The need for a rough working rule to connect
the rival solar and lunar reckonings of time appears to have been a compelling
motive behind the development of a recognizably scientific astronomy by
the Babylonian priestly schools of the last three centuries before Christ.
Upon a sketchy foundation of observations they constructed elaborate numerical
tables (ephemerides) predicting the phenomena of the sun, moon, and planets
with due allowance for the principal inequalities of their motions. They
idealized the paths of these objects into the circle which the Greeks later
called the zodiac, divided into 12 equal "signs". They also established
an approximate common multiple of the month and the year, a 19-year period
since called (after its supposed Greek inventor, Meton) the "Metonic cycle,"
by which the Church calendar is still governed. (See also Calendar;
Zodiac.)

The Babylonians conceived of the earth as
a disc of land surrounded by a moat of sea and surmounted by a solid firmament;
below were the abode of the dead and the great deep. Babylonian astronomy
stemmed partly from the practice of drawing, from the configurations of
the heavenly bodies, omens as to the fortunes of the nations (judicial
astrology) or of individuals (horoscopic astrology). This superstition,
invading the Roman Empire, dominated the life and thought of Christendom
until the rise of modern science; it is not yet wholly extinct.

The ancient Egyptians conceived the cosmos
more crudely as a box-shaped receptacle with the sun-god traveling in a
boat along a celestial river. They introduced the 365-day Egyptian year
which was later adopted as a convenient unit by Greek and medieval astronomers.

Greek Astronomy

The tradition of scientific astronomy which
has come down to us through more than two thousand years originated in
a synthesis of the observations and procedures of the Babylonian ephemerists
with the natural philosophy and geometrical technique of the Greeks. Starting
from prevailing mythological ideas, the Greeks advanced to a naturalistic
conception of the cosmos, referring celestial phenomena either to the properties
of some universal element or to a set of regulative principles to which
nature was held to conform. Pythagoras (sixth century b.c.) and his school
conceived the earth as a sphere, and taught that the paths of the heavenly
bodies could all be resolved into uniform circular motions about the earth.

These doctrines, reduced to mathematical
precision by Eudoxus of Cnidos (fourth century b.c.), were developed by
Aristotle (c. 384-322 b.c.) into a cosmological system which survived almost
unchallenged down to the 16th century. He conceived of a finite, spherical
universe, centered upon the stationary earth. At the moon's distance, the
sphere was divided into a central core constituted by the four elements
(earth, water, air, fire), and an ethereal realm of heavenly bodies. To
account for the planetary motions, he supposed each planet to be attached
to the equator of a uniformly rotating sphere, the poles of which were
embedded in a second exterior sphere concentric with the first but turning
about an independent axis with a period of its own. This sphere was similarly
related to a third, and so on to the number required to give the planet
a compound motion such as it ex hibited when viewed from the earth
at the common center of all the spheres. Aristotle assigned four spheres
each to Saturn and Jupiter and five spheres to each of the other planets
(including the sun and moon). As all these spheres were conceived to form
a single system, it was necessary to interpolate between each pair of planets
a set of "reacting" spheres to prevent all the motions except the diurnal
one (common to the heavenly bodies) from being handed down from the outer
to the inner planets. The total number of spheres therefore amounted to
55. The four elements were supposed to undergo continual transformation
one into another, so that we lived in a region of change and decay; but
Aristotle held the heavenly bodies and their carrying-spheres to be composed
of an incorruptible "ether" and therefore to be incapable of any change
of substance.

In opposition to this view, Heracleides
of Pontus (fourth century b.c.) attributed to the earth a daily axial rotation,
and he taught that Venus and Mercury circulated round the sun. Aristarchus
of Samos (third century b.c.) broadly anticipated the modern heliocentric
(sun-centered) planetary scheme. Aristarchus also formulated a geometrically
correct method for determining the relative distances of the sun and moon;
his results, however, were incorrect because of the primitive instruments
with which he worked. His younger contemporary, Eratosthenes of Cyrene,
successfully applied what has become a classic procedure for estimating
the circumference of the earth. Knowing that Syene in Upper Egypt is under
the Tropic of Cancer, and that the gnomon of a sundial cast no shadow at
the summer solstice, he assumed that Syene and Alexandria had the same
longitude, and ascertained that the distance between the two was 5,000
stadia. The zenith distance for Alexandria was determined to be 1/50 of
the circumference of the meridian, and Eratosthenes assumed that the earth
was spherical, which fixed the circumference of the earth at 250,000 stadia.
In his measurement he erred from modern estimates by only one percent.

The Ptolemaic System

A geocentric (earth-centered) planetary system,
established in the Hellenistic age through the labors of Hipparchus of
Rhodes (second century b.c.), was completed and expounded by Ptolemy of
Alexandria (second century a.d.) in his Almagest (or Syntaxis). This astronomical
classic, which remained authoritative for at least 1,400 years, contains
the oldest surviving star catalog; it records the discovery by Hipparchus
of the precession of the equinoxes; it describes the instruments available
to the ancients for the measurement of celestial angles; and it explains
the epicyclic representation of lunar and planetary motions which was accepted
until the 17th century.

According to this system, a planet was represented
as uniformly describing a circle (the epicycle) about a center which meanwhile
described a larger circle (the deferent) about the earth (see Fig. 1).
By this means a realistic representation could be given of the characteristic
variations in the planet's rate of travel through the constellations and
in its distance from the earth (as suggested by changes in its brightness).
The representation could be refined by introducing additional epicycles
one upon another, or by referring the uniform motion in a circle not to
its center but to some other point, the equant. The representations of
the planetary motions and their reduction to numerical tables represented
the principle contributions of Ptolemy to astronomy.

Islamic Contributions

Following the decay of ancient culture, Greek
science was deviously transmitted to medieval Christendom by way of the
Islamic civilization. The Arabs absorbed the remains of the Hellenistic
tradition in the lands they overran in the seventh century; and Baghdad
became a center for the translation (direct or through Syriac versions)
of scientific classics, including Ptolemy's Almagest, into Arabic. From
there the tradition moved westward to Cairo and to the Muslim universities
of Spain. Observations by the great Muslim astronomer Al-Battani (died
929) indicated the progressive motion of the solar apogee.

The Arabs concentrated upon the redetermination
of astronomical constants and the construction of planetary tables; but
their principal achievement was to maintain the tradition and preserve
the classic texts of ancient astronomy during the time of intellectual
stagnation in the Christian West.

In the twelfth century Latin translations
of Arabic versions of Aristotle and Ptolemy served to introduce Western
Christendom to a system of natural philosophy and a numerical theory of
planetary motions which retained their authority throughout the Middle
Ages. In the fifteenth century these classics became available in the original
Greek; and in Nürnberg (Nuremberg) Johann Müller (1436-1476),
called Regiomontanus, revived the construction and use of astronomical
instruments.