Principle of original horizontality
The Principle of Original Horizontality states that layers of sediment are deposited horizontally under the action of gravity. It is a relative dating technique; the principle is important to the analysis of folded and tilted strata. It was first proposed by the Danish geological pioneer Nicholas Steno. From these observations is derived the conclusion that the Earth has not been static and that great forces have been at work over long periods of time, further leading to the conclusions of the science of plate tectonics; as one of Steno's Laws, the Principle of Original Horizontality served well in the nascent days of geological science. However, it is now known. For instance, coarser grained sediments such as sand may be deposited at angles of up to 15 degrees, held up by the internal friction between grains which prevents them slumping to a lower angle without additional reworking or effort; this is known as the angle of repose, a prime example is the surface of sand dunes. Sediments may drape over a pre-existing inclined surface: these sediments are deposited conformably to the pre-existing surface.
Sedimentary beds may pinch out along strike, implying that slight angles existed during their deposition. Thus the Principle of Original Horizontality is but not universally, applicable in the study of sedimentology and structural geology. Law of superposition Principle of cross-cutting relationships Principle of faunal succession Principle of lateral continuity
Marine isotope stage
Marine isotope stages, marine oxygen-isotope stages, or oxygen isotope stages, are alternating warm and cool periods in the Earth's paleoclimate, deduced from oxygen isotope data reflecting changes in temperature derived from data from deep sea core samples. Working backwards from the present, MIS 1 in the scale, stages with numbers have high levels of oxygen-18 and represent cold glacial periods, while the odd-numbered stages are troughs in the oxygen-18 figures, representing warm interglacial intervals; the data are derived from pollen and foraminifera remains in drilled marine sediment cores and other data that reflect historic climate. The MIS timescale was developed from the pioneering work of Cesare Emiliani in the 1950s, is now used in archaeology and other fields to express dating in the Quaternary period, as well as providing the fullest and best data for that period for paleoclimatology or the study of the early climate of the Earth, representing "the standard to which we correlate other Quaternary climate records".
Emiliani's work in turn depended on Harold Urey's prediction in a paper of 1947 that the ratio between oxygen-18 and oxygen-16 isotopes in calcite, the main chemical component of the shells and other hard parts of a wide range of marine organisms, should vary depending on the prevailing water temperature in which the calcite was formed. Over 100 stages have been identified, going back some 6 million years, the scale may in future reach back up to 15 mya; some stages, in particular MIS 5, are divided into sub-stages, such as "MIS 5a", with 5 a, c, e being warm and b and d cold. A numeric system for referring to "horizons" may be used, with for example MIS 5.5 representing the peak point of MIS 5e, 5.51, 5.52 etc. representing the peaks and troughs of the record at a still more detailed level. For more recent periods precise resolution of timing continues to be developed. In 1957 Emiliani moved to the University of Miami to have access to core-drilling ships and equipment, began to drill in the Caribbean and collect core data.
A further important advance came in 1967, when Nicholas Shackleton suggested that the fluctuations over time in the marine isotope ratios that had become evident by were caused not so much by changes in water temperature, as Emiliani thought, but by changes in the volume of ice-sheets, which when they expanded took up the lighter oxygen-16 isotope in preference to the heavier oxygen-18. The cycles in the isotope ratio were found to correspond to terrestrial evidence of glacials and interglacials. A graph of the entire series of stages revealed unsuspected advances and retreats of ice and filled in the details of the stadials and interstadials. More recent ice core samples of today's glacial ice substantiated the cycles through studies of ancient pollen deposition. A number of methods are making additional detail possible. Matching the stages to named periods proceeds as new dates are discovered and new regions are explored geologically; the marine isotopic records appear more complete and detailed than any terrestrial equivalents, have enabled a timeline of glaciation for the Plio-Pleistocene to be identified.
