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Pre-historic

Pre-historic


- The term prehistory (Greek words προ = before and ιστορία = history) is usually used to describe the period before written history became available. Paul Tournal originally coined the term Pré-historique in describing the finds he had made in the caves of southern France, and was used in French since the 1830s to describe the time before writing, then introduced into English by Daniel Wilson in 1851.
- The term became less meaningful in the 20th century as the boundary between history (strictly the written record) and other disciplines became less rigid and defined. Indeed today most historians rely on evidence from multiple sources and the notion of limiting historical study to a 5000 year span, out of a possible few million years of human existence, and of only those few world cultures that left written records, is no longer taken seriously. For example historians study the Celts, African civilizations and North American civilizations, even though they are by definition "prehistory".
- Prehistory can be said to date back to the beginning of the universe itself, although the term is most often used to describe periods when there was life on Earth; dinosaurs can be described as prehistoric animals and cavemen are described as prehistoric people.
- Because, by definition, there are no written records from prehistoric times, the information we know about the time period is informed by the fields of palaeontology, astronomy, biology, geology, anthropology, archaeology, indeed all the natural sciences.
- Human prehistory differs from history not only in terms of chronology but in the way it deals with the activities of archaeological cultures rather than named nations or individuals. Restricted to material remains rather than written records (and indeed only those remains that have survived), prehistory is anonymous. Because of this, the cultural terms used by prehistorians such as Neanderthal or Iron Age are modern, arbitrary labels, the precise definition of which are often subject to discussion and argument.
- The date marking the end of prehistory, that is the date when written historical records become a useful academic resource, varies from region to region. In Egypt it is generally accepted that prehistory ended around 3500 BC whereas in New Guinea the end of the prehistoric era is set much more recently, AD 1900.

Age systems


- Until the arrival of humans, a geologic time scale defines periods in prehistory. Archaeology has augmented this record and provided more precise divisions during later, human, prehistory.
- Human prehistory in the Old World is often subdivided by the three-age system. This system of classifying human prehistory creates three consecutive time periods, named for their respective predominant tool-making technologies. In the New World other naming schemes have been defined such as that listed in Archeology of the Americas.
- These very general systems of dividing up prehistory are being found to be increasingly inapplicable as archaeological discoveries suggest a much more complex view of prehistory.

External links


- The Neanderthal site at [http://www.geocities.com/patrickbringmans/veldwezelt-hezerwater.html Veldwezelt-Hezerwater], Belgium

See also


- Prehistoric art
- Prehistoric life
- Prehistoric music
- Prehistoric warfare
- Periodization Category:Periods and stages in archaeologyCategory:Anthropology ko:선사시대 ja:先史時代 th:ยุคก่อนประวัติศาสตร์

Greek language

Greek (Greek Ελληνικά, IPA – "Hellenic") is an Indo-European language with a documented history of 3,500 years. Today, it is spoken by 15 million people in Greece, Cyprus, the former Yugoslavia, particularly The Former Yugoslav Republic of Macedonia, Bulgaria, Albania and Turkey. There are also many Greek emigrant communities around the world, such as those in Melbourne, Australia which is the third-largest Greek-populated city in the world, after Athens and Thessaloniki. Greek has been written in the Greek alphabet, the first true alphabet, since the 9th century B.C. and before that, in Linear B and the Cypriot syllabaries. Greek literature has a long and rich tradition.

History

This article does not cover the reconstructed history of Greek prior to the use of writing. For more information, see main article on Proto-Greek language. Greek has been spoken in the Balkan Peninsula since the 2nd millennium BC. The earliest evidence of this is found in the Linear B tablets dating from 1500 BC. The later Greek alphabet (q.v.) is unrelated to Linear B, and was derived from the Phoenician alphabet (abjad); with minor modifications, it is still used today. Greek is conventionally divided into the following periods:
- Mycenean Greek: the language of the Mycenean civilisation. It is recorded in the Linear B script on tablets dating from the 16th century BC onwards.
- Classical Greek (also known as Ancient Greek): In its various dialects was the language of the Archaic and Classical periods of Greek civilisation. It was widely known throughout the Roman empire. Classical Greek fell into disuse in western Europe in the Middle Ages, but remained known in the Byzantine world, and was reintroduced to the rest of Europe with the Fall of Constantinople and Greek migration to Italy.
- Hellenistic Greek (also known as Koine Greek): The fusion of various ancient Greek dialects with Attic (the dialect of Athens) resulted in the creation of the first common Greek dialect, which gradually turned into one of the world's first international languages. Koine Greek can be initially traced within the armies and conquered territories of Alexander the Great, but after the Hellenistic colonisation of the known world, it was spoken from Egypt to the fringes of India. After the Roman conquest of Greece, an unofficial diglossy of Greek and Latin was established in the city of Rome and Koine Greek became a first or second language in the Roman Empire. Through Koine Greek it is also traced the origin of Christianity, as the Apostles used it to preach in Greece and the Greek-speaking world. It is also known as the Alexandrian dialect, Post-Classical Greek or even New Testament Greek (after its most famous work of literature).
- Medieval Greek: The continuation of Hellenistic Greek during medieval Greek history as the official and vernacular (if not the literary nor the ecclesiastic) language of the Byzantine Empire, and continued to be used until, and after the fall of that Empire in the 15th century. Also known as Byzantine Greek.
- Modern Greek: Stemming independently from Koine Greek, Modern Greek usages can be traced in the late Byzantine period (as early as 11th century). Two main forms of the language have been in use since the end of the medieval Greek period: Dhimotikí (Δημοτική), the Demotic (vernacular) language, and Katharévousa (Καθαρεύουσα), an imitation of classical Greek, which was used for literary, juridic, and scientific purposes during the 19th and early 20th centuries. Demotic Greek is now the official language of the modern Greek state, and the most widely spoken by Greeks today. It has been claimed that an "educated" speaker of the modern language can understand an ancient text, but this is surely as much a function of education as of the similarity of the languages. Still, Koinē , the version of Greek used to write the New Testament and the Septuagint, is relatively easy to understand for modern speakers. Greek words have been widely borrowed into the European languages: astronomy, democracy, philosophy, thespian, etc. Moreover, Greek words and word elements continue to be productive as a basis for coinages: anthropology, photography, isomer, biomechanics etc. and form, with Latin words, the foundation of international scientific and technical vocabulary. See English words of Greek origin, and List of Greek words with English derivatives.

