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Diurnal Motion

Diurnal motion

Diurnal motion is an astronomical term referring to the apparent daily motion of stars in "orbit" around the Earth, caused by the Earth's rotation around its axis. It is a rotation around the axis between the two celestial poles. It takes Earth 23 hours, 56 minutes and 4.09 seconds (1 sidereal day) to rotate around the axis connecting the North Pole and the South Pole. Direction of the motion on the Northern hemisphere:
- looking to the north, below the North Star: left-right, west-east
- looking to the north, above the North Star: right-left, east-west
- looking to the south: left-right, east-west Thus northern circumpolar stars move anti-clockwise around the North Star. At the North Pole, north, east and west are not applicable, the motion is simply left-right, or looking vertically upward, anti-clockwise around the zenith. For the southern hemisphere, interchange north/south and left/right, and replace North Star by southern celestial pole. The circumpolar stars move clockwise around it. East/west are not interchanged. At the equator both celestial poles are at the horizon and motion is anti-clockwise (i.e. to the left) around the North Star and clockwise (i.e to the right) around the southern celestial pole. All motion is from east to west, except for the two stationary points. The daily path of an object on the celestial sphere, including the possible part below the horizon, has a length proportional to the cosine of the declination. Thus the speed of the diurnal motion of a celestial object is this cosine times 15 °/hr = 15'/min = 15"/s, i.e. (compare angular diameter):
- up to a Sun or Moon diameter every two minutes
- ca. four seconds for the largest planet
- 2000 diameters of the largest stars per second Diurnal motion can be seen in time-exposure photography [http://www.nao.ac.jp/pio/Conste/diu_nso1.jpg]. Circumpolar stars close to the celestial pole move only slowly. Conversely, following the diurnal motion with the camera, to eliminate it on the photograph, can best be done with an equatorial mount, which requires adjusting the right ascension only; a telescope may have a motor to do that automatically (sidereal drive).

See also


- Celestial sphere
- Positional astronomy. Category:Astrometry Category:Stars

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

Motion

:This article is about motion in physics. See also motion (legal), motion (democracy) and Apple Motion. In physics, motion means a change in the position of a body with respect to time, as measured by a particular observer in a particular frame of reference. Until the end of the 19th century, Newton's laws of motion, which he posited as axioms or postulates in his famous Principia, were the basis of what has since become known as classical physics. Calculations of trajectories and forces of bodies in motion based on Newtonian or classical physics were very successful until physicists began to be able to measure and observe very fast physical phenomena. At very high speeds, the equations of classical physics were not able to calculate accurate values. To address these problems, the ideas of Henri Poincaré and Albert Einstein concerning the fundamental phenomenon of motion were adopted in lieu of Newton's. Whereas Newton's laws of motion assumed absolute values of space and time in the equations of motion, the model of Einstein and Poincaré, now called the special theory of relativity, assumed values for these concepts with arbitrary zero points. Because (for example) the special relativity equations yielded accurate results at high speeds and Newton's did not, the special relativity model is now accepted as explaining bodies in motion (when we ignore gravity). However, as a practical matter, Newton's equations are much easier to work with than those of special relativity and therefore are more often used in applied physics and engineering. In the newtonian model, because motion is defined as the proportion of space to time, these concepts are prior to motion, just as the concept of motion itself is prior to force. In other words, the properties of space and time determine the nature of motion and the properties of motion, in turn, determine the nature of force. In the special relativistic model, motion can be thought of as something like an angle between a space direction and the time direction. In special relativity and euclidean space, only relative motion can be measured and that absolute motion is meaningless.

See also


- Equation of motion
- Molecular dynamics
- Motion perception
- Newton's laws of motion
- Physics
- Trajectory of a projectile Category:Mechanics th:การเคลื่อนที่

Orbit

.]] :For other meanings of the term "orbit", see orbit (disambiguation) In physics, an orbit is the path that an object makes around another object while under the influence of a source of centripetal force, such as gravity.

History

Orbits were first analysed mathematically by Johannes Kepler who formulated his results in his laws of planetary motion. He found that the orbits of the planets in our solar system are elliptical, not circular (or epicyclic), as had previously been believed. Isaac Newton demonstrated that Kepler's laws were derivable from his theory of gravitation and that, in general, the orbits of bodies responding to the force of gravity were conic sections. Newton showed that a pair of bodies follow orbits of dimensions that are in inverse proportion to their masses about their common center of mass. Where one body is much more massive than the other, it is a convenient approximation to take the center of mass as coinciding with the center of the more massive body.

Planetary orbits

Within a planetary system, planets, asteroids, comets and space debris orbit the central star in elliptical orbits. Any comet in a parabolic or hyperbolic orbit about the central star is not gravitationally bound to the star and therefore is not considered part of the star's planetary system. To date, no comet has been observed in our solar system with a distinctly hyperbolic orbit. Bodies which are gravitationally bound to one of the planets in a planetary system, either natural or artificial satellites, follow orbits about that planet. Due to mutual gravitational perturbations, the eccentricities of the orbits of the planets in our solar system vary over time. Pluto and Mercury have the most eccentric orbits. At the present epoch, Mars has the next largest eccentricity while the smallest eccentricities are those of the orbits of Venus and Neptune. As an object orbits another, the periapsis is that point at which the two objects are closest to each other and the apoapsis is that point at which they are the farthest from each other. In the elliptical orbit, the centre of mass of the orbiting-orbited system will sit at one focus of both orbits, with nothing present at the other focus. As a planet approaches periapsis, the planet will increase in velocity. As a planet approaches apoapsis, the planet will decrease in velocity. See also: Kepler's laws of planetary motion

Understanding orbits

There are a few common ways of understanding orbits.
- As the object moves sideways, it falls toward the orbited object. However it moves so quickly that the curvature of the orbited object will fall away beneath it.
- A force, such as gravity, pulls the object into a curved path as it attempts to fly off in a straight line.
- As the object falls, it moves sideways fast enough (has enough tangential velocity) to miss the orbited object. This understanding is particularly useful for mathematical analysis, because the object's motion can be described as the sum of the three one-dimensional coordinates oscillating around a gravitational center. As an illustration of the orbit around a planet (eg Earth), the much-used cannon model may prove useful (see image below). Imagine a cannon sitting on top of a (very) tall mountain, which fires a cannonball horizontally. The mountain needs to be very tall, so that the cannon will be above the Earth's atmosphere and we can ignore the effects of air friction on the cannon ball. 300px If the cannon fires its ball with a low initial velocity, the trajectory of the ball will curve downwards and hit the ground (A). As the firing velocity is increased, the cannonball will hit the ground further (B) and further (C) away from the cannon, because while the ball is still falling towards the ground, the ground is curving away from it (see first point, above). If the cannonball is fired with sufficient velocity, the ground will curve away from the ball at the same rate as the ball falls - it is now in orbit (D). The orbit may be circular like (D) or if the firing velocity is increased even more, the orbit may become more (E) and more (F) elliptical. At a certain even faster velocity (called the escape velocity) the motion changes from an elliptical orbit to a parabola.

