History of astronomy
From this perspective, the study of celestial bodies can be reasonably said to have begun when at some point humanity looked up and began to observe the moon and the stars and the planets regardless of how they may have thought of them.
These ancient beginnings are often indicated by structures studied by archeologists. Stonehenge, constructed some time between 3100 and 2000 BC, may have constituted an astronomical site, possibly an observatory, or the structure may have been designed upon observations previously made. Either way, it seems clear that Stonehenge was meant to take advantage of astronomical phenomena since the "heelstone" in the circle of stones is aligned with the rising Sun on Midsummer's Day (June 21, the Summer Solstice). This represents a true astronomical alignment. Many other Megalithic sites also demonstrate such alignments.
The Megalithic Passage Tomb at Newgrange, built about 3200 B.C. also demonstrates knowledge of astronomical phenomena. The passage and single chamber of the tomb are illuminated by a shaft of sunlight that shines through the roof box over the entrance and penetrates the passage, lighting up the chamber at winter solstice sunrise. This happens at dawn from the 19th to the 23rd of December for 17 minutes.
Earlier evidence of astronomical observations can be found in Vedic India in the Rg Veda which contains a verse observing the winter solstice in the constellation Aries. This would have placed it at around 6500 BC. The Myth of Janus, a four headed god of of the Vedic people of India, presents the possibility of astronomical observations around 4,000 BC. Each head of Janus represented a phase of the moon which in turn represented one of the four seasons: one full moon represented the spring equinox, one full moon represented the autumn equinox, the waning moon the winter solstice and a waxing moon representing the summer solstice.This dating is disputed but it does indicate a very early study of both the constellations and the moon.
Halley's Comet (considered a guest star) was noted by Chinese astronomers as early as 240 BC and perhaps as early as 1059 BC.
Branches and subdisciplines
Celestial mechanics, a subfield of astronomy, began with the application of Newton's theory of mechanics and gravitation (as elucidated in the Principia) to the movement of planets. Eventually Einstein's theory of general relativity and modern computing technology overtook the field of classic physics.
Cosmology is defined as the science of the universe, the branch of astronomy which studies the origin, evolution, and structure of the universe, the study of "the contents, structure, and evolution of the universe from the beginning of time to the future", a branch of astronomy that studies the "origin, large-scale properties, and the evolution of the observable universe.".
Astronomy underwent significant changes in the period following 1970 when a union of particle physics ("the study of the unbelievably small" ) and astronomy ("the study of the incomprehensibly large") had begun to take place. This has had a significant impact on cosmology. The scope of cosmology arguably begins approximately 10-42 seconds following the origin of the universe when the universe was smaller than a proton.
Also referred to as planetary science, this branch of astronomy is involved with the study of other planets, including meteorology, geology, location, orbits, origins. Given the fact that the earth is our primary source of information about other planets, there is a great deal of comparative study of earth and other planets. The primary focus has been on the planets of this solar system but as new planets are discovered, there is a growing amount of data on planets in orbit around other stars.
Astronomy in ancient China
Astronomy in ancient Mesopotamia
Astronomy in ancient India
Astronomy in ancient Greece
Any assertion as to where astronomy began faces the problem of providing dated evidence that supports a reasonable conclusion. With astronomy, there are a great many pieces of evidence in the form of ancient documents and archaeological finds that make such a claim for any place or time difficult to sustain. In other words, it is not really possible to state exactly where astronomy in its earliest forms began. However, it is possible to trace the roots of the study of the skies and the objects visible to the unaided eye with some degree of certainly even if only to establish a theory of its beginnings and where the influences of these early impressions and thoughts eventually spread. For the western world, that is Europe and the European influenced Americas, and the ancient civilisations of North Africa and the Middle East, some of those roots can be traced to the earliest Greek philosophers.
In the centuries before Socrates, Plato and Aristotle, astronomy was concerned with keeping time. The ancient observers had made note of the regularity of the sky and its changing appearance. Stars and particularly the Moon were of concern in noting the passing of time.
Well before Aristotle, the Greeks were observing the heavens and noting the relationship between their lives and the changes in the night sky. Hesiod was a poet living about 700 B.C. who noted what might have been the accepted knowledge of the time when he wrote
- ... when the Pleiades rise it is time to use the sickle, but the plough when they are setting; 40 days they stay away from heaven; when Arcturus ascends from the sea and, rising in the evening, remain visible for the entire night, the grapes must be pruned; but when Orion and Sirius come in the middle of heaven and the rosy fingered Eos sees Arcturus, the grapes must be picked; when the Pleiades, the Hyades, and Orion are setting, then mind the plough; when the Pleiades, fleeing Orion, plunge into the dark sea, storms may be expected; 50 days after the sun's turning is the right time for man to navigate; when Orion appears, Demeter's gift has to be brought to the well-smoothed threshing floor.
Thales of Miletus (624 B.C. – 547 B.C.)
