The roots of astronomy extend to before written records, but
it is clear that humans have always observed the sky. The earliest known
records of astronomical observations come from the Sumerian and Babylonian
cultures (in what is modern-day Iraq) and date from as far back as 3000 B.C.
Careful observations by court-sponsored astronomers led to the first known star
maps, the zodiac, and many of the other constellations still referred to today,
as well as the sexigesimal (base-60) counting system on which our angular
measures are based.
Babylonian astronomers knew the length of the year to high
precision. These measurements were probably used for political and agricultural
purposes. During the same epoch, a number of astronomical monuments, including
Stonehenge, were being constructed as calendar devices around what is now Great
Britain. Egyptian astronomers undertook similar cataloging and mapping work,
using stars as references in alignment of construction projects like the Great
Pyramids.
Greek philosophers were the first to speculate on the
structure of the universe, but were constrained by philosophical traditions
relying on pure geometrical forms. Aristotle (ca.350 B.C.), established a model
of the universe described as nested spheres with the Earth at the center, while
the Sun, Moon, planets and stars moved around the Earth in constant motion each
with its own rate.
Some Greek "natural philosophers," though, did
make more detailed measurements and surmises about the structure of space. For
instance, Pythagoras (ca. 500 B.C.) and Plato (ca. 400 B.C.) made strong
arguments, based on the shape of the Earth's shadow during lunar eclipses and
the changing view of the sky as one traveled north or south, that the Earth was
a sphere. Eratosthenes (ca. 200 B.C.) estimated the diameter of the Earth to
high precision based on the angle of sunlight observed at different locations.
Aristarchus (ca. 280 B.C.) is the first person known to have proposed that the
Earth orbits the Sun. One of the outstanding problems noted by Greek
astronomers was the motions of the planets. The culmination of ancient
astronomical endeavors was the work of Ptolemy (ca. A.D. 140), whose great work
syntaxis was an attempt to match the complicated motions of the planets about
the Earth. In his model, Ptolemy proposed that the Sun, Moon, planets, and
stars moved around the stationary Earth on circular orbits onto which were
placed smaller circles that carried the planets eastward, then briefly westward
("retrograde"), then eastward again relative to the fixed stars. In
melding the tradition of Aristotle with an attempt to match what was actually
observed, Ptolemy ensured his place in astronomical history. (The New York
Times Guide to Essential Knowledge’)
