History of the Moon by Don Wilhelms]
- [http://isthis4real.com/orbit.xml Can you put the moon into orbit? An interactive simulation - (Needs Firefox 1.5)]

Myth and folklore


- [http://www.straightdope.com/classics/a2_337.html Do things get crazy when the moon is full? by Cecil Adams]
- [http://www.infoplease.com/spot/bluemoon1.html Once in a Blue Moon - What is a blue moon? by Ann-Marie Imbornoni]
- [http://www.suite101.com/article.cfm/folklore/10667 The Moon In Folklore - by Virginia Marin]
- [http://www.laputanlogic.com/articles/2004/04/05-0001.html The Rabbit in the Moon - by John Hardy]

Others


- [http://webgis.wr.usgs.gov/the_moon.htm USGS Planetary GIS webserver - the Moon]
- [http://www.perseus.gr/Astro-Lunar-Scenes-Apo-Perigee.htm The Moon at Apogee and Perigee] (striking photographic comparison)
- [http://www.perseus.gr/Astro-Lunar-Scenes-Sounion-01.htm The Full Moon Rising: I] (striking photo - NOT a composite)
- [http://www.perseus.gr/Astro-Lunar-Scenes-Sounion-02.htm The Full Moon Rising: II] (striking photo - NOT a composite)
- [http://www.perseus.gr/Astro-Lunar-Scenes-Sounion-03.htm The Full Moon Rising: III] (striking photo - NOT a composite)
- [http://www.straightdope.com/classics/a2_110.html Why does the Moon appear bigger near the horizon?] (from The Straight Dope)
- [http://www.badastronomy.com Bad Astronomy]: Dr. Philip Plait, an astronomy professor at Sonoma State University, California, runs this site to explain the many cases of incorrect astronomy (and physics) available to the public, including astrology and the Apollo moon landing hoax accusations.
- [http://www.lunarrepublic.com/atlas/index.shtml The Lunar Navigator: Interactive Maps Of The Moon] features free, interactive online access to maps of the Moon's surface
- [http://www.moonpeople.com A comprehensive guide to the Earth's Moon] (Includes a discussion forum)
- [http://www.traipse.com/earth_and_moon/index.html Distance from the Earth to the Moon, illustrated]
- [http://www.ibiblio.org//e-notes/VRML/Globe/Globe.htm 3D VRML Moon globe] zh-min-nan:Go̍eh-niû ko:달 ms:Bulan (satelit) ja:月 simple:Moon th:ดวงจันทร์

Global warming

Global warming is an increase in the average temperature of the Earth's atmosphere and oceans. The term is also used for the more specific scientific theory of anthropogenic global warming, which states that much of the recent observed (and projected) global warming is human-induced and the result of a strengthened greenhouse effect caused by man-made increases in carbon dioxide (through the burning of fossil fuels and deforestation) and other greenhouse gases. The natural greenhouse effect keeps the Earth 30 °C warmer than it otherwise would be; adding carbon dioxide to an atmosphere, with no other changes, will make a planet's surface warmer. Current research tries to uncover more details, e.g. about positive and negative feedback mechanisms, to allow a more precise quantification of the effect. Temperature change is just one aspect of the broader subject of (human-induced) climate change. The scientific opinion on climate change, as expressed by the UN Intergovernmental Panel on Climate Change (IPCC) and explicitly endorsed by the national science academies of the G8 nations, is that the average global temperature has risen 0.6 ± 0.2 °C since the late 19th century, and that "most of the warming observed over the last 50 years is attributable to human activities". A small minority of qualified scientists contest the view that humanity's actions have played a significant role in increasing recent temperatures. Uncertainties do exist regarding how much climate change should be expected in the future, and a hotly contested political and public debate exists over what actions, if any, should be taken in light of global warming. Based on basic science, observational sensitivity studies, and the climate models referenced by the IPCC, temperatures may increase by 1.4 to 5.8 °C between 1990 and 2100 [http://www.sciencemag.org/cgi/reprint/309/5731/100.pdf]. This is expected to result in other climate changes including rises in sea level and changes in the amount and pattern of precipitation. Such changes may increase extreme weather events such as floods, droughts, heat waves, and hurricanes, change agricultural yields, or contribute to biological extinctions. Although warming is expected to affect the frequency and magnitude of these events, it is very difficult to connect any particular event to global warming.