Thales, either a Milesian or a Phoenecian, was an early philosopher, mathematician and engineer. None of his work remains and in fact it may have disappeared by the time of Aristotle. Other sources indicate that he developed a method of navigating by using the constellation Ursa Minor. He is also reputed to have correctly surmised the approximate time of a solar eclipse though there is no evidence that he developed the ability to predict them accurately. At the time of Thales, lunar eclipses were already known and were being predicted. Thales reportedly brought geometry to Greece from Egypt and contributed to the field as well. 
Salon, a contemporary of Thales, introduced a new calendar that employed a cycle of two years with 13 months with 30 days and 12 months of 29 days, a result of 369 days and one month of 29 &1/2 days. The problems of a reliable manner of marking time continued however, motivating further observations accumulating through numerous sources which were instrumental in designing improved calendars. 
Pythagoras (~569 to 474) B.C.
Pythagoras, whose birth and death dates are still uncertain, left no writings and all that is known of him is through biographies written after his death, biographies that attributed divine powers to him.
Pythagoras reportedly studied in various cultures, including Tyre, his father’s original home, and Syria learning from Chaldeans and Syrians. Pythagoras was a pupil of Thales and Anaximander in Miletus, both influencing him, particularly Anaximander who tutored on geometry and cosmology. Upon Thales advice, Pythagoras travelled to Egypt in about 535 B.C. to study mathematics and astronomy. It was in Egypt that Pythagoras reportedly became a priest in the Temple of Diospolis and the influence of the Egyptian priests were demonstrated in later years in the community Pythagoras developed in Italy.
In 525 B.C. Cambyses II King of Persia invaded Egypt and Pythagoras was taken prisoner to Babylon. It was in Babylon that Pythagoras reportedly interacted with the wise men of the city, the Magoi, learning yet more mysticism and mathematics.
Around 520 B.C. Pythagoras was able to return to Samos, journey to Crete for a brief period and then back to Samos where he founded a school called the ‘semicircle’ teaching and working on mathematics in a cave near the city.
Pythagoras left Samos in 518 B.C. to establish a school at Croton (present day Crotone) in southern Italy. The school became popular and he headed the school with his followers known as mathematikoi, vegetarians living permanently in the society without possessions and obeying strict rules.
Pythagoras held to five rules, mystical in nature, of which the first was that reality was mathematical in nature.
Given the total lack of actual documentation of his work and the secrecy of his lifestyle, it is not possible to discern between what Pythagoras achieved and what his followers did. In that sense, to speak of the accomplishments of Pythagoras is to speak of the Pythagorean school at Croton. Their study of mathematics was mystical. But their contribution was to develop the concept of numbers, the concept of mathematical figures and mathematical proofs in the belief that all relationships are reducible to numbers.
These relationships and their numbers were what they brought to astronomy. Pythagoras is credited with a number of other contributions: recognising that the Earth is a sphere, that the orbit of the Moon was inclined to the equator of the Earth and that the Morning Star and the Evening Star, Venus, were one and the same. 
Oenopides of Chios (~490 B.C. to ~ 420 B.C.)
Born in Chios, Greece (present day Khios) what we know of Oenopides is circumstantial evidence from other sources. He evidently lived in Athens in his youth and was known through Plato to have had a reputation as a mathematician. Oenopides is credited by some with having fixed a value for the obliquity of the ecliptic, that is, the angle of the Earth’s ecliptic orbit in relation to the celestial equator, or the Zodiacal circle as it is also known. He settled the value as 24˚ which was considered the proper value until Eratosthenes recalculated the angle. There may, however, be other interpretations of this. For example the zodiacal circle may refer to the motion of the Sun, Moon and the planets each with a diurnal and a zodiacal component. Another possible explanation is that this refers to a motion along the ecliptic or a description of the ecliptic with regard to twelve 30 degree signs which is connected with the zodiacal motion of the Sun, the Moon and the planets.
Aristotle (384 - 322 B.C.)
Aristotle was not alone in the development of the Hellenic foundation of humanity's perspective on the celestial but his name is the most prominent of the early Greeks. He used direct observation and deduction to propose the spherical Earth. He noted that
- Ships disappear or appear on the horizon as they move away or toward the observer;
- The shape of the Earth's shadow on the moon is circular;
- Different stars are visible in the northern and the southern latitudes;
- Since elephants are found both in India (to the east) and Morocco (to the west), both are a reasonable distance on a moderately sized sphere.
Aristotle rejected the idea that the Earth orbited the Sun, apparently because there was no detectable parallax, which was not, in fact, proved until 1838 by Bessel.
Aristotle's perspective of the cosmos was derived from what he thought things should be, it was an aesthetic view of the cosmos rather than a scientifically derived view. For Aristotle, the Prime Mover set the universe in motion both perfect (in Aristotle’s point of view) and eternal. There was no such thing as vacuum, no emptiness. All the nearby objects, the Sun, the Moon and the planets as well as the far distant were set in eight crystalline spheres that revolved around the Earth. For Aristotle there were the four basic elements we have on Earth, fire, and water, earth and air. In the heavens there was a fifth from which the crystalline spheres were composed, aether--a perfect substance that could neither be changed nor destroyed.
