HISTORICAL NOTES

The science of astronomy is perhaps the oldest of the sciences. The first maps were probably of the heavens, since men used the movements of the moon, the sun, and the stars as guides in hunting, fishing, and farming.

The Babylonians studied celestial motions as long ago as 3,800 B.C. and predicted eclipses by tabulating the moon's hour angle. These are the earliest known ephemerides. They grouped the stars by constellations and divided the sun's apparent motion around the earth into 24 parts.

The early Egyptians fixed the dates of their religious festivals astronomically nearly as early as the Babylonians. They could measure angles precisely and oriented the pyramids within one minute of north. The great pyramid of Cheops appears to have been an early astronomical observatory. The light from Sirus shone down a shaft at upper transit., and the light of the pole star shone down the northerly shaft at lower transit. The pole star when the pyramid was bulit was not Polaris but rather Thuban in the constellation Draco.

The early Greeks learned navigation from the Phoenicians. Thales, an early Greek astronomer, was a Phoenician. He measured the length of the year and observed that the sun does not move uniformly between solstices. He is most famous for predicting the solar eclipse of 585 B.C. which ended a battle between Medes and the Lydians.

Aristarchus of Samos of the Alexandrian school who lived in the 1st half of the 3rd Century B.C., was an early proponent of the heliocentric theory of the solar system, i.e., the sun was the center of the universe and the earth revolved about it. Aristyllus and Timocaris, also Alexandrians, measured positions of the principal stars which they catalogued with some numerical precision.

Eratosthenes, also an Alexandrian astronomer, calculated the size of the earth. During the summer solstice he measured the zenith distance of the sun at Alexandria knowing that the sun was in the zenith at Aswan on the Tropic of Cancer. From the distance between the two places he calculated the earth's circumference to be 24,000 miles. The correct value is about 25,900 miles.

Archimedes in the third century B.C. used a glass celestial globe with a smaller terrestrial globe inside to demonstrate the motion of celestial bodies. Cicero described this device. Thus, although many people in the dark ages did not understand that the earth was spherical, astronomers have accepted the fact for at least 25 centuries.

Hipparchus, who was born in 180 B.C. was the greatest of the early astronomers. Unfortunately, he rejected the heliocentric theory of Aristarchus. He compiled a catalogue of more than 1,000 stars, giving their coordinates and divided them into six groups on the basis of brightness. By comparing the positions of these stars with the catalogue of Timocasris and Aristyllus of 250 years earlier, he calculated the precession of the equinoxes and defined the sidereal year and the tropical year.

Ptolemy, who chronicled the work of Hipparchus, lived between 100 and 200 A.D. he wrote the Almagest, a work of 13 books covering all Greek astronomy. He proved the earth to be round, described the position of the ecliptic and two methods of determining the obliquity of the ecliptic. He described the Astrolabe, an instrument for measuring the altitude of an object and a method for constructing celestial globes.

After the burning of the library at Alexandria, astronomy entered a dormant period (476-1500 A.D.). From the publication of the Alamagest to the time of Copernicus there was no astronomical discovery of great importance. Nicholas Copernicus, a Pole, was born in 1473. He was a lawyer and a doctor of medicine and developed a new heliocentric theory in an attempt to bring calculation and observation into agreement. In De Revolutionibus he presented his theory and triggered the birth of modern astronomy. Although the Copernican theory has been greatly modified it was important in that it spurred much new interest in astronomy.

Galileo, born in 1564, developed the telescope as an astronomical device and made precise measurements of the movements of celestial bodies. Galieo also developed the fundamental notions of mechanics upon which Newton later developed his theories. Tycho Brahe, the great Danish astronomer, designed and used special instruments to make precise observations which became the basis for much of Kepler's work. He also developed the sextant.

Johannes Kepler, born in Germany in 1571, worked with the observations of Tycho Brahe and Galileo to evolve his three basic laws of motion of the planets. He determined that the planets moved in elliptical paths about the sun.

Two contemporaries of Copernicus, Casper Vopal and Christian Hayden, designed armillary spheres to display information about the sky. Vopel, in 1500, made an armillary sphere with the earth in the center. Hayden, professor of mathematics at Nurenburg amde a brass shell with a celestial globe on the interior and a terrestial globe on the exterior.

The Gothorp globe, designed by Olearius in 1654, was 11 feet in diameter. It has a terrestrial map on the outside and a celestial map on the inside and a 15 cem. terrestrial sphere supported in the center.

Isaac Newton, born in 1643, developed the first theory of gravitation and a new system of mathematics, the calculus, with which he developed his theory. He derived from his theory the three basic laws previously propounded by Kepler.

Albert Einstein announced his special theory of relativity in 1905 and his general theory of relativity in 1915. He predicted from his theory of gravitation new laws of motion which have been confirmed through observations. His new theory of gravitation has explained minute discrepancies between the observed motions of the planets and those predicted by Newton's theory.