It is now believed that changes in the size of the major ice sheets such as the historical Laurentide ice sheet of North America are the main factor governing variations in the oxygen isotope ratios. The MIS data matches the astronomical data of Milankovitch cycles of orbital forcing or the effects of variations in insolation caused by cyclical slight changes in the tilt of the earth's axis of rotation – the "orbital theory". Indeed, that the MIS data matched Milankovich's theory, which he formed during World War I, so well was a key factor in the theory gaining general acceptance, despite some remaining problems at certain points, notably the so-called 100,000-year problem. For recent periods data from radiocarbon dating and dendrochronology support the MIS data; the sediments acquire depositional remanent magnetization which allows them to be correlated with earth's geomagnetic reversals. For older core samples, individual annual depositions cannot be distinguished, dating is taken from the geomagnetic information in the cores.
Other information as to the ratios of gases such as carbon dioxide in the atmosphere, is provided by analysis of ice cores. The SPECMAP Project, funded by the US National Science Foundation, has produced one standard chronology for oxygen isotope records, although there are others; this high resolution chronology was derived from several isotopic records, the composite curve was smoothed and tuned to the known cycles of the astronomical variables. The use of a number of isotopic profiles was designed to eliminate'noise' errors, that could have been contained within a single isotopic record. Another large research project funded by the US government in the 1970s and 1980s was Climate: Long range Investigation and Prediction, which to a large degree succeeded in its aim of producing a map of the global climate at the Last Glacial Maximum, some 18,000 years ago, with some of the research directed at the climate some 120,000 years ago, during the last interglacial; the theoretical advances and improved data available by the 1970s enabled a "grand synthesis" to be made, best known from the 1976 paper Variations in the earth’s orbit: pacemaker of the ice ages, by J.
D. Hays and John Imbrie, still widely accepted today, covers the MIS timescale and the causal effect of the orbital theory. In 2010 the Subcommission on Quaternary Stratigraphy of the Inte
The Harris matrix is a tool used to depict the temporal succession of archaeological contexts and thus the sequence of depositions and surfaces on a'dry land' archaeological site, otherwise called a'stratigraphic sequence'. The matrix reflects the relative position and stratigraphic contacts of observable stratigraphic units, or contexts; the Matrix was developed in 1973 in Winchester, England, by Dr. Edward C. Harris; the concept of creating seriation diagrams of archaeological strata based on the physical relationship between strata had had some currency in Winchester and other urban centres in England prior to Harris's formalisation. One of the results of Harris's work, was the realisation that sites had to be excavated stratigraphically, in the reverse order to that in which they were created, without the use of arbitrary measures of stratification such as spits or planums. In his Principles of archaeological stratigraphy Harris first proposed the need for each unit of stratification to have its own graphic representation in the form of a measured plan.
In articulating the laws of archaeological stratigraphy and developing a system in which to demonstrate and graphically the sequence of deposition or truncation on a site, Harris, it has been argued, has followed in the footsteps of the great stratigraphic archaeologists such as Mortimer Wheeler, without being a great excavator himself. Harris's work was a vital precursor to the development of single context planning by the Museum of London and the development of land use diagrams, all facets of a suite of archaeological recording tools and techniques developed in the UK which allow in-depth analysis of complex archaeological data sets from urban excavations. In a series of layers and interfacial features, as created, the upper units of stratification are younger and the lower are older, for each must have been deposited on, or created by the removal of, a pre-existing mass of archaeological stratification. Any archaeological layer deposited in an unconsolidated form will tend towards a horizontal disposition.
Strata which are found with tilted surfaces were so deposited, or lie in conformity with the contours of a pre-existing basin of deposition. Any archaeological deposit, as laid down, will be bounded by the edge of the basin of deposition, or will thin down to a feather edge. Therefore, if any edge of the deposit is exposed in a vertical plane view, a part of its original extent must have been removed by excavation or erosion: its continuity must be sought, or its absence explained. Any given unit of archaeological stratification takes its place in the stratigraphic sequence of a site from its position between the undermost of all units which lie above it and the uppermost of all those units which lie below it and with which it has a physical contact, all other superpositional relationships being regarded as redundant; these laws were published in 1979. A fifth law of archaeological stratigraphy has been added following papers presented at the "Interpreting Stratigraphy a Review of the Art" conferences in the UK from 1992 to 2003.