Things moved about Earth, they moved in perfect circles, they were embedded in a perfect substance, they would never stop in their perfect movement--and all of this was based on Aristotle’s vision of perfection.
Aristotle’s view was later incorporated by Ptolemy in Alexandria, North Africa, who made some changes in the Aristotelian perspective to account for anomalies he had observed--the planets occasionally moved in reverse. (Unlike Aristotle, Ptolemy actually observed the phenomena he studied. While he was not the first, this approach to the study of physical phenomena was not required nor evidently even expected of those who made claims about the world.) Ptolemy's work and his writings carried Aristotle’s views forward into the 16th century when Copernicus's work on the calendar led him to make his own changes--in this case a paradigm shift. Copernicus, like Aristotle and unlike Ptolemy, did not make his own observations. However, he did incorporate the work of others and he added his contribution by placing the Sun at the centre of the universe. This heliocentric model of the universe which clearly implied that the Earth itself moved and was not the centre of the universe, was to have a major impact on the study of the celestial, marking the beginning of the end of Aristotelian influence, and the politics of the day.
Aristarchus of Samos (approximately 310 B.C. to 230 B.C.)
Aristarchus was a Greek mathematician and astronomer. He is credited as the first in history to propose a Sun-centred universe and for being one of the first to attempt to determine the sizes and distances of the Sun and Moon. Aristarchus and his theory of a heliocentric cosmos is referred to by Plutarch in his work De facie in orbe lunae. Evidently, its contradiction of Aristotle’s perception of the cosmos was ill received. Archimedes credited Artistarchus with the heliocentric model as well as a much larger universe. Aristarchus is credited by Copernicus as the originator of the heliocentric model as well. His work was also to influence Hipparchus and Ptolemy.
Aristarchus made six hypotheses in his work to determine the size of the moon and the sun and their relative distances. He proposed that
- The Moon receives its light from the Sun, in other words, the moon reflects and does not generate its own light;
- The Earth is the centre of a sphere in which the Moon orbits or rotates;
- The phases of the Moon which change as the Moon rotates around the Earth show a darkened circle in our line of sight—we are looking directly at the Moon in the absence of reflected light;
- Based on Aristarchus' observations, when half the Moon is darkened and appears to be halved, its angular distance from the sun is less 1/30 of a quadrant  which means its angular distance is less than 3 degrees, and is therefore equal to 87 degrees;
- The Width of the Earth’s shadow on the moon is equivalent to twice that of the Moon’s;
- The moon subtends one fifteenth part of a sign of the Zodiac for an angular diameter of 2 degrees.
He believed that he proved a number of propositions. Some of his most notable were:
- The distance from the Earth to Sun is eighteen to twenty times that of the distance between the Earth and the Moon. The average distance between the Sun and the Earth is 150 million kilometres and that of the Earth and the Moon is 384,400 kilometres. Either he thought the Moon was much further away or the Sun was closer, but he was off by a significant amount.
- The diameters of the Sun and Moon have the same ratios as their relative distances between 1:18 and 1:20. Again he was off by a considerable amount. The Sun is about 109 times the diameter of the Earth. The Moon has a diameter of 3,476 kilometres (2,159 miles) or a quarter that of the Earth’s diameter.
- He also proposed that the relative ratios of the diameters of the Sun and the Earth was between 19:3 and 43:6. It is in fact 109:1.
As inaccurate as they were, these attempts were based on real observations and an attempt to apply the mathematical tools of the period. As such, Artistarchus was a positive step forward in the attempt at a rational explanation of the universe. His work was influential for approximately 2,000 years. 
Eratosthenes of Cyrene (276 - 195 B.C.)
Eratosthenes employed observations of the Sun’s shadow and geometry to estimate the circumference of the Earth. By measuring the altitude of the noontime sun at its maximum at Alexandria, North Africa, on June 21st and comparing it with the Sun's altitude at the same time at Syene, in southern Egypt, he determined the angle from the zenith to the point where the Sun was at noon. At Syene, the zenith distance was 0 degrees (directly overhead); at Alexandria it was about 7 degrees. Since Eratosthenes knew how far it was between the two cities, he was able to calculate geometrically the difference in zenith angle and thereby the estimated size of the Earth. Eratosthenes also measured the tilt of the Earth axis by 23.5 degrees. It is the tilt of the Earth’s axis that endows the Earth with seasons.
Aglaonike (c. 200 B.C.)
Also known as Aganice of Thessaly and the daughter of Hegetor of Thessaly, Aglaonike is mentioned as a sorceress in the writings of Plutarch and Apollonius of Rhodes. Possibly the first recorded woman astronomer, she was apparently familiar with the metonic cycle (periods of the full moon and the cycles of eclipses) because she reportedly developed the ability to predict lunar eclipses. She lived sometime around the early 2nd century B.C., but exact dates are unknown. 