Classification

Greek is an independent branch of the Indo-European language family. The ancient languages which were probably most closely related to it, Ancient Macedonian language (which may be regarded as a dialect of Greek) and Phrygian, are not well enough documented to permit detailed comparison. Among living languages, Armenian seems to be the most closely related to it.

Geographic distribution

Modern Greek is spoken by about 15 million people mainly in Greece and Cyprus. There are also Greek-speaking populations in Georgia, Ukraine, Egypt, Turkey, Albania, Former Yugoslav Republic of Macedonia and Southern Italy. The language is spoken also in many other countries where Greeks have settled, including Armenia, Australia, Austria, Belgium, Bulgaria, Canada, Denmark, France, Germany, Netherlands, Sweden, United Kingdom, and the United States.

Official status

Greek is the official language of Greece where it is spoken by about 99.5% of the population. It is also, alongside Turkish, the official language of Cyprus. Due to the membership of Greece and Cyprus, Greek is one of the 20 official languages of the European Union.

Phonology

This section generally describes the post-Classic phonology of the Greek language. :All phonetic transcriptions in this section use the International Phonetic Alphabet

Vowel sounds

Greek has 5 vowel sounds, all phonemic:



1830s

Events and Trends


- Electromagnetic induction discovered by Michael Faraday
- Dutch-speaking farmers known as Voortrekkers emigrate northwards from the Cape Colony
- Croquet invented in Ireland
- Railroad construction begins in earnest in the United States
- Egba refugees fleeing the Yoruba civil wars found the city of Abeokuta in south-west Nigeria

World Leaders


- Emperor Francis II (Austria)
- Emperor Ferdinand I (Austria)
- Chancellor Klemens Wenzel von Metternich (Austria)
- King Louis-Philippe (France)
- Pope Gregory XVI
- Emperor Nicholas I (Russia)
- King William IV (United Kingdom)
- Queen Victoria (United Kingdom)
- Prime Minister Lord Grey (United Kingdom)
- Prime Minister Lord Melbourne (United Kingdom)
- President Andrew Jackson (United States)
- President Martin Van Buren (United States)

See also

1830s in sports

External links


- [http://www.mccord-museum.qc.ca/en/keys/webtours/GE_P1_1_EN.html The Aftermath of the Rebellions] — The Rebellions of 1837-1838 : the most dramatic political event in Canadian history Category:1830s ko:1830년대 ja:1830年代

1851

1851 was a common year starting on Wednesday (see link for calendar).

Events


- January 23 - The flip of a coin determines whether a new city in Oregon is named after Boston, Massachusetts, or Portland, Maine, with Portland winning.
- March 1 - Victor Hugo gives speech at the French national assembly and uses the phrase United States of Europe several times
- March 27 - First reported case of white men seeing Yosemite Valley.
- March 30 - A population census was taken of all people living in the United Kingdom.
- May 1 - The Great Exhibition of the Works of Industry of All Nations in the Crystal Palace, Hyde Park, London is opened by Queen Victoria. It runs until October 18.
- May 15 - Rama IV is crowed King of Thailand.
- July - The immortal game, a famous chess game, is played.
- July 1 - Colony of Victoria separates from New South Wales.
- July 1 - Serial poisoner Helene Jegado is arrested in Rennes, France
- July 29 - Annibale de Gasparis, in Naples, Italy discovers asteroid 15 Eunomia.
- August 5 - Mount Pelee erupts and kills 30 people.
- August 22 - The yacht America wins the first America's Cup race.
- September 15 - Saint Joseph's University is founded in Philadelphia, Pennsylvania.
- September 18 - The New York Times is founded.
- October - Reuters news service founded.
- October 18 - The Great Exhibition in London is closed.
- October 24 - Ariel and Umbriel, moons of Uranus, discovered by William Lassell.
- November 13 - The Denny Party lands at Alki Point, the first settlers of what will become Seattle, Washington.
- November 14 - Herman Melville's novel Moby-Dick is published in the U.S. by Harper & Brothers, New York - after it was first published on October 18, by Richard Bentley, London.
- December 2 - Louis Napoleon, president of France, dissolves French National Assembly and declares a new constitution to extend his term. Later he declares himself as an emperor Napoleon III. End of the Second Republic.
- December 6 - Trial of Helene Jegado begins; she is eventually sentenced to death and executed in a guillotine.
- December 9 - The first YMCA in North America is established in Montreal, Quebec.
- December 24 - The Library of Congress burns.
- December 26-27 - Royal Navy warship bombards Lagos island; Oba Kosoko is wounded and flees to Epe.
- December 29 - The first YMCA opens, in Boston, Massachusetts.