In constructing a matrix, the latest contexts sit on top of the matrix and the earliest at the bottom with the lines that link them together representing direct stratigraphic contact. The matrix thus demonstrates the temporal relationship between any two units of archaeological stratification. While excavating, it is best practice to compile the area and site stratigraphic matrices during the progress of an excavation through reference to both the drawn and written record. Regular daily checking of the record and the compilation of the matrix itself both help inform the individual archaeologist on the physical processes of site formation and highlight any areas where dubious relationships such as H relationships or loops in the recorded sequence may occur. Loops are sequences in the matrix that produce temporal anomalies so that the earliest context in a sequence of context appears to be than the latest context by virtue of errors in excavation or recording. Urban archaeological sites are complex affairs generating thousands of units of archaeological stratigraphy.
It is of more vital importance when excavating such sites to compile the matrix as the excavation progresses. Such sites by definition produce multi-linear sequences of succession and to date the best way to get a handle of these sequences is to compile the matrix by hand, based on the drawings and the context sheets; this ensures an internally consistent record and that the complexity of the site is given due regard. Computer programmes do exist which can aid the production of a matrix, though at the moment these tend towards articulating linear sequences rather than multi-linear sequences; the Harris matrix is a tool that aids the accurate and consistent excavation of a site and articulates complex sequences in a clear and understandable way. Harris matrices play an invaluable role in the articulation of sequence and provide the building blocks from which higher order units of stratigraphically related events can be constructed. Take this hypothetical section as an example of matrix formation.
Here there are twelve contexts, numbered thus: A horizontal layer Masonry wall remnant Backfill of the wall construction cut A horizontal layer the same as 1 Construction cut for wall 2 A clay floor abutting wall 2 Fill of shallow cut 8 Shallow pit cut A horizontal layer A horizontal layer the same as 9 Natural sterile ground formed before human occupation of the site Trample in the base of cut 5 formed by workmen's boots constructing the structure
Relative dating is the science of determining the relative order of past events, without determining their absolute age. In geology, rock or superficial deposits and lithologies can be used to correlate one stratigraphic column with another. Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating and geologists used relative dating to determine ages of materials. Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred, it remains a useful technique. Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate; the Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of'relative dating' as observed in geology from the 17th century to the early 20th century. The regular order of the occurrence of fossils in rock layers was discovered around 1800 by William Smith.
While digging the Somerset Coal Canal in southwest England, he found that fossils were always in the same order in the rock layers. As he continued his job as a surveyor, he found the same patterns across England, he found that certain animals were in only certain layers and that they were in the same layers all across England. Due to that discovery, Smith was able to recognize the order. Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras. Methods for relative dating were developed when geology first emerged as a natural science in the 18th century. Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events; the principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past."
In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now." 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, batholiths 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. 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 explains that, with sedimentary rocks, if inclusions are found in a formation 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. These foreign bodies are picked up as magma or lava flows, are incorporated to cool in the matrix; as a result, xenoliths are older than the rock. The principle of original horizontality states that the deposition of sediments occurs as horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization; the law of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. This is because it is not possible for a younger layer to slip beneath a layer deposited; the only disturbance that the layers experience is bioturbation, in which animals and/or plants move things in the layers. However, this process is not enough to allow the layers to change their positions; this principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.
The principle of faunal succession is based on the appearance of fossils in sedimentary rocks. As organisms exist at the same time period throughout the world, their presence or absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith a hundred years before the publication of Charles Darwin's theory of evolution, the principles of succession were developed independently of evolutionary thought; the principle becomes quite complex, given the uncertainties of fossilization, the localization of fossil types due to lateral changes in habitat, that not all fossils may be found globally at the same time. The principle of lateral continuity states that layers of sediment extend laterally in all directions; as a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be continuous. Layers of sediment do not extend indefinitely.