Hipparchus of Nicea (190-120 B.C.)
Hipparchus, an astronomer born in Bithynia, lived on the island of Rhodes where he did most of the work known to us. Rhodes, near the coast of Anatolia, was by that time famous for its schools in philosophy and art. Very little of Hipparchus's work actually survives. Most of what is known about Hipparchus is through other works. Hipparchus’ only remaining work is a commentary written in the third century B.C., the Commentary on the Phainomena of Eudoxus and Aratus. Ptolemy’s work, the Almagest, is the largest source of information on Hipparchus. Ptolemy credits Hipparchus as his most important predecessor.
Hipparchus is possibly the first person in history to use numerical data from observations to construct geometric models to explain astronomical motions. He is credited with discovering the precession of the equinoxes and his work in mathematics was significant. Besides his work in geometry, he is considered the founder of trigonometry.
Hipparchus took a practical approach to his work rather than rely on Aristotelian type models constructed through what he thought things should be. Hipparchus made recorded observations over a period from 147 to 127 BC. He then drew upon earlier works including Apollonius's deferent-epicycle and eccentric, as well as his own to construct the geometric models.
Hipparchus developed or invented some of his instruments used in his observations and models. Ptolemy described an instrument Hipparchus developed called a dioptra and one he may have invented, the planispheric astrolabe, used to tell the time at night from stellar observations. John Philoponus (sixth century AD) provided the earliest surviving description of the planispheric astrolabe, a considerable time after Hipparchus. However, the underlying mathematical theory for the stereographic projection used in the astrolabe is found in Ptolemy’s work, the Planispherium.
Hipparchus attempted to determine the distance between the Moon and the Earth by comparing concurrently a solar eclipse as viewed from two positions, a total eclipse in Syene and the other, a partial eclipse in Alexandria. While an observer at Syene observed the total eclipse of the Sun blocked by the Moon, an observer at Alexandria observed that 1/5th of the Sun's disk was visible. This meant that the angular size of the visible Sun observed from Alexandria is 1/10th of a degree (0.1 degree). Expressing this angle in radians and applying the small angle approximation gave the ratio of the Syene-Alexandria distance to the Earth-Moon distance.
Hipparchus also calculated the precession of Earth’s rotational axis. Currently, the North Celestial Pole is closely aligned with the Polaris. At the time of Hipparchus it was not as closely aligned. Five thousand years ago when the pyramids of Egypt were being constructed, it was more closely aligned with the star Thuban in the constellation Draco. In another twelve thousand years it will be more closely aligned with the star Vega in the constellation Lyra. In other words, as the Earth slowly shifts like a spinning gyroscope, the North Star will change as the direction of the Earth’s axis shifts or precesses. The complete cycle or precession back to today’s current alignment with Polaris takes 26,000 years.
Hipparchus also created a numbering system for stellar magnitude, today referred to as apparent magnitude. Hipparchus designated six magnitudes: first magnitude for the brightest star and sixth for the faintest visible stars.
Hipparchus also used Babylonian methods and observations in his work. The influence of Babylonian astronomy on Greece is not clear, probably dating to the time of Pythagoras if not earlier, but Hipparchus work does provide a clear historical link between the two cultures which we can only surmise from the biographies about Pythagoras. 
Astronomy in ancient Persia
Astronomy in ancient Egypt
Astronomy in the Medieval Middle East
The Middle East here refers to the eastern Mediterranean; from Turkey to northern Africa and eastward to Iran including the site of such ancient civilizations as Phoenicia and Babylon and Egypt.
Translations of Indian and Persian astronomical texts were translated into Arabic in the 8th century. It was not until the beginning of the 9th however that the major Greek texts were translated into Arabic. Work in practical and theoretical astronomy, publications, and the numerous observatories built strongly suggest that astronomical activity was extensive and widespread in the Middle East from the 9thuntil the 16th centuries.
Astronomy of the Mayan civilisation
Astronomy of the Aztec civilisation
Astronomy of the Incan civilisation
Claudius Ptolemy (~85-165 AD)
Omar Khayyam (1048-1131)
Georg Peurbach (1423-1461)
Peurbach was a fifteenth century reformer who addressed errors in astronomy texts, errors that predated Ptolemy and went as far back as the most ancient Greek texts. Peurbach, with his student Regiomantus, wrote a new textbook and guide to Ptolemy’s Almagest, thereby advancing the work in theoretical astronomy. Peurbach's New Theory of the Planets (published 1454) was an attempt to resolve earlier models employing descriptive geometrics to predict planetary motions with his homocentric celestial spheres--nested, concentric spheres.
Their new book was to influence Copernicus as an undergraduate student at the University of Cracow and eventually his work, De Revolutionibus Orbium Coelestium ("On the Revolutions of the Celestial Orbs").
