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 degrees 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.