In biology, phylogenetics is the study of the evolutionary history and relationships among individuals or groups of organisms. These relationships are discovered through phylogenetic inference methods that evaluate observed heritable traits, such as DNA sequences or morphology under a model of evolution of these traits; the result of these analyses is a phylogeny – a diagrammatic hypothesis about the history of the evolutionary relationships of a group of organisms. The tips of a phylogenetic tree can be living organisms or fossils, represent the "end", or the present, in an evolutionary lineage. Phylogenetic analyses have become central to understanding biodiversity, evolution and genomes. Taxonomy is the identification and classification of organisms, it is richly informed by phylogenetics, but remains a methodologically and logically distinct discipline. The degree to which taxonomies depend on phylogenies differs depending on the school of taxonomy: phenetics ignores phylogeny altogether, trying to represent the similarity between organisms instead.
Usual methods of phylogenetic inference involve computational approaches implementing the optimality criteria and methods of parsimony, maximum likelihood, MCMC-based Bayesian inference. All these depend upon an implicit or explicit mathematical model describing the evolution of characters observed. Phenetics, popular in the mid-20th century but now obsolete, used distance matrix-based methods to construct trees based on overall similarity in morphology or other observable traits, assumed to approximate phylogenetic relationships. Prior to 1950, phylogenetic inferences were presented as narrative scenarios; such methods are ambiguous and lack explicit criteria for evaluating alternative hypotheses. The term "phylogeny" derives from the German Phylogenie, introduced by Haeckel in 1866, the Darwinian approach to classification became known as the "phyletic" approach. During the late 19th century, Ernst Haeckel's recapitulation theory, or "biogenetic fundamental law", was accepted, it was expressed as "ontogeny recapitulates phylogeny", i.e. the development of a single organism during its lifetime, from germ to adult, successively mirrors the adult stages of successive ancestors of the species to which it belongs.
But this theory has long been rejected. Instead, ontogeny evolves – the phylogenetic history of a species cannot be read directly from its ontogeny, as Haeckel thought would be possible, but characters from ontogeny can be used as data for phylogenetic analyses. 14th century, lex parsimoniae, William of Ockam, English philosopher and Franciscan friar, but the idea goes back to Aristotle, precursor concept 1763, Bayesian probability, Rev. Thomas Bayes, precursor concept 18th century, Pierre Simon first to use ML, precursor concept 1809, evolutionary theory, Philosophie Zoologique, Jean-Baptiste de Lamarck, precursor concept, foreshadowed in the 17th century and 18th century by Voltaire and Leibniz, with Leibniz proposing evolutionary changes to account for observed gaps suggesting that many species had become extinct, others transformed, different species that share common traits may have at one time been a single race foreshadowed by some early Greek philosophers such as Anaximander in the 6th century BC and the atomists of the 5th century BC, who proposed rudimentary theories of evolution 1837, Darwin's notebooks show an evolutionary tree 1843, distinction between homology and analogy, Richard Owen, precursor concept 1858, Paleontologist Heinrich Georg Bronn published a hypothetical tree to illustrating the paleontological "arrival" of new, similar species following the extinction of an older species.
