74 ASTRONOMY. 
and the observations made with it: 2. Calculating Astronomy, which teaches 
the method of obtaining from the observations the calculated results. For 
a well-grounded study of astronomy, a knowledge of pure mathematics, 
of mathematical physics, and of the sciences of optics and mechanics, is 
necessary. ‘T'o become a practical astronomer, talent for observing, and a 
readiness in calculating, are required. 
Astronomy, from its very nature, gives quite an abundant opportunity 
for pictorial representations, which have for their object either to explain 
theoretical propositions, or to make a visible exhibition of the objects and 
phenomena of the starry heavens. It might, however, be a not uninteresting 
preliminary, to take a historical survey of this science, showing how the 
investigating disposition of man has been occupied in its endeavor to obtain 
a more accurate and perfect knowledge of the Universe. 
It is highly probable that the earliest nations, incited by the beauty of the 
starry heavens, and the necessities of life, gradually learned the changes of 
the days and seasons, the course of the sun, the moon, and some planets. The 
Chaldeans introduced the twelve signs of the zodiac, the sun-dial, and the 
clepsydra or water clock ; and also laid the foundations of Astrology. The 
Persians and Babylonians determined the sun’s year at 3654 days; the 
Pheenicians applied Astronomy to navigation; and that the Egyptians 
possessed astronomical knowledge is abundantly shown by characters on 
their obelisks and temples. The Chinese seem to have known and used the 
gnomon from the most remote antiquity, and the Emperor Yao (2367 sz. c.) 
determined the moon’s year to be 354 days. At the time of Hoang Ti, 
they were acquainted with the equinoctial and solsticial points. The 
Greeks, however, first raised Astronomy to a higher level: Thales 
(640 B. c.) knew the cause of eclipses of the sun and moon ; Anaximander 
constructed a geography and the first map; and Meton introduced the 
moon's cycle of nineteen years, to reconcile the moon’s year with the course 
of the sun. Aristarchus (born 267 s. c.) was the most celebrated astronomer 
among the Greeks; he even guessed at the motion of the earth around the 
sun. He was followed by Hipparchus, who, by means of the equinoxes, 
determined the sun’s year to be 365 days, 5 hours, 55 minutes, 12 seconds; 
and also constructed a catalogue of fixed stars. Ptolemy likewise sketched 
out a catalogue of fixed stars, as also a system of the universe, called after 
him the Ptolemaic system. The Romans had too little taste for mathe- 
matics to become good astronomers: Seneca alone, with a few other Roman 
philosophers, had just ideas of the heavenly bodies ; the Romans were, how- 
ever, so much the more given to Astrology. Julius Cesar introduced the 
calendar invented by the Greek, Sosigenes, and called, after him, the Julian. 
Later than this, when learning fled to the newly established cloister, Astro- 
nomy was cultivated almost exclusively by the Arabians, particularly from 
A.D. 650. Albatari prepared astronomical tables; and in the time of 
Almansor, the moon’s year was introduced, which is still in use among the 
Turks ; but after the death of Almansor (in the 10th century), the study of 
Astronomy among the Arabians died away, and during the middle ages 
scarcely anything was esteemed but Astrology. Charlemagne, Gerbert 
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