Newton's laws of motion

For a system of only two bodies that are only influenced by their mutual gravity, their orbits can be exactly calculated by Newton's laws of motion and gravity. Briefly, the sum of the forces will equal the mass times its acceleration. Gravity is proportional to mass, and falls off proportionally to the square of distance. To calculate, it is convenient to describe the motion in a coordinate system that is centered on the heavier body, and we can say that the lighter body is in orbit around the heavier body. An unmoving body that's far from a large object has more energy than one that's close. This is because it can fall farther. This is called "potential energy" because it is not yet actual. With two bodies, an orbit is a flat curve. The orbit can be open (so the object never returns) or closed (returning), depending on the total kinetic + potential energy of the system. In the case of an open orbit, the speed at any position of the orbit is at least the escape velocity for that position, in the case of a closed orbit, always less. The path of a free-falling (orbiting) body is always a conic section. An open orbit has the shape of a hyperbola (or in the limiting case, a parabola); the bodies approach each other for a while, curve around each other around the time of their closest approach, and then separate again forever. This is often the case with comets that occasionally approach the Sun. A closed orbit has the shape of an ellipse (or in the limiting case, a circle). The point where the orbiting body is closest to Earth is the perigee, called periapsis (less properly, "perifocus" or "pericentron") when the orbit is around a body other than Earth. The point where the satellite is farthest from Earth is called apogee, apoapsis, or sometimes apifocus or apocentron. A line drawn from periapsis to apoapsis is the line-of-apsides. This is the major axis of the ellipse, the line through its longest part. Orbiting bodies in closed orbits repeat their path after a constant period of time. This motion is described by the empirical laws of Kepler, which can be mathematically derived from Newton's laws. These can be formulated as follows: # The orbit of a planet around the Sun is an ellipse, with the Sun in one of the focal points of the ellipse. Therefore the orbit lies in a plane, called the orbital plane. The point on the orbit closest to the attracting body is the periapsis. The point farthest from the attracting body is called the apoapsis. There are also specific terms for orbits around particular bodies; things orbiting the Sun have a perihelion and aphelion, things orbiting the Earth have a perigee and apogee, and things orbiting the Moon have a perilune and apolune (or, synonymously, periselene and aposelene). An orbit around any star, not just the Sun, has a periastron and an apastron # As the planet moves around its orbit during a fixed amount of time, the line from Sun to planet sweeps a constant area of the orbital plane, regardless of which part of its orbit the planet traces during that period of time. This means that the planet moves faster near its perihelion than near its aphelion, because at the smaller distance it needs to trace a greater arc to cover the same area. This law is usually stated as "equal areas in equal time." # For each planet, the ratio of the 3rd power of its semi-major axis to the 2nd power of its period is the same constant value for all planets. Except for special cases like Lagrangian points, no method is known to solve the equations of motion for a system with four or more bodies. The 2-body solutions were published by Newton in Principia in 1687. In 1912, K. F. Sundman developed a converging infinite series that solves the 3-body problem, however it converges too slowly to be of much use. Instead, orbits can be approximated with arbitrarily high accuracy. These approximations take two forms. One form takes the pure elliptic motion as a basis, and adds perturbation terms to account for the gravitational influence of multiple bodies. This is convenient for calculating the positions of astronomical bodies. The equations of motion of the moon, planets and other bodies are known with great accuracy, and are used to generate tables for celestial navigation. The differential equation form is used for scientific or mission-planning purposes. According to Newton's laws, the sum of all the forces will equal the mass times its acceleration (F = ma). Therefore accelerations can be expressed in terms of positions. The perturbation terms are much easier to describe in this form. Predicting subsequent positions and velocities from initial ones corresponds to solving an initial value problem. Numerical methods calculate the positions and velocities of the objects a tiny time in the future, then repeat this. However, tiny arithmetic errors from the limited accuracy of a computer's math accumulate, limiting the accuracy of this approach. Differential simulations with large numbers of objects perform the calculations in a hierarchical pairwise fashion between centers of mass. Using this scheme, galaxies, star clusters and other large objects have been simulated.

Analysis of orbital motion

(see also orbit equation and Kepler's first law) To analyse the motion of a body moving under the influence of a force which is always directed towards a fixed point, it is convenient to use polar coordinates with the origin coinciding with the centre of force. In such coordinates the radial and transverse components of the acceleration are, respectively: :\frac - r\left( \frac \right)^2 and :\frac\frac\left( r^2\frac \right). Since the force is always radial, the transverse acceleration is zero, and it follows that: :\frac = hu^2, where h is a constant of integration and we have introduced the auxiliary variable u defined as 1/r. If magnitude of the radial force is f(r) per unit mass of the orbiting body, then the elimination of the time variable from the radial component of the equation of motion yields: :\frac + u = \frac. In the case of an inverse square force law the right hand side of the equation becomes a constant and the equation is seen to be the harmonic equation (up to a shift of origin of the dependent variable). The equation of the orbit described by the particle is thus: :r = \frac = \frac, where φ and e are constants of integration and L is the Semi-latus rectum. This can be recognised as the equation of a conic section in polar coordinates.

Orbital parameters

See: Orbital elements For a general elliptic orbit, the relations between the axis, eccentricity, and least and largest distance are: :Semimajor axis = (periapsis + apoapsis)/2 = mean of the extreme radii :Periapsis = semimajor axis × (1 - eccentricity) = least distance :Apoapsis = semimajor axis × (1 + eccentricity) = largest distance Note that there are alternative definitions for a "mean radius" or "average distance": if you average the radius over time for one orbit (mean anomaly), or over the orbital angle as observed by the primary (true anomaly), then you get a different result. See here for details.