Undated


- Dictator Rosas overthrown in Brazil. New government recognizes independent Paraguay. New Blanco government in Uruguay
- Florida State University is founded.
- Gold discovered in Australia.
- St. Paul's College, Hong Kong is founded.

Births


- January 17 - A. B. Frost, American illustrator (d. 1928)
- January 19 - Jacobus Kapteyn, Dutch astronomer (d. 1922)
- February 8 - Kate Chopin, American writer (d. 1904)
- March 19 - William Henry Stark, Business Leader (d. 1936)
- March 27 - Vincent d'Indy, French composer and teacher (d. 1931)
- March 28 - Bernardino Machado, Portuguese President (d. 1944)
- April 21 - Charles Barrois, French geologist (d. 1939)
- May 6 - Aristide Bruant, French cabaret singer and comedian (d. 1925)
- May 20 - Emil Berliner, telephone and recording pioneer (d. 1929)
- May 21 - Léon Bourgeois, French statesman, recipient of the Nobel Peace Prize (d. 1925)
- August 14 - Doc Holliday, American gambler and gunfighter (d. 1887)
- September 7 - David King Udall, American politician (d. 1938)
- October 2 - Ferdinand Foch, French commander of allied forces in World War I (d. 1929)
- Robert Abbe, American surgeon (d. 1928)
- Tom Morris, Jr., Scottish golfer (d. 1875)

Deaths


- January 10 - Karl Freiherr von Müffling, Prussian field marshal (b. 1775)
- January 27 - John James Audubon, French-American naturalist and illustrator (b. 1785)
- January 31 - David Spangler Kaufman, Congressman from Texas (b. 1813)
- February 1 - Mary Shelley, English author (b. 1797)
- February 18 - Carl Gustav Jakob Jacobi, German mathematician (b. 1804)
- March 9 - Hans Christian Ørsted, Danish scientist (b. 1777)
- September 10 - Thomas Hopkins Gallaudet, American educator (b. 1787)
- September 11 - Sylvester Graham, American nutritionist and inventor (b. 1794)
- September 14 - James Fenimore Cooper, American writer (b. 1789)
- October 4 - Manuel de Godoy, Spanish statesman (b. 1767)
- November 26 - Nicolas Jean de Dieu Soult, French marshal (b. 1769)
- December 19 - Joseph Mallord William Turner, English artist (b. 1775) ko:1851년 ms:1851 simple:1851 th:พ.ศ. 2394

Universe

The terms known Universe, observable Universe, or visible Universe are often used to describe the part of the Universe that we can see or otherwise observe. Those who believe it is impossible to observe the whole continuum may use our Universe, referring only to that knowable by human beings in particular.

Expansion and age, and the Big Bang theory

The most important result of cosmology, that the Universe is expanding, is derived from redshift observations and quantified by Hubble's law. Extrapolating this expansion back in time, one approaches a gravitational singularity, a rather abstract mathematical concept, which may or may not correspond to reality. This gives rise to the Big Bang theory, the dominant model in cosmology today. The age of the Universe was estimated to be about 13.7 billion (13.7 × 109) years, with a margin of error of about 1 percent (± 200 million years), according to NASA's Wilkinson Microwave Anisotropy Probe (WMAP). However, this is based on the assumption that the underlying model used for data analysis is correct. Other methods of estimating the age of the Universe give different ages. A fundamental aspect of the Big Bang can be seen today in the observation that the farther away from us galaxies are, the faster they move away from us. It can also be seen in the cosmic microwave background radiation which is the much-attenuated radiation that originated soon after the Big Bang. This background radiation is remarkably uniform in all directions, which cosmologists have attempted to explain by an initial period of rapid inflation following the Big Bang.

