A Brief History of Astronomy
The Science of Astronomy
The greatest influence on western astronomy comes from ancient Greece. Greek philosophers were the first to apply mathematics to attempt to understand the universe more deeply than for simple predictive purposes. Although the civilization of ancient Greece declined, the Greek ideas that were passed to Indian and Asian colonies were subsequently adopted by Islamic scholars, and were later introduced into Europe by Arab invaders in the fifteenth century Table
1 gives a listing of the Greek alphabet. These letters note scientific quantities and are used in the naming of stars.
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Pythagoras
Pythagoras (circa 580–500 B.C.), who is credited with summarizing previously developed ideas of geometry, is also credited with proposing the idea of a spherical Earth and Moon. He developed this concept from studying the pattern of shadows on the Moon during eclipses. Pythagoras also correctly deduced the cause of the phases of the Moon.
Aristotle
Aristotle (circa 384–322 B.C.) believed that the Sun moved around Earth. His belief in a stationary Earth was probably the result of his familiarity with the concept of parallax and his interpretation of observed celestial movement. Parallax refers to the apparent change in the position of an object resulting from the change in the direction or position from which it is viewed. Understanding parallax, Aristotle would have expected that if Earth rotated and moved about the Sun, then parallax should be observable for nearby celestial objects. His inability to observe the parallax effect led to the conclusion that Earth is stationary, rather than the correct deduction that the objects are so far away that the parallax angles are too small to detect. Aristotle also argued for the philosophical concepts of perfect celestial shapes (spheres) and motions (uniform on circular paths).
Aristarchus
Aristarchus (fl. circa 270 B.C.) used a trigonometric interpretation of a few simple observations to develop the first valid idea of the scale of the universe. He first noticed that the apparent angular sizes of the Sun and Moon are the same, hence their sizes (radii) are in proportion to their distances. Second, he estimated that the angle between the direction to the quarter Moon and the Sun is 87 degrees. Recognizing that the Sun-Moon-Earth angle must be a right angle (90 degrees) when the Moon is exactly half illuminated, Aristarchus determined that the distance from Earth to the Sun is 20 times that to the Moon. Third, by timing a lunar eclipse, Aristarchus could establish that three moon diameters fit within the diameter of Earth's shadow. Fourth, the ratio of the eclipse time to the lunar orbital period gives the lunar size relative to its orbital distance [eclipse time/orbital period = 3 diameter (moon)/2π (moon)]. Taken together, these observations place the Moon at a distance of 10 Earth diameter (as compared to the true value of 30 Earth diameter) and the Sun at 200 Earth diameter (true value 11,700), with the Moon one-third (true value 0.27 times the size of Earth) and the Sun seven times the size of Earth (true value 109 times the size of the Earth). Although his values differ from accepted values, his effort was a significant advance. Aristarchus was also the only significant Greek philosopher to believe that Earth moved, although he ultimately discarded this idea.
Eratosthenes
Eratosthenes (circa 276–194 B.C.) determined the circumference of Earth (40,000 kilometers) from observations at Alexandria and Syene, Egypt, by noting that the seven-degree angle between the direction to the Sun at these two sites on the same date is the same fraction of a full circle (360 degrees) as is the distance between the two towns to the circumference of Earth.
Hipparchus
The work of Hipparchus (fl. 146–127 B.C.), often thought of as the greatest of the ancient Greek astronomers, shows the transition from basic naked-eye astronomy to systematic observational work with a long series of written records being kept. From long-term records, he could determine the length of a year to an accuracy of 6 minutes and note that the seasons were of unequal length, now known to be an effect of the ellipticity of Earth's orbit. He was successful in the prediction of eclipses. Comparison of his observations of the sky with the writings of Eudoxus three centuries earlier led to the realization that Earth's rotation axis had moved, an effect known as precession. (As it spins, Earth is like a top for which the rotational axis moves about in a circle.)
Hipparchus was also the first to compile a catalog of the 850 brightest stars, with a numerical estimate of their brightness estimates. In Hipparchus's scale, the brightest naked-eye stars were designated category 1. Somewhat fainter stars were designated 2, still fainter 3, and so forth, to 6, the faintest naked-eye stars. These categories, called magnitudes in modern terminology, are based on the physiological response of the eye and brain, which perceives brightness ratios. On average, a magnitude 1 star is about 2.5 times as bright as a magnitude 2 star, a magnitude 2 star is 2.5 times as bright as a magnitude 3 star, and so forth. Magnitudes are still used as a measurement of brightness. The scale is extended to brighter objects (Sun, full Moon) by going to negative numbers (–26, –12.5, respectively) and to fainter objects by going to larger numerical magnitudes (the largest telescope with the best light detection devices and integration over time can detect stars as faint as magnitude 30).
Ptolemy and the Geocentric hypothesis
The writings of Ptolemy (fl. 140 A.D.), the last of the ancient Greek astronomers, show continued numerical advance over his predecessors. By his era, for example, trigonometric and geometric analysis of lunar observations had refined the Moon's distance to essentially the value accepted today.
Ptolemy attempted to explain what astronomers of his era could see occurring in the sky in terms of a geocentric model, at the center of which was a stationary Earth. He first assumed that the whole system revolved once per day around a stationary Earth. Each of the planets moved at uniform rates on small circles (epicycles), which in turn moved uniformly around larger circles ( deferents), with the center of each deferent offset slightly from the position of Earth. The Sun and Moon, however, used no epicycles (both have almost uniform motion about the sky). For Mercury and Venus, Ptolemy needed to lock the centers of their epicycles to the direction of the Sun. The position of each outer planet on its epicycle was such that the center-planet direction is parallel to the Earth-Sun direction. Finally, the distances of all celestial objects must be based on the apparent motions relative to the stars and on sizes of the retrograde loops, thus the order outward from Earth is the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn. The stars were placed exterior to Saturn.