Orbital period

See: orbital period

Orbital decay

If some part of a body's orbit enters an atmosphere, its orbit can decay because of drag. At each periapsis, the object scrapes the air, losing energy. Each time, the orbit grows less eccentric (more circular) because the object loses kinetic energy precisely when that energy is at its maximum. Eventually, the orbit circularises and then the object spirals into the atmosphere. The bounds of an atmosphere vary wildly. During solar maxima, the Earth's atmosphere causes drag up to a hundred kilometres higher than during solar minimums. Some satellites with long conductive tethers can also decay because of electromagnetic drag from the Earth's magnetic field. Basically, the wire cuts the magnetic field, and acts as a generator. The wire moves electrons from the near vacuum on one end to the near-vacuum on the other end. The orbital energy is converted to heat in the wire. Another method of artificially influencing an orbit is through the use of solar sails or magnetic sails. These forms of propulsion require no propellant or energy input, and so can be used indefinitely. See statite for one such proposed use. Orbital decay can also occur due to tidal forces for objects below the synchronous orbit for the body they're orbiting. The gravity of the orbiting object raises tidal bulges in the primary, and since below the synchronous orbit the orbiting object is moving faster than the body's surface the bulges lag a short angle behind it. The gravity of the bulges is slightly off of the primary-satellite axis and thus has a component along the satellite's motion. The near bulge slows the object more than the far bulge speeds it up, and as a result the orbit decays. Conversely, the gravity of the satellite on the bulges applies torque on the primary and speeds up its rotation. Artificial satellites are too small to have an appreciable tidal effect on the planets they orbit, but several moons in the solar system are undergoing orbital decay by this mechanism. Mars' innermost moon Phobos is a prime example, and is expected to either impact Mars' surface or break up into a ring within 50 million years. Finally, orbits can decay via the emission of gravitational waves. This mechanism is extremely weak for most stellar objects, only becoming significant in cases where there is a combination of extreme mass and extreme acceleration, such as with black holes or neutron stars that are orbiting each other closely.

Earth orbits

See Earth orbit for more details.
- Low Earth orbit
- High Earth Orbit
- Intermediate circular orbit
- Geostationary orbit
- Geosynchronous orbit
- Geostationary transfer orbit
- Molniya orbit
- Polar orbit
- Polar Sun Synchronous Orbit (this is not a complete list).

Scaling in gravity

The gravitational constant G is defined to be:
- 6.6742 × 10−11 N·m2/kg2
- 6.6742 × 10−11 m3/(kg·s2)
- 6.6742 × 10−11(kg/m3)-1s-2. Thus the constant has dimension density-1 time-2. This corresponds to the following properties. Scaling of distances (including sizes of bodies, while keeping the densities the same) gives similar orbits without scaling the time: if for example distances are halved, masses are divided by 8, gravitational forces by 16 and gravitational accelerations by 2. Hence orbital periods remain the same. Similarly, when an object is dropped from a tower, the time it takes to fall to the ground remains the same with a scale model of the tower on a scale model of the earth. When all densities are multiplied by four, orbits are the same, but with orbital velocities doubled. When all densities are multiplied by four, and all sizes are halved, orbits are similar, with the same orbital velocities. These properties are illustrated in the formula : GT^2 \sigma = 3\pi \left( \frac \right)^3, for an elliptical orbit with semi-major axis a, of a small body around a spherical body with radius r and average density σ, where T is the orbital period.

Role in the evolution of atomic theory

When atomic structure was first probed experimentally early in the twentieth century, an early picture of the atom portrayed it as a miniature solar system bound by the coulomb force rather than by gravity. This was inconsistent with electrodynamics and the model was progressively refined as quantum theory evolved, but there is a legacy of the picture in the term orbital for the wave function of an energetically bound electron state.

See also


- Astrojax
- Circular orbit
- Clarke orbit
- Difference between sub-orbital and orbital spaceflights
- Elliptic orbit
- Escape velocity
- Gravitational slingshot
- Hohmann transfer orbit
- Hyperbolic trajectory
- Kepler's laws of planetary motion
- Orbit equation
- Orbital maneuver
- Orbital period
- Orbital spaceflight
- Orbital speed
- Parabolic trajectory
- Retrograde motion
- Specific orbital energy
- Sub-orbital spaceflight
- Trajectory

External links


- An on-line orbit plotter: http://www.bridgewater.edu/departments/physics/ISAW/PlanetOrbit.html
- [http://www.braeunig.us/space/orbmech.htm Orbital Mechanics] (Rocket and Space Technology) Category:Celestial mechanics Category:Solar System als:Umlaufbahn ja:軌道 (力学) simple:Orbit th:วงโคจร



Sidereal day

. At time 1, the sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the sun is overhead again (1→3 = one solar day).]]
An apparent sidereal day is the time it takes for the Earth to turn 360 degrees in its rotation; more precisely, is the time it takes a typical star to make two successive upper meridian transits. This is slightly shorter than a solar day. There are 366.2422 sidereal days in a tropical year, but 365.2422 solar days, resulting in a sidereal day of 86,164.091 seconds (or: 23 hours, 56 minutes, 4.091 seconds). The reason there is one more sidereal day than "normal" days in a year is that the Earth's orbit around the Sun offsets one sidereal day, giving observers on Earth 365 1/4 days, even though the planet itself rotated 366 1/4 times (the Earth rotates in the same direction around its axis as it does around the Sun: seen from the northern sky, counter-clockwise). Midnight, in sidereal time, is when the First Point of Aries crosses the upper meridian. A mean sidereal day is reckoned, not from the actual transit, but from the transit of the mean vernal equinox (see: mean sun).

See also


- sidereal month
- sidereal time
- time Category:Units of time ja:恒星日

North Pole

:This is about the geographic meaning of "North Pole." For the cities, see North Pole, Alaska and North Pole, New York. The North Pole is the northernmost point on our planet. There are various ways of defining our North Pole. The North Pole lies in the Arctic Ocean.

Defining the North Pole

The North Pole, is very, very cold. It is just a giant piece of ice on top of the world.