Size of Universe and observable Universe

There is disagreement over whether the Universe is finite or infinite in spatial extent and volume. However, the observable Universe, consisting of all locations that could have affected us since the Big Bang given the finite speed of light, is certainly finite. The edge of the cosmic light horizon is 13.7 billion light years (4.19 gpc) distant. The present distance (comoving distance) to the edge of the observable Universe is larger, since the Universe has been expanding; it is estimated to be about 78 billion light years (7.8 × 1010 light years, or 7.4 × 1023 km). This would make the comoving volume, of the known Universe, equal to 1.9 × 1033 cubic light years (assuming this region is perfectly spherical). The observable Universe contains about 7 × 1022 stars, organized in about 100 billion galaxies, which themselves form clusters and superclusters. The number of galaxies may be even larger, based on the Hubble Deep Field observed with the Hubble Space Telescope. The Hubble Space Telescope discovered galaxies such as Abell 1835 IR1916, which are over 13 billion light years from Earth. Both popular and professional research articles in cosmology often use the term "Universe" when they really mean "observable Universe". This is because unobservable physical phenomena are scientifically irrelevant; that is, they cannot affect any events that we can perceive. See also Causality (physics). We live in the centre of the Universe that we observe, in apparent contradiction to the Copernican principle which says that the Universe is more or less uniform and it has no distinguished centre. This is simply because light does not travel infinitely fast, and we make observations of the past. As we look further and further away, we see things from epochs (times) closer and closer to the limit of time, which equals zero, according to the Big bang model. Since light travels at the same speed in any direction towards us, it is reasonable to suggest that we live at the center of our observable Universe.

Shape of the Universe

An important open question of cosmology is the shape of the Universe. Mathematically, which 3-manifold represents best the spatial part of the Universe? Firstly, whether the Universe is spatially flat, i.e. whether the rules of Euclidean geometry are valid on the largest scales, is unknown. Currently, most cosmologists believe that the observable Universe is very nearly spatially flat, with local wrinkles where massive objects distort spacetime, just as a lake is (nearly) flat. This opinion was strengthened by the latest data from WMAP, looking at "acoustic oscillations" in the cosmic microwave background radiation temperature variations. Secondly, whether the Universe is multiply connected, is unknown. The Universe has no spatial boundary according to the standard Big Bang model, but nevertheless may be spatially finite (compact). This can be understood using a two-dimensional analogy: the surface of a sphere has no edge, but nonetheless has a finite area. It is a two-dimensional surface with constant curvature in a third dimension. The 3-sphere is a three-dimensional equivalent in which all three dimensions are constantly curved in a fourth. If the Universe is indeed spatially finite, as described, then traveling in a "straight" line, in any given direction, would theoretically cause one to eventually arrive back at the starting point. Strictly speaking, we should call the stars and galaxies "views" of stars and galaxies, since it is possible that the Universe is multiply-connected and sufficiently small (and of an appropriate, perhaps complex, shape) that we can see once or several times around it in various, and perhaps all, directions. (Think of a house of mirrors.) If so, the actual number of physically distinct stars and galaxies would be smaller than currently accounted. Although this possibility has not been ruled out, the results of the latest cosmic microwave background research make this appear very unlikely.

Fate of the Universe

Depending on the average density of matter and energy in the Universe, it will either keep on expanding forever or it will be gravitationally slowed down and will eventually collapse back on itself in a "big crunch". Currently the evidence suggests not only that there is insufficient mass/energy to cause a recollapse, but that the expansion of the Universe seems to be accelerating and will accelerate for the whole of eternity (see accelerating Universe). Other ideas of the fate of our Universe include the Big Rip, the Big Freeze, and Heat Death of the Universe theory. For a more detailed discussion of other theories, see the ultimate fate of the Universe.

Multiverse

There is some speculation that multiple universes exist in a higher-level multiverse (also known as a megaverse), our Universe being one of those universes (lower case). For example, matter that falls into a black hole in our Universe could emerge as a "Big Bang," starting another universe. However, all such ideas are currently untestable and cannot be regarded as anything more than speculation. The concept of parallel universes are understood only when related to string theory.

Other terms

Different words have been used throughout history to denote "all of space", including the equivalents in various languages of "heavens", "cosmos" and "world". Although words like world and its equivalents in other languages now almost always refer to the planet Earth, they previously referred to everything that exists—see Copernicus, for example—and still sometimes do (as in "the whole wide world"). Some languages use the word for "world" as part of the word for "outer space", e.g. in the German word "Weltall".

See also


- Cosmic latte - the color of the Universe
- Cosmology
- Cosmos
- Parallel universe

References


- Albert Einstein (1952). Relativity: The Special and the General Theory (Fifteenth Edition), ISBN 0-517-88441-0

External links


- [http://www.anzwers.org/free/universe/index.html Richard Powell: An Atlas of the Universe] - a series of images at various scales, with explanations.
- [http://www.shekpvar.net/~dennis/Elib/Astronomicon/Astronomicon/Cosmos/cosmos.html Cosmos - an Illustrated Dimensional Journey from microcosmos to macrocosmos]
- [http://universe-review.ca/ A Review of the Universe - Structures, Evolutions, Observations, and Theories]
- [http://www.space.com/scienceastronomy/age_universe_030103.html Age of the Universe at Space.Com]
- [http://slate.msn.com/id/2087206/ My So-Called Universe] by Jim Holt, on various arguments for and against an infinite Universe and parallel universes
- [http://www.hep.upenn.edu/~max/multiverse1.html Parallel Universes] by Max Tegmark
- [http://www.astro.princeton.edu/~mjuric/universe/ Logarithmic Maps of the Universe]
- [http://setiathome.ssl.berkeley.edu/ Seti@Home - the Search for Extraterrestrial Intelligence]
- [http://www.exploreuniverse.com/ic Universe - Space Information Centre ] by Exploreuniverse.com
- [http://hypertextbook.com/facts/1999/TopazMurray.shtml Number of Galaxies in the Universe] Category:Environments ko:우주 ms:Alam Semesta ja:宇宙 simple:Universe