Bronn did not propose a mechanism responsible for precursor concept. 1858, elaboration of evolutionary theory and Wallace in Origin of Species by Darwin the following year, precursor concept 1866, Ernst Haeckel, first publishes his phylogeny-based evolutionary tree, precursor concept 1893, Dollo's Law of Character State Irreversibility, precursor concept 1912, ML recommended and popularized by Ronald Fisher, precursor concept 1921, Tillyard uses term "phylogenetic" and distinguishes between archaic and specialized characters in his classification system 1940, term "clade" coined by Lucien Cuénot 1949, Jackknife resampling, Maurice Quenouille, precursor concept 1950, Willi Hennig's classic formalization 1952, William Wagner's groundplan divergence method 1953, "cladogenesis" coined 1960, "cladistic" coined by Cain and Harrison 1963, first attempt to use ML for phylogenetics and Cavalli-Sforza 1965 Camin-Sokal parsimony, first parsimony criterion and first computer program/algorithm for cladistic analysis both by Camin and Sokal character compatibility method called clique analysis, introduced independently by Camin and Sokal and E. O. Wilson 1966 English translation of Hennig "cladistics" and "cladogram" coined 1969 dynamic and successive wei
Radiocarbon dating is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. The method was developed in the late 1940s by Willard Libby, who received the Nobel Prize in Chemistry for his work in 1960, it is based on the fact that radiocarbon is being created in the atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting 14C combines with atmospheric oxygen to form radioactive carbon dioxide, incorporated into plants by photosynthesis; when the animal or plant dies, it stops exchanging carbon with its environment, from that point onwards the amount of 14C it contains begins to decrease as the 14C undergoes radioactive decay. Measuring the amount of 14C in a sample from a dead plant or animal such as a piece of wood or a fragment of bone provides information that can be used to calculate when the animal or plant died; the older a sample is, the less 14C there is to be detected, because the half-life of 14C is about 5,730 years, the oldest dates that can be reliably measured by this process date to around 50,000 years ago, although special preparation methods permit accurate analysis of older samples.
Research has been ongoing since the 1960s to determine what the proportion of 14C in the atmosphere has been over the past fifty thousand years. The resulting data, in the form of a calibration curve, is now used to convert a given measurement of radiocarbon in a sample into an estimate of the sample's calendar age. Other corrections must be made to account for the proportion of 14C in different types of organisms, the varying levels of 14C throughout the biosphere. Additional complications come from the burning of fossil fuels such as coal and oil, from the above-ground nuclear tests done in the 1950s and 1960s; because the time it takes to convert biological materials to fossil fuels is longer than the time it takes for its 14C to decay below detectable levels, fossil fuels contain no 14C, as a result there was a noticeable drop in the proportion of 14C in the atmosphere beginning in the late 19th century. Conversely, nuclear testing increased the amount of 14C in the atmosphere, which attained a maximum in about 1965 of twice what it had been before the testing began.
Measurement of radiocarbon was done by beta-counting devices, which counted the amount of beta radiation emitted by decaying 14C atoms in a sample. More accelerator mass spectrometry has become the method of choice; the development of radiocarbon dating has had a profound impact on archaeology. In addition to permitting more accurate dating within archaeological sites than previous methods, it allows comparison of dates of events across great distances. Histories of archaeology refer to its impact as the "radiocarbon revolution". Radiocarbon dating has allowed key transitions in prehistory to be dated, such as the end of the last ice age, the beginning of the Neolithic and Bronze Age in different regions. In 1939, Martin Kamen and Samuel Ruben of the Radiation Laboratory at Berkeley began experiments to determine if any of the elements common in organic matter had isotopes with half-lives long enough to be of value in biomedical research, they synthesized 14C using the laboratory's cyclotron accelerator and soon discovered that the atom's half-life was far longer than had been thought.
This was followed by a prediction by Serge A. Korff employed at the Franklin Institute in Philadelphia, that the interaction of thermal neutrons with 14N in the upper atmosphere would create 14C, it had been thought that 14C would be more to be created by deuterons interacting with 13C. At some time during World War II, Willard Libby, at Berkeley, learned of Korff's research and conceived the idea that it might be possible to use radiocarbon for dating. In 1945, Libby moved to the University of Chicago, he published a paper in 1946 in which he proposed that the carbon in living matter might include 14C as well as non-radioactive carbon. Libby and several collaborators proceeded to experiment with methane collected from sewage works in Baltimore, after isotopically enriching their samples they were able to demonstrate that they contained 14C. By contrast, methane created from petroleum showed no radiocarbon activity because of its age; the results were summarized in a paper in Science in 1947, in which the authors commented that their results implied it would be possible to date materials containing carbon of organic origin.
