History of Astronomy | Page 8

113 of
that work he records the real opinions of Socrates as set forth by
Xenophon; and the reader will, perhaps, sympathise with Socrates in
his views on contemporary astronomy:--
With regard to astronomy he [Socrates] considered a knowledge of it
desirable to the extent of determining the day of the year or month, and
the hour of the night, ... but as to learning the courses of the stars, to be
occupied with the planets, and to inquire about their distances from the
earth, and their orbits, and the causes of their motions, he strongly
objected to such a waste of valuable time. He dwelt on the
contradictions and conflicting opinions of the physical philosophers, ...
and, in fine, he held that the speculators on the universe and on the laws
of the heavenly bodies were no better than madmen (_Xen. Mem_, i. 1,
11-15).

Plato (born 429 B.C.), the pupil of Socrates, the fellow-student of
Euclid, and a follower of Pythagoras, studied science in his travels in
Egypt and elsewhere. He was held in so great reverence by all learned
men that a problem which he set to the astronomers was the keynote to
all astronomical investigation from this date till the time of Kepler in
the sixteenth century. He proposed to astronomers the problem of
representing the courses of the planets by circular and uniform
motions.
Systematic observation among the Greeks began with the rise of the
Alexandrian school. Aristillus and Timocharis set up instruments and
fixed the positions of the zodiacal stars, near to which all the planets in
their orbits pass, thus facilitating the determination of planetary
motions. Aristarchus (320-250 B.C.) showed that the sun must be at
least nineteen times as far off as the moon, which is far short of the
mark. He also found the sun's diameter, correctly, to be half a degree.
Eratosthenes (276-196 B.C.) measured the inclination to the equator of
the sun's apparent path in the heavens--i.e., he measured the obliquity
of the ecliptic, making it 23° 51', confirming our knowledge of its
continuous diminution during historical times. He measured an arc of
meridian, from Alexandria to Syene (Assuan), and found the difference
of latitude by the length of a shadow at noon, summer solstice. He
deduced the diameter of the earth, 250,000 stadia. Unfortunately, we do
not know the length of the stadium he used.
Hipparchus (190-120 B.C.) may be regarded as the founder of
observational astronomy. He measured the obliquity of the ecliptic, and
agreed with Eratosthenes. He altered the length of the tropical year
from 365 days, 6 hours to 365 days, 5 hours, 53 minutes--still four
minutes too much. He measured the equation of time and the irregular
motion of the sun; and allowed for this in his calculations by supposing
that the centre, about which the sun moves uniformly, is situated a little
distance from the fixed earth. He called this point the excentric. The
line from the earth to the "excentric" was called the line of apses. A
circle having this centre was called the equant, and he supposed that a
radius drawn to the sun from the excentric passes over equal arcs on the
equant in equal times. He then computed tables for predicting the place

of the sun.
He proceeded in the same way to compute Lunar tables. Making use of
Chaldæan eclipses, he was able to get an accurate value of the moon's
mean motion. [Halley, in 1693, compared this value with his own
measurements, and so discovered the acceleration of the moon's mean
motion. This was conclusively established, but could not be explained
by the Newtonian theory for quite a long time.] He determined the
plane of the moon's orbit and its inclination to the ecliptic. The motion
of this plane round the pole of the ecliptic once in eighteen years
complicated the problem. He located the moon's excentric as he had
done the sun's. He also discovered some of the minor irregularities of
the moon's motion, due, as Newton's theory proves, to the disturbing
action of the sun's attraction.
In the year 134 B.C. Hipparchus observed a new star. This upset every
notion about the permanence of the fixed stars. He then set to work to
catalogue all the principal stars so as to know if any others appeared or
disappeared. Here his experiences resembled those of several later
astronomers, who, when in search of some special object, have been
rewarded by a discovery in a totally different direction. On comparing
his star positions with those of Timocharis and Aristillus he round no
stars that had appeared or disappeared in the interval of 150 years; but
he found that all the stars seemed to have changed their places with
reference to that point in the heavens where the ecliptic is