Caveman

ad]] A caveman is a popular stylized characterization of what early humans or hominids may have looked and behaved like. The term is sometimes used colloquially to refer to Neanderthals or to Homo sapiens of the Palaeolithic era, although popular descriptions of cavemen are usually highly inaccurate. The term has been discouraged recently for its inaccuracy, as it mostly refers to modern misconceptions of early man. In the past, many people shared the view of the 17th century philosopher Thomas Hobbes that the life of early man (the state of nature) was "...solitary, poore, nasty, brutish and short". The image still lives on as an icon in the minds of many, and is seen in many items of popular culture, that are now appearing outdated. Examples include the comic strips B.C. (starting in the 1950s) and Alley Oop (starting in the 1930s) and the 1960s cartoon The Flintstones. Cavemen are usually portrayed as being hairy, clothed in animal skins, armed with clubs, unintelligent, and aggressive. They are often shown as living contemporaneously with dinosaurs, a situation totally contradicted by archaeological and paleontological evidence. The lack of habitable caves also implies that the commonly depicted cave homes of such people is, in most cases, incorrect.

See also


- Captain Caveman
- Walking with Cavemen Category:Prehistory

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

Biology

Biology is the study, or science, of life. It is concerned with the characteristics and behaviors of organisms, how species and individuals come into existence, and the interactions they have with each other and with the environment. Biology encompasses a broad spectrum of academic fields that are often viewed as independent disciplines. However, together they address the phenomenon of life over a wide range of scales. At the atomic and molecular scale, life is studied in the disciplines of molecular biology, biochemistry, and molecular genetics. At the level of the cell, it is studied in cell biology, and at multicellular scales, it is examined in physiology, anatomy, and histology. Developmental biology studies life at the level of an individual organism's development or ontogeny. Moving up the scale towards more than one organism, genetics considers how heredity works between parent and offspring. Ethology considers group behavior of more than one individual. Population genetics looks at the level of an entire population, and systematics considers the multi-species scale of lineages. Interdependent populations and their habitats are examined in ecology and evolutionary biology. A speculative new field is astrobiology (or xenobiology), which examines the possibility of life beyond the Earth.
Biology studies the variety of life (clockwise from top-left) E. coli, tree fern, gazelle, Goliath beetle

Principles of biology

Unlike physics, biology does not usually describe systems in terms of objects which obey immutable physical laws described by mathematics. Nevertheless, the biological sciences are characterized and unified by several major underlying principles and concepts: universality, evolution, diversity, continuity, homeostasis, and interactions.

Universality: Biochemistry, cells, and the genetic code

mathematics]] Main articles: Life The most salient example of biological universality is that all living things share a common carbon-based biochemistry and in particular pass on their characteristics via genetic material, which is based on nucleic acids such as DNA and which uses a common genetic code with only minor variations. Another universal principle is that all organisms (that is, all forms of life on Earth except for viruses) are made of cells. Similarly, all organisms share common developmental processes. For example, in most metazoan organisms, the basic stages of early embryonic development share similar morphological characteristics and include similar genes.

Evolution: The central principle of biology

Main article: Evolution The central organizing concept in biology is that all life has a common origin and has changed and developed through the process of evolution (see Common descent). This has led to the striking similarity of units and processes discussed in the previous section. Charles Darwin established evolution as a viable theory by articulating its driving force, natural selection (Alfred Russell Wallace is recognized as the co-discoverer of this concept). Genetic drift was embraced as an additional mechanism of evolutionary development in the modern synthesis of the theory. The evolutionary history of a species— which describes the characteristics of the various species from which it descended— together with its genealogical relationship to every other species is called its phylogeny. Widely varied approaches to biology generate information about phylogeny. These include the comparisons of DNA sequences conducted within molecular biology or genomics, and comparisons of fossils or other records of ancient organisms in paleontology. Biologists organize and analyze evolutionary relationships through various methods, including phylogenetics, phenetics, and cladistics (The major events in the evolution of life, as biologists currently understand them, are summarized on this evolutionary timeline).

Diversity: The variety of living organisms

evolutionary timeline, based on rRNA gene data, showing the separation of the three domains bacteria, archaea, and eukaryotes as described initially by Carl Woese. Trees constructed with other genes are generally similar, although they may place some early-branching groups very differently, presumably owing to rapid rRNA evolution. The exact relationships of the three domains are still being debated.]] Despite its underlying unity, life exhibits an astonishingly wide diversity in morphology, behavior, and life histories. In order to grapple with this diversity, biologists attempt to classify all living things. Scientific classification seeks to reflect the evolutionary trees (phylogenetic trees) of the organism being classified. Classification is the province of the disciplines of systematics and taxonomy. Taxonomy places organisms in groups called taxa, while systematics seeks to define their relationships with each other. This clasification technique has evolved to reflect advances in cladistics and genetics, shifting the focus from physical similarities and shared characteristics to phylogenetics. Traditionally, living things have been divided into five kingdoms: :Monera -- Protista -- Fungi -- Plantae -- Animalia However, many scientists now consider this five-kingdom system to be outdated. Modern alternative classification systems generally begin with the three-domain system: :Archaea (originally Archaebacteria) -- Bacteria (originally Eubacteria) -- Eukaryota These domains reflect whether the cells have nuclei or not, as well as differences in the cell exteriors. There is also a series of intracellular parasites that are progressively "less alive" in terms of metabolic activity: :Viruses -- Viroids -- Prions