Libby and James Arnold proceeded to test the radiocarbon dating theory by analyzing samples with known ages. For example, two samples taken from the tombs of two Egyptian kings and Sneferu, independently dated to 2625 BC plus or minus 75 years, were dated by radiocarbon measurement to an average of 2800 BC plus or minus 250 years; these results were published in Science in 1949. Within 11 years of their announcement, more than 20 radiocarbon dating laboratories had been set up worldwide. In 1960, Libby was awarded the Nobel Prize in Chemistry for this work. In nature, carbon exists as two stable, nonradioactive isotopes: carbon-12, carbon-13, a radioactive isotope, carbon-14 known as "radiocarbon"; the half-life
Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics and chemistry in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, stars, nebulae and comets. More all phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A related but distinct subject is physical cosmology, the study of the Universe as a whole. Astronomy is one of the oldest of the natural sciences; the early civilizations in recorded history, such as the Babylonians, Indians, Nubians, Chinese and many ancient indigenous peoples of the Americas, performed methodical observations of the night sky. Astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, the making of calendars, but professional astronomy is now considered to be synonymous with astrophysics. Professional astronomy is split into theoretical branches. Observational astronomy is focused on acquiring data from observations of astronomical objects, analyzed using basic principles of physics.
Theoretical astronomy is oriented toward the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain observational results and observations being used to confirm theoretical results. Astronomy is one of the few sciences in which amateurs still play an active role in the discovery and observation of transient events. Amateur astronomers have made and contributed to many important astronomical discoveries, such as finding new comets. Astronomy means "law of the stars". Astronomy should not be confused with astrology, the belief system which claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin, they are now distinct. Both of the terms "astronomy" and "astrophysics" may be used to refer to the same subject. Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the Earth's atmosphere and of their physical and chemical properties," while "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, dynamic processes of celestial objects and phenomena."
In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject. However, since most modern astronomical research deals with subjects related to physics, modern astronomy could be called astrophysics; some fields, such as astrometry, are purely astronomy rather than astrophysics. Various departments in which scientists carry out research on this subject may use "astronomy" and "astrophysics" depending on whether the department is affiliated with a physics department, many professional astronomers have physics rather than astronomy degrees; some titles of the leading scientific journals in this field include The Astronomical Journal, The Astrophysical Journal, Astronomy and Astrophysics. In early historic times, astronomy only consisted of the observation and predictions of the motions of objects visible to the naked eye.
In some locations, early cultures assembled massive artifacts that had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops and in understanding the length of the year. Before tools such as the telescope were invented, early study of the stars was conducted using the naked eye; as civilizations developed, most notably in Mesopotamia, Persia, China and Central America, astronomical observatories were assembled and ideas on the nature of the Universe began to develop. Most early astronomy consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, the nature of the Sun and the Earth in the Universe were explored philosophically; the Earth was believed to be the center of the Universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the Ptolemaic system, named after Ptolemy.
A important early development was the beginning of mathematical and scientific astronomy, which began among the Babylonians, who laid the foundations for the astronomical traditions that developed in many other civilizations. The Babylonians discovered. Following the Babylonians, significant advances in astronomy were made in ancient Greece and the Hellenistic world. Greek astronomy is characterized from the start by seeking a rational, physical explanation for celestial phenomena. In the 3rd century BC, Aristarchus of Samos estimated the size and distance of the Moon and Sun, he proposed a model of the Solar System where the Earth and planets rotated around the Sun, now called the heliocentric model. In the 2nd century BC, Hipparchus discovered precession, calculated the size and distance of the Moon and inven