Overview

The scientific consensus on global warming is that the Earth is warming, and that humanity's greenhouse gas emissions are making a significant contribution. This consensus is summarized by the findings of the Intergovernmental Panel on Climate Change (IPCC). In the Third Assessment Report, the IPCC concluded that "most of the warming observed over the last 50 years is attributable to human activities". This position was recently supported by an international group of science academies from the G8 countries and Brazil, China and India [http://www.royalsoc.ac.uk/displaypagedoc.asp?id=13057]. Over the past century or so the global (land and sea) temperature has increased by 0.6 ± 0.2 °C [http://www.grida.no/climate/ipcc_tar/wg1/figspm-1.htm]. The effects of global warming are increasingly visible. At the same time, atmospheric carbon dioxide has increased from around 280 parts per million (by volume) in 1800 to around 315 in 1958 and 367 in 2000, a 31% increase over 200 years. Other greenhouse gas emissions have also increased. Future CO2 levels are expected to continue rising due to ongoing fossil fuel usage, though the actual trajectory will depend on uncertain economic, sociological, technological, and natural developments. The IPCC SRES gives a wide range of future CO2 scenarios [http://www.grida.no/climate/ipcc_tar/wg1/123.htm], ranging from 540-970 parts per million by 2100. Climate models, driven by estimates of increasing carbon dioxide and to a lesser extent by generally decreasing sulphate aerosols, predict that temperatures will increase (with a range of 1.4 to 5.8 °C for change between 1990 and 2100 [http://www.grida.no/climate/ipcc_tar/wg1/339.htm]). Much of this uncertainty results from not knowing future CO2 emissions, but there is also uncertainty about the accuracy of climate models. Climate commitment studies predict that even if levels of greenhouse gases and solar activity were to remain constant, the global climate is committed to 0.5 °C of warming (some model results are as high as 1.0 °C) over the next one hundred years due to the lag in warming caused by the oceans. Although the combination of scientific consensus and economic incentives (especially for Russia) were enough to persuade the governments of more than 150 countries to ratify the Kyoto Protocol - there are issues about just how much greenhouse gas emissions warm the planet. Uncertainties remain and have been emphasized by some politicians, and others questioning the costs needed to reduce future global warming; however, the business position on climate change is increasingly changing to accept global warming as both real and anthropogenic, and that action such as carbon emissions trading and carbon taxes is needed. The scientific consensus is questioned by a small minority of scientists.

Warming of the Earth

carbon tax Relative to 1860-1900 the global (land and sea) temperature has increased by 0.75 °C. Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C per decade since 1979. Over the past 1-2 thousand years before 1850 the temperature is believed to have been relatively stable, with various (possibly local) fluctuations, such as the Medieval Warm Period or the Little Ice Age. The length of time over which one is interested in change may vary according to the focus of the user of the term and the datasets available for investigation. Temperature record holds a discussion of the various records. An approximately global instrumental temperature record begins in about 1860; contamination from the urban heat island is believed to be small. A longer-term perspective is available from various proxy records for recent millenia; see Temperature record of the past 1000 years for a discussion of these records and their differences. Attribution of recent climate change is clearest for the most recent period (the last 50 years) for which the most detailed data is available. Satellite temperature measurements of the tropospheric temperature date from 1979.

Causes of global warming

Satellite temperature measurements]] The climate system varies both through natural, "internal" processes as well as in response to variations in external "forcing" from both human and non-human causes, including changes in the Earth's orbit around the Sun (Milankovitch cycles), solar activity, and volcanic emissions as well as greenhouse gases. See Climate change for further discussion of these forcing processes. Climatologists accept that the earth has warmed recently. Somewhat more controversial is what may have caused this change. See attribution of recent climate change for further discussion. Atmospheric scientists know that adding carbon dioxide (CO2) or methane (CH4) to an atmosphere, with no other changes, will tend to make a planet's surface warmer. Indeed, greenhouse gases create a natural greenhouse effect without which temperatures on Earth would be 30 °C lower, and the Earth uninhabitable. It is therefore not correct to say that there is a debate between those who "believe in" and "oppose" the theory that adding CO2 or CH4 to the Earth's atmosphere will result in warmer surface temperatures on Earth, on average. Rather, the debate is about what the net effect of the addition of CO2 and CH4 will be, and whether changes in water vapor, clouds, the biosphere and various other climate factors will cancel out its warming effect. The observed warming of the Earth over the past 50 years appears to be at odds with the skeptics' theory that climate feedbacks will cancel out the warming.

Greenhouse gas emissions

greenhouse effect Coal-burning power plants, automobile exhausts, factory smokestacks, and other waste vents of the human environment contribute about 22 billion tons of carbon dioxide and other greenhouse gases into the earth's atmosphere each year. Animal agriculture, manure, natural gas, rice paddies, landfills, coal, and other sources contribute about 250 million tons of methane each year. About half of human emissions have remained in the atmosphere. The atmospheric concentrations of CO2 and CH4 have increased by 31% and 149% respectively above pre-industrial levels since 1750. This is considerably higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. From less direct geological evidence it is believed that CO2 values this high were last attained 40 million years ago. About three-quarters of the anthropogenic emissions of CO2 to the atmosphere during the past 20 years is due to fossil fuel burning. The rest is predominantly due to land-use change, especially deforestation [http://www.grida.no/climate/ipcc_tar/wg1/006.htm]. The longest continuous instrumental measurement of CO2 mixing ratios began in 1958 at Mauna Loa. Since then, the annually averaged value has increased monotonically from 315 ppmv (see the Keeling Curve). The concentration reached 376 ppmv in 2003. South Pole records show similar growth [http://www.cmdl.noaa.gov/info/spo2000.html]. The monthly measurements display small seasonal oscillations. Note that anthropogenic emissions of other pollutants - notably sulphate aerosol - can exert a cooling effect; this accounts for the plateau/cooling seen in the temperature record in the middle of the century [http://www.grida.no/climate/ipcc_tar/wg1/462.htm].