Johannes Regiomontanus (1436-1476)
Johannes Müller von Königsberg, also known as Regiomontanus, was a student of Georg Peurbach. Johannes completed his work with Peurbach (after Peurbach's death) publishing Epitome of the Almagest in 1496, the revision of Ptolemy's work, Syntaxis, commissioned by Cardinal Johannes Bessarion. Epitome was later employed by such astronomers as Copernicus and Galileo. Regiomantanus continued his work of observation and critique, improving translations of the ancient Greek works, and openly pointing out the discrepancies between observations and current astronomical theory. The significance of his work was such that those who followed, including Nicholas Copernicus, Tycho Brahe and Johannes Kepler, worked to reform astronomy under his influence.
Regiomantus developed a method of determining longitude at sea from the position of the Moon which he published in his work, Ephemerides. He also constructed a calendar which he published in Kalendarium. These and other works he published with the movable type invented by Guttenburg, making Regiomontanus the first to introduce this medium to science. His work was very influential at the time and Christopher Columbus and Amerigo Vespucci were both influenced by him. 
Nikolas Kopernig (Copernicus, 1473-1543)
Leonard Digges (1520-1559)
Leonard Digges was a writer of mathematics and science in English, one of the first people to popularise work in either field. He was also a surveyor who invented the theodolite, the telescope, the reflecting telescope and possibly the refractive telescope. He published a number of works during his lifetime but his achievements were expanded and revised by his son Thomas and published after Leonard's death, some of the work for the first time.
Thomas Digges (1543-1595)
Thomas Digges's contribution to astronomy is notable for two things: his ability to write for the layman and thereby inform the public of some of the great advances in science; his comprehension of Copernicus's cosmological model that led him to postulate a much larger cosmos than previously perceived.
Digges work, the Perfit Description of the Cælestiall Orbes printed in 1576, elaborated, in English, the most important ideas of Book 1 of Copernicus’s De Revolutionibus which had been printed just thirty-three years before in 1543. Digges's historical significance was not widely known until his book, the Perfit Description was reprinted in the 1930's. Since then, Digges has been identified as the first public advocate of Copernicanism in England.
Digges's father Leonard was also an author and had published science and mathematics in English, which was a little unusual at the time. The result being that his works became popular and set the stage, arguably for others to continue to write for the general public. Thomas Digges's family was heavily penalised when his father was sentenced to death for his part in the rebellion of Sir Thomas Wyatt, and then having had his sentence commuted was stripped of all assets and holdings. After his father's death in in 1559, Thomas was raised and educated by philosopher and mathematician John Dee, his guardian and the astrologer to Queen Elizabeth I. Dee had a substantial library and evidently supported Copernicus' view of cosmology although he published nothing on the subject. These resources and perspectives were not lost on Thomas and he read extensively and cooperated with Dee on some work.
In 1571, Thomas published Leonard Digges's book on the telescope, Pantometria, twelve years after his father's death. Panometria was the first publications to discuss the invention of the telescope in English. Thomas had extended, revised and enhanced the book and he wrote the preface. Thomas continued his studies and his research and in 1576 he then published a revised edition of his father's book Prognostication Everlasting. Thomas's revision included the first ever discussion in English of Copernicus's model of the universe. He also asserted that the universe is infinite and he included a diagramme showing a heliocentric universe with the stars stretching into infinity. Apparently this was a leap of imagination fed by the potential capacity of the cosmos provided by his telescopic observations of the Milky Way as well as influence from others. He did not state specifically what led him to this position but he is apparently the first to postulate an infinite universe.
Thomas's publication, Alae seu scalae mathematicae, in 1573, was a Latin text prompted by the new star of 1572, a supernova. Thomas's observations were employed by Tycho Brahe in his work. The supernova created quite a stir worldwide and certainly in Europe. There was a tremendous increase in astronomical and astrological work and publications. Tycho Brahe's supernova was significant because it encouraged astronomers in the 16th-century to question their perception that the heavens were immutable, that is, unchanging. Thomas's contribution was to determined the nova's position and his conclusion that its appearance was a challenge to traditional cosmology of the day.
Galileo Galilei (1564-1642)
Johannes Kepler (1571-1630)
Tycho Brahe (1546-1601)
Isaac Newton (1642-1727)
Charles Messier (1730-1817)
Jacobus Kapteyn (1851-1922)
Friedrich Wilhelm (William) Herschel (1738 - 1822)
In his mid to late 30s, Herschel was influenced by James Ferguson's book, Astronomy (1756), Robert Smith's Opticks (1738) and Harmonics (1749) and eventually learned to make specula mirrors which were of the highest quality, becoming known as a skilled maker of the most powerful telescopes of his day. Herschel turned his labours to actual observations and discovered the planet Uranus March 13, 1781 and eventually discovered two of Uranus's moons, Titania and Oberon, in 1787.
He received a copy of Messier's and Méchain's Catalog of Nebulae and Star Clusters in 1781 and began to make his own observations of nebulae. He was to discover approximately 2500 new "nebulae" and star clusters over a period of 20 years.
