History of astronomy

Astronomy is the branch of physics that studies celestial bodies and the universe as a whole.

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 archeologist. Stonehenge, constructed sometime between 3100 to 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

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

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.

Planetology

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 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.

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 sperical 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 Jun 21st and comparing it with the Sun's altitude at the same time at Syene, in southern Egypt, he determined that 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, she 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 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.