Geographic North Pole

The Geographic North Pole, also known as True North, is close to the northern point at which the Earth's axis of rotation meets the surface. Geographic North defines latitude 90° North. In whichever direction you travel from here, you are always heading south. The pole is located in the Arctic Ocean, which at this point has a depth of 4087 metres (13,410 feet). Classically (19th century) this pole was exactly where people believed the pole of rotation met the Earth's surface, but soon astronomers noticed a small apparent variation of latitude as determined for a fixed point on Earth by observing stars. This variation had a period of about 435 days and the periodic part of it is now called the Chandler wobble after its discoverer. It is desirable to tie the system of Earth coordinates (latitude, longitude, and elevations or orography) to fixed landforms. Of course, given continental drift and the rising and falling of land due to volcanos, erosion and so on, there is no system in which all geographic features are fixed. Yet the International Earth Rotation and Reference Systems Service and the International Astronomical Union have defined a framework called the International Terrestrial Reference System that does an admirable job. The North pole of this system now defines geographic North and it does not quite coincide with the rotation axis. Also see polar motion. On the basis of the sector principle, Canada claims its sovereignty to extend all the way to the Geographic North Pole. There is no land at this location, which is usually covered by sea ice. The theory under which Canada has claimed sovereignty to the North Pole is controversial as there is in fact 770 km of ocean between the pole and Canada's northernmost land point, and several nations, most notably the United States, have challenged the notion that the North Pole does not lie in international waters.

Expeditions

The first expedition to the pole is generally accepted to have been made by, African-American Matthew Henson, Navy engineer, Anglo-American Robert Edwin Peary and four Inuit men (Ootah, Seegloo, Egingway, and Ooqueah) on April 6, 1909. Polar historians believe that Peary honestly thought he had reached the pole. However a 1996 analysis of a newly-discovered copy of Peary's record indicates that Peary must have been in fact 20 nautical miles (40 km) short of the Pole. The first undisputed sight of the pole was in 1926 by Norwegian explorer Roald Amundsen and his American sponsor Lincoln Ellsworth from the airship Norge, designed and piloted by the Italian Umberto Nobile, in a flight from Svalbard to Alaska. On May 3, 1952 U.S. Air Force Lieutenant Colonel Joseph O. Fletcher and Lieutenant William P. Benedict landed a plane at the geographic North Pole. Flying with them was scientist Albert P. Crary. The United States Navy submarine USS Nautilus (SSN-571) crossed the North Pole on August 3, 1958, and on March 17, 1959, the USS Skate (SSN-578) surfaced at the pole, becoming the first naval vessel to reach it. Ralph Plaisted made the first confirmed surface conquest of the North Pole on April 19, 1968. The Soviet nuclear-powered icebreaker Arktika on August 17, 1977, completed the first surface vessel journey to the pole. On April 6, 1992 Robert Schumann became the youngest person to visit the north pole.

Magnetic North

Magnetic North is one of several locations on the Earth's surface known as the "North Pole". Its definition, as the point where the geomagnetic field points vertically downwards, i.e. the dip is 90°, was proposed in 1600 by Sir William Gilbert, a courtier of Queen Elizabeth I, and is still used. It should not be confused with the less frequently used Geomagnetic North Pole. Magnetic North is the place to which all magnetic compasses point, although since the pole marked "N" on a bar magnet points north, and only opposite magnetic poles are attracted to each other, the Earth's magnetic north is actually a south magnetic pole. The orientation of magnetic fields of planets can flip over, an event which is called a geomagnetic reversal. The Earth's poles have done this repeatedly throughout history, and 500,000 years ago, the south magnetic pole was at the South Pole. It is thought that this occurs when the circulation of liquid nickel/iron in the Earth's outer core is disrupted and then reestablishes itself in the opposite direction. It is not known what causes these disruptions. The first expedition to reach this pole was led by James Clark Ross, who found it at Cape Adelaide on the Boothia Peninsula on June 1, 1831. Roald Amundsen found Magnetic North in a slightly different location in 1903. The third observation of Magnetic North was by Canadian government scientists Paul Serson and Jack Clark, of the Dominion Astrophysical Observatory, who found the pole at Allen Lake on Prince of Wales Island. The Canadian government has made several measurements since, which show that Magnetic North is continually moving northwest. Its location (in 2003) is 78°18' North, 104° West, near Ellef Ringnes Island, one of the Queen Elizabeth Islands, in Canada. During the 20th century it has moved 1100 km, and since 1970 its rate of motion has accelerated from 9 km/a to 41 km/a (2001-2003 average; see also Polar drift). If it maintains its present speed and direction it will reach Siberia in about 50 years, but it is expected to veer from its present course and slow down. This movement is on top of a daily or diurnal variation in which Magnetic North describes a rough ellipse, with a maximum deviation of 80 km from its mean position. This effect is due to disturbances of the geomagnetic field by the sun. A line drawn from one magnetic pole to the other does not go through the centre of the Earth, it actually misses it by about 530 km. The angular difference between Magnetic North and true North varies with location, and is called the magnetic declination.

Geomagnetic North Pole

The Geomagnetic North Pole is the pole of the Earth's geomagnetic field closest to true north. The first-order approximation of the Earth's magnetic field is that of a single magnetic dipole (like a bar magnet), tilted about 11° with respect to Earth's rotation axis and centered at the Earth's core. The residuals form the nondipole field. The Geomagnetic poles are the places where the axis of this dipole intersects the Earth's surface. Because the dipole approximation is far from a perfect fit to the Earth's magnetic field, the magnetic field is not quite vertical at the geomagnetic poles. The locations of true vertical field orientation are the magnetic poles, and these are about 30 degrees of longitude away from the geomagnetic poles. Like the Magnetic North Pole, the geomagnetic north pole is a south magnetic pole, because it attracts the north pole of a bar magnet. It is the centre of the region in the magnetosphere in which the Aurora Borealis can be seen. Its present location is 78°30' North, 69° West, near Qaanaaq in Greenland, however it is now drifting away from North America and toward Siberia [http://news.yahoo.com/s/ap/20051209/ap_on_sc/earth_magnetic_pole]. The first voyage to this pole was by David Hempleman-Adams in 1992.

Northern Pole of Inaccessibility

The Northern Pole of Inaccessibility, located at 84°03' north, 174°51' west, is the point farthest from any northern coastline, about 1100 km from the nearest coast. It is a geographic construct, not an actual physical phenomenon. It was first reached by Sir Hubert Wilkins, who flew by aircraft in 1927; in 1958 a Russian icebreaker reached this point.