Continuity: The common descent of life

Main article: Common descent Up into the 19th century, it was commonly believed that life forms could appear spontaneously under certain conditions (see abiogenesis). This misconception was challenged by William Harvey's diction that "all life [is] from [an] egg" (from the Latin "Omne vivum ex ovo"), a foundational concept of modern biology. It simply means that there is an unbroken continuity of life from its initial origin to the present time. A group of organisms is said to share a common descent if they share a common ancestor. All organisms on the Earth have been and are descended from a common ancestor or an ancestral gene pool. This last universal common ancestor of all organisms is believed to have appeared about 3.5 billion years ago. Biologists generally regard the universality of the genetic code as definitive evidence in favor of the theory of universal common descent (UCD) for all bacteria, archaea, and eukaryotes (see: origin of life).

Homeostasis: Adapting to change

Main article: Homeostasis Homeostasis is the ability of an open system to regulate its internal environment to maintain a stable condition by means of multiple dynamic equilibrium adjustments controlled by interrelated regulation mechanisms. All living organisms, whether unicellular or multicellular, exhibit homeostasis. Homeostasis manifests itself at the cellular level through the maintenance of a stable internal acidity (pH); at the organismic level, warm-blooded animals maintain a constant internal body temperature; and at the level of the ecosystem, as when atmospheric carbon dioxide levels rise and plants are theoretically able to grow healthier and remove more of the gas from the atmosphere. Tissues and organs can also maintain homeostasis.

Interactions: Groups and environments

organ of the genus Amphiprion that dwell among the tentacles of tropical sea anemones. The territorial fish protects the anemone from anemone-eating fish, and in turn the stinging tentacles of the anemone protects the anemone fish from its predators]] Every living thing interacts with other organisms and its environment. One reason that biological systems can be difficult to study is that so many different interactions with other organisms and the environment are possible, even on the smallest of scales. A microscopic bacterium responding to a local sugar gradient is responding to its environment as much as a lion is responding to its environment when it searches for food in the African savannah. For any given species, behaviors can be co-operative, aggressive, parasitic or symbiotic. Matters become more complex when two or more different species interact in an ecosystem. Studies of this type are the province of ecology.

Scope of biology

Main article: List of biology disciplines Biology has become such a vast research enterprise that it is not generally regarded as a single discipline, but as a number of clustered sub-disciplines. This article considers four broad groupings. The first group consists of those disciplines that study the basic structures of living systems: cells, genes etc.; the second group considers the operation of these structures at the level of tissues, organs, and bodies; the third group considers organisms and their histories; the final constellation of disciplines focuses on their interactions. It is important to note, however, that these boundaries, groupings, and descriptions are a simplified characterization of biological research. In reality, the boundaries between disciplines are fluid, and most disciplines frequently borrow techniques from each other. For example, evolutionary biology leans heavily on techniques from molecular biology to determine DNA sequences, which assist in understanding the genetic variation of a population; and physiology borrows extensively from cell biology in describing the function of organ systems.

Structure of life

DNA sequences and structures]] Main articles: Molecular biology, Cell biology, Genetics, Developmental biology Molecular biology is the study of biology at a molecular level. This field overlaps with other areas of biology, particularly with genetics and biochemistry. Molecular biology chiefly concerns itself with understanding the interactions between the various systems of a cell, including the interrelationship of DNA, RNA, and protein synthesis and learning how these interactions are regulated. Cell biology studies the physiological properties of cells, as well as their behaviors, interactions, and environment. This is done both on a microscopic and molecular level. Cell biology researches both single-celled organisms like bacteria and specialized cells in multicellular organisms like humans. Understanding cell composition and how they function is fundamental to all of the biological sciences. Appreciating the similarities and differences between cell types is particularly important in the fields of cell and molecular biology. These fundamental similarities and differences provide a unifying theme, allowing the principles learned from studying one cell type to be extrapolated and generalized to other cell types. Genetics is the science of genes, heredity, and the variation of organisms. In modern research, genetics provides important tools in the investigation of the function of a particular gene, or the analysis of genetic interactions. Within organisms, genetic information generally is carried in chromosomes, where it is represented in the chemical structure of particular DNA molecules. Genes encode the information necessary for synthesizing proteins, which in turn play a large role in influencing (though, in many instances, not completely determining) the final phenotype of the organism. Developmental biology studies the process by which organisms grow and develop. Originating in embryology, modern developmental biology studies the genetic control of cell growth, differentiation, and "morphogenesis," which is the process that gives rise to tissues, organs, and anatomy. Model organisms for developmental biology include the round worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, the zebrafish Brachydanio rerio, the mouse Mus musculus, and the weed Arabidopsis thaliana.