Herschel published his own observations that led to the discovery of Solar Motion in 1783, determining that our solar system is moving towards the star Lambda Herculis. He also introduced the term Solar Apex.
In 1789, using a large telescope, 48-inch (1.2-meter) aperture, of his own construction, he discovered Saturn’s sixth known moon Enceladus. He then went on to discover Saturn's seventh moon, Mimas. His telescope was to remain the largest in the world until Lord Rosse assembled his 72-inch "Leviathan" at Parsonstown, in Ireland in 1845. The size of his 48 inch telescope however made it difficult to handle and he did not employ it as often as his smaller telescopes. Lord Rosse continued his work with the larger telescope and was subsequently the first to discover a spiral nebulae.
Herschel's contributions to astronomy were very important. He studied stellar motion and the solar system's movement in the direction of constellation Hercules. He produced a model of the Milky Way galaxy from current stellar statistics, and proposed various aspects of the nature of nebulae, including a possibility of external galaxies (island universes) outside the Milky Way, first postulated by Immanuel Kant. His contributions included work in physics (particularly optics) and he was the first to discover infrared light.
W. H.Pickering and Annie J. Cannon
Fred Hoyle (1915-2001)
Arno Penzias and Robert Wilson
Jocelyn Bell (Burnell) and Anthony Hewish
The Infinite Universe
The break through in humanity's concept of the size of the universe came in small steps. Copernicus's cosmology provided the rational support for a universe of much greater size.
Thomas Digges (1543-1595)
In 1576, English author and astronomer Thomas Digges proposed the idea of a vast, even infinite, universe. His description, "the orb of stars fixed infinitely up . . . . perpetually shining glorious lights innumerable far excelling our sun both in quantity and quality."Digges spent considerable time with the telescope, had read Copernicus's De Revolutionibus, and was one of the few people who understood at the time the implications of Copernicus's cosmology. 
A similar proposal was made by a contemporary, Italian philosopher and Dominican monk, Giordano Bruno (1548-1600), who asserted that, "there are innumerable suns, and an infinite number of earths revolve around those suns." In his book, Atom, renown theoretical physicist Lawrence M. Krauss cites the passage in Bruno’s De l’infinito universe e mondi (On the Infinite Universe and Worlds) (1584) as follows:
There are countless suns and countless earths all rotating around their suns in exactly the same way as the seven planets of our system. We see only the suns because they are the largest bodies and are luminous, but their planets remain invisible to us because they are smaller and non-luminous. The countless worlds in the universe are no worse, and no less inhabited than our Earth.
Johann Kepler is apparently the first to write about the puzzle of the dark night sky. If the number of stars is infinite, and they are uniformly distributed, why is the night sky dark? If they are infinite in number and distributed evenly the night sky should glow brilliantly and possibly create a devastating heat. Kepler concluded that the universe was finite.
Issac Newton, working from Galileo's data and Kepler's work, provided insight into how Copernicus's cosmology was supported by the laws of motion and his concept of gravity provided a means to determine and predict celestial mechanics. However, Newton's work presented a paradox, If gravity prevailed, then why did the entire universe not fall in on itself? Newton's answer to this was that the stars were uniformly distributed across an infinite space and their mutual gravity kept them suspended as they were. He was not able to explain how these stable stars being pulled upon equally from all directions would maintain the eternal status quo if a star were to deviate from its position and thereby lead to a cascade of stars falling in toward each other. Nor was he able to explain why the night sky was dark.
Heinrich W. M. Olbers
In the early 19th century, nocturnal darkness led Hienrich Olbers to propose that interstellar light was obstructed by clouds of interstellar matter. The question of the nocturnal darkness remained however since Olber was unable to explain how in a infinite universe the clouds themselves had not been heated by the star light and were glowing. This is Olber's Paradox.
- If the universe is static and infinite and uniform;
- Then every line of sight from Earth must end at a star;
- And every line of sight would show a point of light and thus the sky would be brilliantly lit and the heat from the stars would be overwhelming.
Edgar Allan Poe
Later, in the 18th century, the writer Edgar Allan Poe attempted to resolve Olber's paradox. He proposed that "[The] distance of the invisible background [is] so immense that no ray from it has yet been able to reach us at all." This meant that the age of the universe was finite, it had a beginning and was not eternal.
- Archeoastronomy is the study of ancient and prehistoric astronomy; methods and interpretations.
- A Brief History of Astronomy Gene Smith, University of California, San Diego Center for Astrophysics & Space Sciences
- Newgrange Megalithic Passage Tomb
- Astronomy of Vedic India Eirik L. Harris, Pamona College
- Introduction and Mathematics ReviewCollins, George (1989) The Foundations of Celestial Mechanics
- Celestial Mechanics James B. Calvert, Associate Professor Emeritus of Engineering, University of Denver (2003). Mechanics and Thermodynamics
- Smoot, George, Davidson, Keay (1993). Wrinkles in time: The imprint of creation. London: Abacus Books
- Glossary George Mason University
- Glossary Contemporary Physics Education Project
- Introductory Astronomy Glossary Astronomical Society of the Pacific
- Comparative Planetology University of Washington Astronomy Dept.