Defining North Poles in astronomy

Astronomers define the north "geographic" pole of a planet or other object in the solar system by the planetary pole that is in the same ecliptic hemisphere as the Earth's north pole. More accurately, «The north pole is that pole of rotation that lies on the north side of the invariable plane of the solar system» [http://www.hnsky.org/iau-iag.htm]. This means some objects will have directions of rotation opposite the "normal" (i.e., not counter-clockwise as seen from above the north pole). Another frequently used definition uses the right-hand rule to define the north pole: it is then the pole around which the object rotates counterclockwise [http://nssdc.gsfc.nasa.gov/planetary/factsheet/index.html]. When using the first definition (the IAU's), an object's axial tilt will always be 90° or less, but its rotation period may be negative (retrograde rotation); when using the second definition, axial tilts may be greater than 90° but rotation periods will always be positive. For the magnetic poles, their names are decided upon by the direction that their field lines emerge or enter the planet's crust. If they enter the same way as they do for Earth at the north pole, we call this the planet's north magnetic pole. Some bodies in the solar system, including Saturn's moon Hyperion and the asteroid 4179 Toutatis, lack a stable geographic north pole. They rotate chaotically because of their irregular shape and gravitational influences from nearby planets and moons, and as a result the instantaneous pole wanders over their surface, and may vanish altogether for brief periods (when the object comes to a complete standstill with respect to the distant stars). The projection of a planet's north geographic pole onto the celestial sphere gives its north celestial pole. In the particular (but frequent) case of synchronous satellites, four more poles can be defined. They are the near, far, leading, and trailing poles. Take Io for example; this moon of Jupiter rotates synchronously, so its orientation with respect to Jupiter stays constant. There will be a single, unmoving point of its surface where Jupiter is at the zenith, exactly overhead —this is the near pole, also called the sub- or pro-Jovian point. At the antipode of this point is the far pole, where Jupiter lies at the nadir; it is also called the anti-Jovian point. There will also be a single unmoving point which is furthest along Io's orbit (best defined as the point most removed from the plane formed by the north-south and near-far axes, on the leading side) —this is the leading pole. At its antipode lies the trailing pole. Io can thus be divided into north and south hemispheres, into pro- and anti-Jovian hemispheres, and into leading and trailing hemispheres. Note that these poles are mean poles because the points are not, strictly speaking, unmoving: there is constant jiggling about the mean orientation, because Io's orbit is slightly eccentric and the gravity of the other moons disturbs it regularly.

Day and night

During the summer months, the North Pole experiences twenty four hours of daylight but during the winter months the North Pole experiences twenty four hours of darkness. Sunrise and sunset do not occur in a twenty four hour cycle. At the north pole, sunrise begins at the Vernal equinox taking three months for the sun to reach its highest point at the summer solstice when sunset begins, taking three months to reach sunset at the Autumnal equinox. A similar effect can be observed at the South Pole, with a six month difference. This day/night effect is in stark contrast to what is observed at the Equator. This effect is caused by a combination of the Earth's axial tilt and its rotation around the sun. The direction and angle of axial tilt of the Earth remains fairly constant (on a yearly basis) in its plane of rotation around the sun. Hence during the summer, the North Pole is always facing the sun's rays but during the winter, it always faces away from the sun.

Territorial claims to the North Pole (Arctic)

Until 1999, the North Pole (Arctic) had been considered international territory. However, as the polar ice has begun to recede at a rate higher than expected (see global warming), several countries have made moves to claim the water or seabed at the Pole. Russia made its first claim in 2001, claiming Lomonosov Ridge, an underwater mountain ridge underneath the Pole, as a natural extension of Siberia. This claim was contested by Norway, Canada, the United States and Denmark in 2004. Denmark's territory of Greenland has the nearest coastline to the North Pole, and Denmark argues that the Lomonosov Ridge is in fact an extension of Greenland. Canada claims sovereignty in a sector continuing to the North Pole between 60°W and 141°W longitude, a claim that is not universally recognized. In addition, Canada claims the water between its Arctic Islands as internal waters, a claim that is not recognized by the United States (Denmark, Russia and Norway have made claims similar to those of Canada and are opposed by the EU and the United States). The potential value of the North Pole and the area around resides in any possible potential petroleum and gas below the underlying sea-bed, the exploration for which in the near future might become more feasible after the opening of the Northwest Passage.

North Pole in culture

In Christmas stories, the North Pole is sometimes regarded as the place where Santa lives, and where his workshop is located.

See also


- South Pole
- Arctic Ocean
- List of firsts

Magnetic Declination

Magnetic north is determined by the earth’s magnetic field and is not the same as true (or geographic) north. The location of the magnetic north pole changes slowly over time, but it is currently northwest of Hudson’s Bay in northern Canada (approximately 700 km [450 mi] from the true north pole). Maps are based on the geographic north pole because it does not change over time, so north is always at the top of a quadrangle map. However, if you were walk a straight line following the direction your compass needle indicates as north, you would find that you didn’t go from south to north on the map. How far your path varied from true north depends on where you started from; the angle between a straight north-south line and the line you walked is the magnetic declination in the area you were walking. In the example below, if you walked 1.25 miles toward magnetic north (i.e. you followed your compass without adjusting for magnetic declination) you would end up 1/3 of a mile away from where you would be if you walked 1.25 miles toward true north. Fortunately, magnetic declination has been measured throughout the U.S. and can be corrected for on your compass (see below). The map below shows lines of equal magnetic declination throughout the U.S. and Canada. The line of zero declination runs from magnetic north through Lake Superior and across the western panhandle of Florida. Along this line, true north is the same as magnetic north. If you are working west of the line of zero declination, your compass will give a reading that is east of true north. Conversely, if you are working east of the line of zero declination, your compass reading will be west of true north. The exact amount that you need to adjust the declination on your compass to reconcile magnetic north to true north is given in the map legend to the left of the map scale.

External links


- [http://www.arctic-council.org Arctic Council]
- [http://www.northernforum.org The Northern Forum]
- [http://www.arctic.noaa.gov/gallery_np.html North Pole scenery observed by Web Cams]
- [http://www.arctic.noaa.gov/essay_untersteiner3.html The short Arctic summer of 2004]
- [http://www.arctic.noaa.gov/essay_untersteiner2.html The puzzling Arctic summer of 2003]
- [http://www.arctic.noaa.gov/faq.html FAQ on the Arctic and the North Pole]
- [http://www.monolith.com.au/travel/polar_contro.html Polar Controversies Still Rage] article by Roderick Eime
- Category:Navigation Category:Arctic Category:Geography of Canada Category:Poles ko:북극점 ja:北極点 simple:North Pole

North Star

The North Star is a title of the star best suited for navigation northwards. A candidate must be visible from Earth and circumpolar to the north celestial pole. The current one is Polaris. The North Star has been historically used by explorers to determine their latitude. At any point north of the equator the angle from the horizon to the North Star (its altitude) is the same as the latitude from which that angle was taken. For example, the angle to the North Star for a person at 30° latitude will be about 30°. Polaris has a visual magnitude of only 1.97. On the other hand, in 3000 BC the faint star Thuban in the constellation Draco was the North Star; and at magnitude 3.67 it is five times fainter than Polaris. The bright Vega will be the North Star by AD 14,000. In comparison the brightest star, Sirius, has a magnitude of −1.46 (assuming that we exclude the Sun at −26.8). Currently, there is no South Star as useful as Polaris; the faint star σ Octantis is closest to the south celestial pole. However, the constellation Crux, the Southern Cross, points towards the south pole.