Physiology of organisms

Main articles: Physiology, Anatomy Physiology studies the mechanical, physical, and biochemical processes of living organisms by attempting to understand how all of the structures function as a whole. The theme of "structure to function" is central to biology. Physiological studies have traditionally been divided into plant physiology and animal physiology, but the principles of physiology are universal, no matter what particular organism is being studied. For example, what is learned about the physiology of yeast cells can also apply to human cells. The field of animal physiology extends the tools and methods of human physiology to non-human species. Plant physiology also borrows techniques from both fields. Anatomy is an important branch of physiology and considers how organ systems in animals, such as the nervous, immune, endocrine, respiratory, and circulatory systems, function and interact. The study of these systems is shared with medically oriented disciplines such as neurology and immunology.

Diversity and evolution of organisms

immunology of a population of organisms is sometimes depicted as if travelling on a fitness landscape. The arrows indicate the preferred flow of a population on the landscape, and the points A, B, and C are local optima. The red ball indicates a population that moves from a very low fitness value to the top of a peak]] Main articles: Evolutionary biology, Botany, Zoology Evolutionary biology is concerned with the origin and descent of species, as well as their change over time, and includes scientists from many taxonomically-oriented disciplines. For example, it generally involves scientists who have special training in particular organisms such as mammalogy, ornithology, or herpetology, but use those organisms as systems to answer general questions about evolution. Evolutionary biology also makes use of paleontologists, who use the fossil record to answer questions about the mode and tempo of evolution, as well as theoreticians in areas such as population genetics and evolutionary theory. In the 1990s, developmental biology re-entered evolutionary biology from its initial exclusion from the modern synthesis through the study of evolutionary developmental biology. Related fields which are often considered part of evolutionary biology are phylogenetics, systematics, and taxonomy. The two major traditional taxonomically-oriented disciplines are botany and zoology. Botany is the scientific study of plants. Botany covers a wide range of scientific disciplines that study the growth, reproduction, metabolism, development, diseases, and evolution of plant life. Zoology involves the study of animals, including the study of their physiology within the fields of anatomy and embryology. The common genetic and developmental mechanisms of animals and plants is studied in molecular biology, molecular genetics, and developmental biology. The ecology of animals is covered under behavioral ecology and other fields.

Classification of life

The dominant classification system is called Linnaean taxonomy, which includes ranks and binomial nomenclature. How organisms are named is governed by international agreements such as the International Code of Botanical Nomenclature (ICBN), the International Code of Zoological Nomenclature (ICZN), and the International Code of Nomenclature of Bacteria (ICNB). A fourth Draft BioCode was published in 1997 in an attempt to standardize naming in these three areas, but it has yet to be formally adopted. The International Code of Virus Classification and Nomenclature (ICVCN) remains outside the BioCode.

Interactions of organisms

International Code of Virus Classification and Nomenclature]] Main articles: Ecology, Ethology, Behavior, Biogeography Ecology studies the distribution and abundance of living organisms, and the interactions between organisms and their environment. The environment of an organism includes both its habitat, which can be described as the sum of local abiotic factors such as climate and geology, as well as the other the organisms that share its habitat. Ecological systems are studied at several different levels, from individuals and populations to ecosystems and the biosphere. As can be surmised, ecology is a science that draws on several disciplines. Ethology studies animal behavior (particularly of social animals such as primates and canids), and is sometimes considered a branch of zoology. Ethologists have been particularly concerned with the evolution of behavior and the understanding of behavior in terms of the theory of natural selection. In one sense, the first modern ethologist was Charles Darwin, whose book The expression of the emotions in animals and men influenced many ethologists. Biogeography studies the spatial distribution of organisms on the Earth, focusing on topics like plate tectonics, climate change, dispersal and migration, and cladistics.

History of the word "biology"

Formed by combining the Greek βίος (bios), meaning 'life', and λόγος (logos), meaning 'study of', the word "biology" in its modern sense seems to have been introduced independently by Gottfried Reinhold Treviranus (Biologie oder Philosophie der lebenden Natur, 1802) and by Jean-Baptiste Lamarck (Hydrogéologie, 1802). The word itself is sometimes said to have been coined in 1800 by Karl Friedrich Burdach, but it appears in the title of Volume 3 of Michael Christoph Hanov's Philosophiae naturalis sive physicae dogmaticae: Geologia, biologia, phytologia generalis et dendrologia, published in 1766.

History

Main articles: History of biology, History of medicine, History of genetics Major discoveries in biology include:
- Cell theory
- Germ theory of disease
- Genetics
- Evolution
- DNA

Related topics

Main articles: List of biology topics

External links


- [http://www.rom.on.ca/biodiversity/biocode/biocode1997.html BioCode]: A proposal for organism naming.
- [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=Books NCBI Open-Access Books]
- PhyloCode, [http://www.ohiou.edu/phylocode/index.html]
- [http://tolweb.org/tree/phylogeny.html The Tree of Life]: A multi-authored, distributed Internet project containing information about phylogeny and biodiversity.
- [http://www.bioone.org/perlserv/?request=index-html BioOne] Bioscience research journals.
- [http://www.bionews.in/biologynews.htm Biology News] Biology News, Articles and Research discoversies.