Alternative theories

Solar variation theory

Keeling Curve Direct variations in solar output appear too small to have substantially affected the climate; nonetheless some researchers (e.g. [http://www.dsri.dk/~hsv/SSR_Paper.pdf]) have proposed that feedbacks from clouds or other processes enhance the effect. In the IPCC Third Assessment Report (TAR), it was reported that volcanic and solar forcings might account for half of the temperature variations prior to 1950, but that the net effect of such natural forcings was roughly neutral since then [http://www.grida.no/climate/ipcc_tar/wg1/450.htm]. In particular, the change in climate forcing from greenhouse gases since 1750 was estimated to be 8 times larger than the change in forcing due to increasing solar activity over the same period [http://www.grida.no/climate/ipcc_tar/wg1/251.htm#tab611]. Since the TAR various studies (Lean et al., 2002, Wang et al., 2005) have suggested that irradiance changes over pre-industrial are less by a factor of 3-4 than in the reconstructions of, e.g. Hoyt and Schatten (1993), Lean (2000) used in the TAR. Stott et al. [http://climate.envsci.rutgers.edu/pdf/StottEtAl.pdf] estimated solar forcing to be 16% or 36% of greenhouse warming.

Other theories

Various other hypotheses have been proposed, including but not limited to:
- The warming is within the range of natural variation and needs no particular explanation.
- The warming is a consequence of coming out of a prior cool period — the Little Ice Age — and needs no other explanation.
- The warming trend itself has not been clearly established, and therefore does not need any explanation. At present, none of these has more than a small number of supporters within the climate science community.

Climate models

Little Ice Age A2 emissions scenario, one of the IPCC scenarios that assumes no action is taken to combat global warming.]] Little Ice Age Scientists have studied this issue with computer models of the climate (see below). These models are accepted by the scientific community as being valid only after it has been shown that they do a good job of simulating known climate variations, such as the difference between summer and winter, the North Atlantic Oscillation, or El Niño. All climate models that pass these tests also predict that the net effect of adding greenhouse gases will be a warmer climate in the future. The amount of predicted warming varies by model, however, which probably reflects the way different models depict clouds differently. As noted above, climate models have been used by the IPCC to anticipate a warming of 1.4 °C to 5.8 °C between 1990 and 2100 [http://www.grida.no/climate/ipcc_tar/wg1/339.htm]. They have also been used to help determine the causes of recent climate change by comparing the observed changes to those that the models predict from various natural and human derived forcing factors. The most recent climate models can produce a good match to observations of global temperature changes over the last century. These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions. Adding simulation of the ability of the environment to sink carbon dioxide suggested that rising fossil fuel emissions would decrease absorption from the atmosphere, amplifying climate warming beyond previous predictions, although "Globally, the amplification is small at the end of the 21st century in this model because of its low transient climate response and the near-cancellation between large regional changes in the hydrologic and ecosystem responses" [http://www.pnas.org/cgi/reprint/0504949102v1]. Another suggested mechanism whereby a warming trend may be amplified involves the thawing of tundra, which can release the potent greenhouse gas, methane, that is trapped in large quantities in permafrost and ice clathrates [http://www.newscientist.com/article.ns?id=mg18725124.500]. Uncertainties in the representation of clouds are a dominant source of uncertainty in existing models, despite clear progress in modeling of clouds [http://www.grida.no/climate/ipcc_tar/wg1/271.htm]. There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability. Further, all such models are limited by available computational power, so that they may overlook changes related to small scale processes and weather (e.g. storm systems, hurricanes). However, despite these and other limitations, the IPCC considers climate models "to be suitable tools to provide useful projections of future climates" [http://www.grida.no/climate/ipcc_tar/wg1/309.htm].

Issues

The relation between global warming and ozone depletion

Although they are often interlinked in the popular press, the connection between global warming and ozone depletion is not strong. There are four areas of linkage:
- Global warming from CO2 radiative forcing is expected (perhaps somewhat surprisingly) to cool the stratosphere. This, in turn, would lead to a relative increase in ozone depletion and the frequency of ozone holes.
- Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposed effects: reduced ozone allows more solar radiation to penetrate, thus warming the troposphere. But a colder stratosphere emits less long-wave radiation, tending to cool the troposphere. Overall, the cooling dominates: the IPCC concludes that observed stratospheric O3 losses over the past two decades have caused a negative forcing of the surface-troposphere system [http://www.grida.no/climate/ipcc_tar/wg1/223.htm] of about –0.15 ± 0.10 W m–2 [http://www.ipcc.ch/press/SPM.pdf].
- One of the strongest predictions of the GW theory is that the stratosphere should cool. However, although this is observed, it is difficult to use it for attribution (for example, warming induced by increased solar radiation would not have this upper cooling effect) because similar cooling is caused by ozone depletion.
- Ozone depleting chemicals are also greenhouse gases, representing 0.34 ± 0.03 W/m2, or about 14% of the total radiative forcing from well-mixed GHG's [http://www.ipcc.ch/press/SPM.pdf].

The relation between global warming and global dimming

Some scientists now consider that the ef