See also


- Pole star Category:Navigation Category:Star name disambiguations ja:北極星

Zenith

:For other uses, see Zenith (disambiguation). The zenith, in astronomy, is the point in the sky which appears directly above the observer. More precisely, it is the point on the sky with an altitude of +90 degrees, and it is the pole of the horizontal coordinate system. Geometrically, it is the point on the celestial sphere intersected by a line drawn from the center of the Earth through your location on the Earth's surface. The point opposite the zenith is the nadir. The zenith is, by definition, a point along the local meridian.

References

This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for KDE. See http://edu.kde.org/kstars/index.phtml Category:Astrological factors Category:Spherical astronomy Category:Arabic words ja:天頂

Equator

The equator is an imaginary circle drawn around a planet (or other astronomical object) at a distance halfway between the poles. The equator divides the planet into a Northern Hemisphere and the Southern Hemisphere. The latitude of the equator is, by definition, 0°. The length of Earth's equator is about 40,075.0 km, or 24,901.5 miles. The equator is one of the five main circles of latitude based on the relationship of the Earth's rotation and plane of orbit around the sun. Additionally, the equator is the only line of latitude which is also a great circle The Sun, in its seasonal movement through the sky, passes directly over the equator twice each year on the Vernal and Autumnal Equinoxes, which occur in March and September (respectively). At the equator, the rays of the sun are perpendicular to the surface of the earth on these dates. Places near the equator experience the quickest rates of sunrise and sunset in the world, taking minutes. Such places also have a relatively constant amount of day/night time on every day throughout the year compared with more northerly or southerly places.

Equatorial climate

In many tropical regions people identify two seasons, wet and dry, but most places very close to the equator are wet throughout the year, although seasons can vary depending on a variety of factors including elevation and proximity to an ocean. ocean The surface of the Earth at the equator is mainly ocean. The highest point on the Equator is 4,690 m, at 77° 59' 31" W on the south slopes of Volcán Cayambe (summit 5,790 m) in Ecuador. This is a short distance above the snow line, and is the only point on the Equator where snow lies on the ground (Google Earth satellite data and photos).

Equatorial countries

The equator traverses the land and/or water of 13 countries in total:
- São Tomé and Príncipe - passing through Ilhéu das Rolas, an islet in this archipelago
- Gabon
- Republic of the Congo
- Democratic Republic of Congo
- Uganda
- Kenya
- Somalia
- Maldives - misses every island, passing between Gaafu Dhaalu Atoll and Gnaviyani Atoll
- Indonesia
  - Sumatra - also small islands Tanah Masa to the West and Lingga to the East
  - Borneo - Kalimantan
  - Sulawesi
  - Halmahera - also small islands Kayoa to the West and Gebe to the East
  - Kawe, a small island near Waigeo - and other islets throughout Indonesia
- Kiribati - misses every island
  - Gilbert Islands - passing between Aranuka and Nonouti Atolls
  - Line Islands - passing between Kiritimati Island and Malden Island, though neither is very close to the equator
- Ecuador
  - Galapagos Islands - passing through Isabela Island.
  - Mainland Ecuador
- Colombia
- Brazil

See also


- Thermal equator Category:Lines of latitude Category:Spherical astronomy zh-min-nan:Chhiah-tō ms:Garisan Khatulistiwa ja:赤道 th:เส้นศูนย์สูตร

Celestial sphere

In astronomy and navigation, the celestial sphere is an imaginary rotating sphere of "gigantic radius", concentric with the Earth. All objects in the sky can be thought of as lying upon the sphere. Projected, from their corresponding terrain equivalents, are the celestial equator and the celestial poles. Many ancient societies believed that the stars were equidistant from the Earth and that this sphere was a real model of the universe. This model is a useful abstraction, but not correct. Everything we see in the sky is so very far away that their distances are impossible to gauge just by looking at them. Since their distances are indeterminate, you need only know the direction toward the object to locate it in the sky. In this sense, the celestial sphere model is a very practical tool for positional astronomy. As the Earth rotates on its axis, the objects on the celestial sphere will appear to rotate around the celestial poles every 24 hours; this is diurnal motion. For example the Sun will typically appear to rise in the east and set in the west, as will the stars, planets and moon. On each subsequent night, a given star will rise ~4 minutes earlier than it rose the previous night. Superimposed on diurnal motion is; intrinsic motion as the objects change their relative positions, with respect to Earth. For example, over the course of a year the Sun, relative to the background stars, will follow a bisecting great circle (known as the ecliptic). The celestial sphere is divided by projecting the equator into space. This divides the sphere into the north celestial hemisphere and the south celestial hemisphere. Likewise, one can locate the Celestial Tropic of Cancer, Celestial Tropic of Capricorn, North Celestial Pole, and South Celestial Pole. As the earth rotates from west to east, the celestial sphere appears to rotate from east to west. If a star is sufficiently near the celestial pole that is above the horizon, the star is also always above the horizon, encircling the pole; such stars are circumpolar. The directions toward various objects in the sky can be quantified by constructing a celestial coordinate system.