Further reading


- Lynn Margulis, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 3rd ed., St. Martin's Press, 1997, paperback, ISBN 0805072527 (many other editions)
- Neil Campbell, Biology (7th edition), Benjamin-Cummings Publishing Company, 2004, hardcover, ISBN 080537146X
- Category:School subjects als:Biologie ko:생물학 ms:Biologi ja:生物学 simple:Biology th:ชีววิทยา

Geology

Geology (from Greek γη- (ge-, "the earth") and λογος (logos, "word", "reason")) is the science and study of the Earth, its composition, structure, physical properties, history, and the processes that shape it. It is one of the Earth sciences. Geologists have helped establish the age of the Earth at about 4.5 billion (4.5x109) years, and have determined that the Earth's lithosphere, which includes the crust, is fragmented into tectonic plates that move over a rheic upper mantle (asthenosphere) via processes that are collectively referred to as plate tectonics. Geologists help locate and manage the earth's natural resources, such as petroleum and coal, as well as metals such as iron, copper, and uranium. Additional economic interests include gemstones and many minerals such as asbestos, perlite, mica, phosphates, zeolites, clay, pumice, quartz, and silica, as well as elements such as sulfur, chlorine, and helium. Astrogeology refers to the application of geologic principles to other bodies of the solar system. However, specialised terms such as selenology (studies of the Moon), areology (of Mars), etc., are also in use. The word "geology" was first used by Jean-André Deluc in the year 1778 and introduced as a fixed term by Horace-Bénédict de Saussure in the year 1779. An older meaning of the word was first used by Richard de Bury. He used it to distinguish between earthly and theological jurisprudence.

History

In China, the polymath Shen Kua (1031 - 1095) formulated a hypothesis for the process of land formation: based on his observation of fossil shells in a geological stratum in a mountain hundreds of miles from the ocean, he inferred that the land was formed by erosion of the mountains and by deposition of silt. The work on rocks Peri lithon by Theophrastus, a student of Aristotle, remained authoritative for millennia. However, its interpretation of fossils was not overturned until after the Scientific Revolution. It was translated into Latin and the other languages of Europe such as French. Georg Bauer (Georg Agricola), a physician, summarised the knowledge of mining and metallurgy in 1556. Georg Agricola (1494-1555) wrote the first systematic treatise about mining and smelting works, De re metallica libri XII, with an appendix Buch von den Lebewesen unter Tage (book of the creatures beneath the earth). He covered subjects like wind energy, hydrodynamic power, melting cookers, transport of ores, extraction of soda, sulfur and alum, and administrative issues. The book was published in 1556. By the 1700s Jean-Etienne Guettard and Nicolas Desmarest hiked central France and recorded their observations on geological maps; Guettard recorded the first observation of the volcanic origins of this part of France. James Hutton recorded his Theory of the Earth in the 1788 Transactions of the Royal Society of Edinburgh, later called uniformitarianism. William Smith (1769-1839) drew some of the first geological maps and began the process of ordering rock strata (layers) by examining the fossils contained in them. James Hutton is often viewed as the first modern geologist. In 1785 he presented a paper entitled Theory of the Earth to the Royal Society of Edinburgh. In his paper, he explained his theory that the Earth must be much older than had previously been supposed in order to allow enough time for mountains to be eroded and for sediment to form new rocks at the bottom of the sea, which in turn were raised up to become dry land. Followers of Hutton were known as Plutonists because they believed that some rocks were formed by vulcanism which is the deposition of lava from volcanoes, as opposed to the Neptunists, who believed that all rocks had settled out of a large ocean whose level gradually dropped over time. In 1811 Georges Cuvier and Alexandre Brongniart published their explanation of the antiquity of the Earth, inspired by Cuvier's discovery of fossil elephant bones in Paris. To prove this, they formulated the principle of stratigraphic succession of the layers of the earth. They were independently anticipated by William Smith's stratigraphic studies on England and Scotland. Sir Charles Lyell first published his famous book, Principles of Geology, in 1830 and continued to publish new revisions until he died in 1875. He successfully promoted the doctrine of uniformitarianism. This theory states that slow geological processes have occurred throughout the Earth's history and are still occurring today. In contrast, catastrophism is the theory that Earth's features formed in single, catastrophic events and remained unchanged thereafter. Though Hutton believed in uniformitarianism, the idea was not widely accepted at the time. catastrophism illustrated on relief globe of the Field Museum ]] By 1827 Charles Lyell's Principles of Geology reiterated Hutton's uniformitarianism, which influenced the thought of Charles Darwin. 19th Century geology revolved around the question of the Earth's exact age. Estimates varied from a few 100,000 to billions of years. The most significant advance in 20th century geology has been the development of the theory of plate tectonics in the 1960s. Plate tectonic theory arose out of two separate geological observations: seafloor spreading and continental drift. The theory revolutionised the Earth sciences. The theory of continental drift was proposed by Alfred Wegener in 1912 and by Arthur Holmes, but wasn't broadly accepted until the 1960s when the theory of plate tectonics was developed.

Important principles of geology

There are a number of important principles in geology. Many of these involve the ability to provide the relative ages of strata or the manner in which they were formed. The Principle of Intrusive Relationships concerns crosscutting intrusions. In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes. The Principle of Cross-cutting Relationships pertains to the formation of faults and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault. The Principle of Inclusions and Components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a