- Greek Astronomy O'Connor J J & Robertson E F University of St. Andrews, School of Mathematical and Computational Science. Citing A F Aveni, Empires of time: Calendars, clocks and cultures (New York, 1989).; B Hetherington, A chronicle of pre-telescope astronomy (Chichester, 1996); and A Pannekoek, A history of astronomy (New York, 1989).
- Thales University of St. Andrews School of Mathematical and Computational Sciences
- Greek Astronomy O'Connor J J & Robertson E F University of St. Andrews, School of Mathematical and Computational Science.
- The Society was open to men and women. The society had the inner circle, the mathematikoi, and an outer circle, the akousmatics who lived in their own houses, came to the Society during the day were allowed possessions and not required to be vegetarians.
- Pythagoras O'Connor, J J & Robertson, E F University of St. Andrews School of Mathematical and Computational Sciences
- I Bulmer-Thomas, Biography in Dictionary of Scientific Biography (New York 1970-1990) citing Plato’s Erastae in of Chios O'Connor, J. J. & Robertson, E. F. University of St. Andrews School of Mathematical and Computational Sciences
- There is some dispute about his having discovered this. It may be attributed to Pythagoras or it may be attributed to Egyptian priests
- There are a number of cycles of this nature, for example the precession of the equinoxes, the period of time in which the sun moves through an entire cycle relative to the constellations, is about 26,000 years, 1.5 degrees a century. In other words, if in 1000 B.C. Odysseus began a voyage (i.e. as related in Homer’s Odyssey) when he viewed the star Orion in the sky—Orion is in the constellation Scorpious—then the precession of the equinox would have positioned the Earth beneath Orion in October 3,000 years ago. Today, Orion appears in December. What month was it when Odysseus saw Orion in 1000BC? Astronomy Department, Cornell University.
- Oenopides of Chius: A survey of the modern literature with a collection of the ancient testimonia István M Bodnár, Eötvös University/Central European University, Max Planck Institute for the History of Science
- Oenopides O'Connor, J J & Robertson, E F University of St. Andrews School of Mathematical and Computational Sciences
- Aristotle (384 - 322 B.C.) History of Astronomy, Astronomy, Cornell University
- Gribbin, J. (2002) Science: A history. London: Penguin
-  Plutarch, De facie in orbe lunae , c. 6 “Only do not, my good fellow, enter an action against me for impiety in the style of Cleanthes, who thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the Universe, this being the effect of his attempt to save the phenomena by supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis.” Attributed to Dercyllides, a contemporary of Aristarchus.
- Archimedes, Sand-Reckoner, Chapter 1 "Now you are aware that "universe" is the name given by most astronomers to the sphere the center of which is the center of the earth, and the radius of which is equal to the straight line between the center of the sun and the center of the earth; this you have seen in the treatises written by astronomers. But Aristarchus of Samos brought out writings consisting of certain hypotheses, in which it appears, as a consequence of the assumptions just made, that the universe is many times greater than the "universe" just mentioned. His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun in the circumference of a circle, the sun lying in the middle of the orbit, and that the sphere of the fixed stars, situated about the same center as the sun, is so great that the circle in which he supposes the earth to revolve bears such a proportion to the distance of the fixed stars as the center of the sphere bears to its surface."
- Copernican System Galileo Project. Rice University
- one quadrant = 90 degrees
- be opposite to; of angles and sides, in geometry used to determine dimensions from known quantities
- The twelve signs of the Zodiac occupy equivalent portions of 30 degrees each of the 360 degrees of the celestial sphere. So the moon, therefore, has 1/15 of 30 degrees.
- The average distance between the Earth and the Sun is about 93,000,000 miles (150 million kilometres). Perihelion, the closest distance, is 91.4 million miles (147.1 million km) away from us. Aphelion, its farthest, is 94.5 million miles (152.1 million km) away. The average distance of the Moon and the Earth is 238,855 miles (384,400 kilometres), the width of 30 Earths. Because of its elliptical orbit, its distance from Earth varies between 225,700 miles (363,300 kilometres) and 252,000 miles (405,500 kilometres).
- Aristarchus of Samos University of St. Andrews School of Mathematical and Computational Sciences
- Aristarchus of Samos Riley, Kristen (1995) Paper prepared for Greek Science Course taught by Prof. Gregory Crane, Tufts University
- How far away is the sun? How far away is the moon? How large is the Sun? How small is the moon compared to Earth? Ask an Astronomer, NASA
- Aristarchus Cornell Astronomy
- latitude = 23.5 degrees north
- Eratosthenes (276 - 195 B.C.) Cornell Astronomy
- AGLAONIKE Deborah Crocker (University of Alabama), Sethanne Howard (US Naval Observatory retired). "4,000 years of women in science," University of Alabama, Dept of Physics and Astronomy
- Ogilvie, M. B. (1986). Women in Science. The MIT Press.