See also


- Armillary sphere
- Celestial coordinate system
- Celestial horizon
- Equinox
- Geocentric universe
- Prograde and retrograde motion
- Solstice
- Zodiac

External link


- [http://skyandtelescope.com/observing/skychart/# SkyandTelescope.com SkyChart] Category:Spherical astronomy Category:Celestial coordinate system ja:天球 th:ทรงกลมฟ้า

Angular diameter

The angular diameter of an object as seen from a given position is the diameter measured as an angle. It satisfies the formula \delta = \arctan (diameter/ distance). In astronomy the size of objects in the sky is often measured in terms of their angular diameter as seen from Earth, rather than their actual size. The angular diameter of Earth's orbit around the Sun, from a distance of one parsec, is 2" (two arcseconds). The angular diameter of the Sun, from a distance of one light year, is 0.03", of the Earth 0.0003". This table shows the angular sizes of the most important Solar System bodies as seen from the Earth.
Sun30'
Moon29' - 33'
Venus10" - 58"
Jupiter 32" - 49"
Saturn 16" - 20"
Mars 4" - 16"
Uranus 3" - 4"
Neptune 2"

- Alpha Centauri A: ca. 0.007"
- Sirius: ca. 0.007" Thus the angular diameter of the Sun is ca. 250,000 that of Sirius (it has twice the diameter and the distance is 500,000 times as much; the Sun is 10,000,000,000 times as bright, corresponding to an angular diameter ratio of 100,000, so Sirius is roughly 6 times as bright per unit solid angle). The angular diameter of the Sun is also ca. 250,000 that of Alpha Centauri A (it has the same diameter and the distance is 250,000 times as much; the Sun is 40,000,000,000 times as bright, corresponding to an angular diameter ratio of 200,000, so Alpha Centauri A is a little brighter per unit solid angle). The angular diameter of the Sun is about the same as that of the Moon (the diameter is 400 times as large and the distance also; the Sun is 200,000-500,000 times as bright as the full Moon (figures vary), corresponding to an angular diameter ratio of 450-700, so a celestial body with a diameter of 2.5-4" and the same brightness per unit solid angle would have the same brightness as the full Moon).

See also


- Angular resolution
- Apparent size Category:Elementary geometry Category:Astrometry Category:Astronomy Category:Angle

Photography

Photography is the process of making pictures by means of the action of light. It involves recording light patterns, as reflected from objects, onto a sensitive medium through a timed exposure. The process is done through mechanical, chemical or digital devices commonly known as cameras. The word comes from the Greek words φως phos ("light"), and γραφις graphis ("stylus", "paintbrush") or γραφη graphê, together meaning "drawing with light" or "representation by means of lines" or "drawing."

Photographic image forming devices

Most commonly a camera or camera obscura is the image forming device and photographic film or a digital storage card is the recording medium, although other methods are available. For instance, the photocopy or xerography machine forms permanent images but uses the transfer of static electrical charges rather than photographic film, hence the term electrophotography. The rayographs published by Man Ray in 1922 are images produced by the shadows of objects cast on the photographic paper, without the use of a camera. And one can place objects directly on the glass of a scanner to produce pictures electronically. Photographers control the camera to expose the light recording material (usually film or a charge-coupled device) to light. After processing, this produces an image whose contents are acceptably sharp, bright and composed to achieve the objective of taking the photograph. The controls include:
- Focus
- Aperture of the lens
- Duration of exposure (or shutter speed)
- Focal length of the lens (telephoto, macro, wide angle, or zoom)
- Sensitivity of the medium to light intensity and color The controls are usually inter-related, for example brightness is aperture multiplied by shutter speed, and varying the focal length of the lens will allow greater control over the depth of field. Depth of field is the area of the image that is in focus. The larger the depth of field, the larger the area of the image that is in focus. The smaller the depth of field, the smaller the area that is in focus. A higher aperture setting, like f16 or f22, gives the photographer a smaller depth of field. A lower aperture setting, like f1.4 or f2.8, gives a larger depth of field.

Uses of photography

Photography can be classified under imaging technology and has gained the interest of scientists and artists from its inception. Scientists have used its capacity to make accurate recordings, such as Eadweard Muybridge in his study of human and animal locomotion (1887). Artists have been equally interested by this aspect but have also tried to explore other avenues than the photo-mechanical representation of reality, such as the pictorialist movement. Military, police and security forces use photography for surveillance, recognition and data storage.

History of photography

pictorialist pictorialist

Invention

Chemical photography

Projecting images onto surfaces has been done for centuries. The camera obscura and the camera lucida were used by artists to trace scenes as early as the 16th century. These early cameras did not fix an image in time; they only projected what was before an opening in the wall of a darkened room onto a surface. In effect, the entire room was turned into a large pinhole camera. Indeed, the phrase camera obscura literally means "darkened room," and it is after these darkened rooms that all modern cameras have been named. The first photograph is considered to be an image produced in 1826 by the French inventor Nicéphore Niépce on a polished pewter plate covered with a petroleum derivative called bitumen of Judea. It was produced with a camera, and required an eight hour exposure in bright sunshine. However, this process turned out to be a dead end and Niépce began experimenting with silver compounds based on a Johann Heinrich Schultz discovery in 1724 that a silver and chalk mixture darkens when exposed to light. Niépce, in Chalon-sur-Saône, and the artist Jacques Daguerre, in Paris, refined the existing silver process in a partnership. In 1833 Niépce died unexpectedly of a stroke, leaving his notes to Daguerre. While he had no scientific background, Daguerre made two pivotal contributions to the process. He discovered that by exposing the silver firstly to iodine vapour, before exposure to light, and then to mercury fumes after the photograph was taken, a latent image could be formed and made visible. By then bathing the plate in a salt bath the image could be fixed. In 1839 Daguerre announced that he had invented a process using silver on a copper plate called the Daguerreotype. A similar process is still used today for Polaroids®. The French government bought the patent and immediately made it public domain. Across the English Channel, William Fox Talbot had earlier discovered another means to fix a silver process image but had kept it secret. After reading about Daguerre's invention, Talbot refined his process, so that it might be fast enough to take photographs of people as Daguerre had done, and by 1840 he had invented the calotype process. He coated paper sheets with silver chloride to create an intermediate negative image. Unlike a daguerreotype, a calotype negative could be used to reproduce positive prints, like most chemical films do today. Talbot patented this process, which greatly limited its adoption. He spent the rest of his life in lawsuits defending the patent until he gave up on photography all together. But later this process was refined by George Eastman and is today the basic technology used by chemical film cameras. Hippolyte Bayard also developed a method of photography, but delayed announcing it and so was not recognized as its inventor. Hippolyte Bayard

Reference


- Coe, Brian. The Birth of Photography. Ash & Grant, 1976.

Social history

Popularization

The Daguerreotype proved popular as it responded to the demand for portraiture emerging from the middle classes during the Industrial Revolution. This demand, that could not be met in volume and in cost by oil painting, may well have been the push for the development of photography. But still daguerreotypes, while beautiful, were fragile and difficult to copy. A single photograph taken in a portrait studio could cost $1000 in 2005 dollars. Photographers also encouraged chemists to refine the process of making many copies cheaply, which eventually lead them back to Talbot's process. Ultimately, the modern photographic process came about from a series of refinements and improvements in the first 20 years. In 1884 George Eastman, of Rochester, New York, developed dry gel on paper, or film, to replace the photographic plate, so that a photographer no longer needed to carry boxes of plates and toxic chemicals around. In July of 1888 Eastman's Kodak camera went on the market with the slogan "You press the button, we do the rest". Now anyone could take a photograph and leave the dangerous portions of the process to others. Photography became available for the mass-market in 1901 with the introduction of Kodak Brownie. Very little has changed in chemical photography since then, though color film has become the standard, as well as automatic focus and automatic exposure. Digital recording of images is becoming increasingly prevalent, as digital cameras allow instant previews on LCD screens among other benefits, and the resolution of top of the range models has exceeded high quality 35mm film while lower resolution models have become affordable. For the enthusiast photographer processing black and white film, little has changed since the introduction of the 35mm film Leica camera in 1925.

Economic history

In the nineteenth century, photography developed rapidly as a commercial service. In the U.S. in 1890, the number of professional photographers was about the same as the number of accountants, artists, and dentists, respectively, and about ten times greater than the number of authors. End-user supplies of photographic equipment accounted for only about 20% of industry revenue. Several trends characterize the photographic industry from the end of the nineteenth century to the end of the twentieth century. The ratio of revenue from end-user photographic supplies to revenue from professional services rose by an order of magnitude. The prevalence of personal cameras and the ratio of end-user photographs rose closely in tandem with the prevalence of telephone and the telephone conversation minutes. However, the ratio of photographic industry revenue to telephone industry revenue dropped sharply.[http://www.galbithink.org/sense-s6.htm#wpp1] Given the development of new digital technologies for creating and sharing images, and of new communications devices, e.g. camera phones, understanding the economics of image use are becoming increasingly important for understanding the evolution of the communications industry as a whole. Resources Jenkins, Reese V. Images & Enterprise: Technology and the American Photographic Industry 1839-1925. Baltimore, The Johns Hopkins University Press, 1975. The book provides a fine overview of the economics of photography and is especially strong on the growth and development of the Eastman Kodak Company.

Color photography

Main article: color photography Color photography was explored throughout the 1800s. Initial experiments in color could not fix the photograph and prevent the color from fading. The first permanent color photo was taken in 1861 by the physicist James Clerk Maxwell. One of the early methods of taking color photos was to use three cameras. Each camera would have a color filter in front of the lens. This technique provides the photographer with the three basic channels required to recreate a color image in a darkroom or processing plant. Practical application of the technique was held back by the very limited colour response of early film; however, in the early 1900s, following the work of photo-chemists such as H. W. Vogel, emulsions with adequate sensitivity to green and red light at last became available. The first color film, Autochrome, thus did not reach the market until 1907; it was based on a 'screen-plate' filter made of dyed dots of potato starch. The first modern ('integrated tri-pack') color film, Kodachrome, was introduced in 1935 based on three colored emulsions. Most modern color films, except Kodachrome, are based on technology developed for Agfacolor (as 'Agfacolor Neue') in 1936. Instant color film was introduced by Polaroid in 1963. Color photography may form images as a positive transparency, intended for use in a slide projector or as color negatives, intended for use in creating positive color enlargements on specially coated paper. The latter is now the most common form of film (non-digital) color photography, owing to the introduction of automated photoprinting equipment.

Digital photography

Main article: digital photography digital photography Traditional photography was a considerable burden for photographers working at remote locations (such as press correspondents) without access to processing facilities. With increased competition from television, there was pressure to deliver their images to newspapers with greater speed. Photo-journalists at remote locations would carry a miniature photo lab with them, and some means of transmitting their images down the telephone line. In 1981, Sony unveiled the first consumer camera to use a CCD for imaging, and which required no film -- the Sony Mavica. While the Mavica did save images to disk, the images themselves were displayed on television, and therefore the camera could not be considered fully digital. In 1990, Kodak unveiled the DCS 100, the first commercially available digital camera. Its cost precluded any use other than photojournalism and professional applications, but commercial digital photography was born. Digital photography uses an electronic sensor such as a charge-coupled device to record the image as a piece of electronic data rather than as chemical changes on film. Some other devices, such as cell phones, now include digital photography features. In 10 years, digital cameras have become widespread consumer products. Digital cameras now outsell film cameras, and many include features not found in film cameras such as the ability to shoot video and record audio. Kodak announced in January 2004 that it would no longer produce reloadable 35-millimeter cameras after the end of that year. This was interpreted as a sign of the end of film photography. However, Kodak was at that time a minor actor on the reloadable film cameras market. The price of 35mm and APS compact cameras have dropped, probably due to direct competition from digital and the resulting growth of the offer of second-hand film cameras. However, "wet" photography may endure, as dedicated amateurs and skilled artists often prefer the use of traditional and familiar materials and techniques.

Commercial photography

The commercial photographic world is traditionally broken down to:
- Advertising photography: photographs done to illustrate a service or product. These images are generally done with an Advertising Agency, Design Firm or with an in-house Corporate design team.
- Editorial photography: photographs done to illustrate a story or idea within the context of a magazine. These are usually assigned by the magazine.
- Photojournalism: this can be considered a subset of Editorial. Photographs done in this context are accepted as a truthful documentation of a news story.
- Portrait and wedding photography: photographs done and sold directly to the end user of the images.
- Fine art photography: photographs created to fulfill a vision, and reproduced to be sold directly to the end user. The market for photographic services demonstrates the aphorism "one picture is worth a thousand words," which has an interesting basis in the history of photography. Magazines and newspapers, companies putting up Web sites, advertising agencies and other groups pay for photography. Many people take photographs for self-fulfillment or for commercial purposes. Organizations with a budget and a need for photography have several options: they can assign a member of the organization, hire someone, run a public competition, or obtain rights to stock photographs.

Terminology

Traditionally, the product of photography has been called a photograph. The term photo is a convenient abbreviation. Many people also call them pictures. In digital photography, the term image has begun to replace photograph. This term is neither more nor less correct than photograph, either in film or digital photography. (The term image is traditional in geometric optics.) Although not viewed by all photographers as true photography, digital photography in fact meets all requirements to be called such. Even though there are no chemical processes, a digital camera captures a frame of whatever it happens to be pointed at, which can be viewed later.

Photography as an art form

stock photographs settings can achieve unusual results]] During the twentieth century, both fine art photography and documentary photography became accepted by the English-speaking art world and the gallery system.