- (2001). Encyclopedia of Astronomy and Astrophysics, Edited by Paul Murdin, Bristol: Institute of Physics Publishing
- Hipparchus on a poem Dept. of History and Philosophy of Science, University of Cambridge.
- The Astrolabe James E. Morrison, Astrolab.org
- Scientific instruments of Medieval and Renaissance Europe Guide to the Astrolabe Museum of the History of Science at Oxford. Site containing a large collection of documents, graphics and explanations on astrolabes of Medieval and Renaissance Europe
- Ptolemy used a completely different type of instrument, the 'armillary astrolabe' or star-taker made of rings or bracelets, which he described in the Almagest Book 5, chapter 1 Hipparchus Dept. of History and Philosophy of Science, University of Cambridge.
- In other words, 1/5th of 30 minutes of arc of the Sun's disk was visible. The Sun's angular diameter is 30 arc minutes or ½ a degree.
- See Hipparchus (190 - 120 B.C.) for an illustration of this.
- The Precession of the Earth's Axis Cornell University Dept. of Astronomy
- Paul J. Green, (Astrophysicist, Smithsonian Astrophysical Observatory) (2005) "Star." World Book Online Reference Center. World Book, Inc.  Reprinted by NASA at  Paul J. Green, PhD. is an Astrophysicist with Smithsonian Astrophysical Observatory.
- Hipparchus (190 - 120 B.C.) Cornell University Dept. of Astronomy
- Hipparchus and the Astrolabe Dept. of History and Philosophy of Science, University of Cambridge.
- Mathematical Techniques in Astronomy Dept. of History and Philosophy of Science, University of Cambridge.
- Word Net Cognitive Science Laboratory, Princeton University
- The Interplay of Science and Theology in the Fourteenth-century Kalam Dallal, Ahmad (2002) Draft for The 2001-2002 Sawyer Seminar at the University of Chicago
- The birth of scientific controversies. The dynamics of the Arabic tradition and its impact on the development of science: Ibn al-Haytham’s challenge of Ptolemy’s Almagest Hassan Tahiri Université de Lille 3, MSH Nord-Pas de Calais
- Copernican System Galileo Project, Rice University
-  Robert Hatch, University of Florida
- Regiomontanus Starry Messenger. Department of History and Philosophy of Science, University of Cambridge
- Regiomontanus BiographyO'Connor, J J & Robertson, E F (2004) School of Math and Statistics, University of St. Andrews
- Johannes Regiomontanus: Calendar The University of Glasgow, Special Collections
- Johann Müller (Regiomontanus) J.G. Hagen. (1911) The Catholic Encyclopedia, Volume X. New York: Robert Appleton Company.
- Gribbin, J. (2002) Science: A history. London: Penguin
- Thomas Digges: Gentleman and mathematician Stephen Johnston (1994) chapter 2 (pp. 50-106) of, ‘Making mathematical practice: gentlemen, practitioners and artisans in Elizabethan England’ Ph.D. thesis, Cambridge. Available through University of Oxford, Museum of History of Science
- Thomas Digges O'Connor, J. J. and Robertson, E. F. (2002) MacTutor History of Mathematics Archive, School of Math and Statistics, University of St. Andrews.
- Leonard Digges Richard S. Westfall, Department of History and Philosophy of Science, Indiana University for the Galileo Project, Rice University
- Thomas was publishing his father's work with his contributions which he stated in the text, However, there was an amended section to the book, Mathematical Discourse of Geometrical Solids, a study of Platonic and Archimedean bodies, which was entirely his own work. Refer to PhD thesis, Stephen Johnston cited above.
-  Galileo Project. Rice University
- sometimes referred to as Tycho's Supernova See reference to NASA/ESA Space Telescope cited below.
- Thomas Digges O'Connor, J. J. and Robertson, E. F. (2002) MacTutor History of Mathematics Archive, School of Math and Statistics, University of St. Andrews.
- Thomas Digges Richard S. Westfall, Department of History and Philosophy of Science, Indiana University for the Galileo Project, Rice University
- heic0415: Stellar survivor from 1572 A.D. NASA/ESA Space Telescope. On Nov. 11, 1572, Tycho Brahe observed a star in the constellation Cassiopeia as bright as Jupiter which eventually equaled Venus in brightness. It was visible during daylight for about two weeks and eventually faded from unaided view altogether after about 16 months.
- Friedrich Wilhelm (William) Herschel (November 15, 1738 - August 25, 1822) Students for the Exploration and Development of Space
- William Herschel (1738-1822) High Altitude Observatory
- Gatti H. (1999) Giordano Bruno and Renaissance Science. Cornell University Press: Ithaca, NY.
- Krauss LM. (2001) Atom. Little Brown and Company. ISBN 0-7595-8324-2.
- http://krauss.faculty.asu.edu/bio.html Lawrence M. Krauss: Biography