   It is the scientific study of the individual celestial bodies (such as moons, planets, stars,
    nebulae, and galaxies) and of the universe as a whole.
   Its various branches include astrometry, astrodynamics, cosmology, and astrophysics.
Thales (624-547 B.C., Ionian) was a Greek philosopher who believed that the Earth is a disk
floating on an endless ocean. Legend has it that he correctly predicted a solar eclipse in the
year 585 B.C.
Pythagoras (569-475 B.C., Ionian) was a mathematician who put forward the idea that the
universe is made of crystal spheres that encircle the Earth. According to him, the Sun, the
Moon, the planets, and the stars travel in separate spheres.
Aristotle (384-322 B.C., Greek), the great philosopher, proved that the Earth is spherical, and
believed that it was at the center of the universe. According to him, the Sun, planets, and stars
were located in spheres that revolved around the Earth.
Aristarchus (310-230 B.C., Greek) was the first to put forward the idea that the Sun was
actually in the center of the universe.
Hipparchus (190-120 B.C., Greek) is compiled the first known star catalog to organize
astronomical objects, and also came up with a scale to define the brightnesses of stars. A
version of this magnitude system is still used today. He measured the distance from the Earth to
the Moon to be 29.5 Earth diameters (we know today that the real value is 30 Earth diameters).
Perhaps his greatest discovery was the precession, or wobble, of the Earth"s axis, which is
caused by the gravitational pull of the Sun and Moon.
Claudius Ptolemy (85-165 A.D., Greek) was an astronomer who used Hipparchus" extensive
observations to develop a model that predicted the movements of the Sun, Moon, planets, and
stars. His model, called the Ptolemaic system, visualized an Earth-centered universe and
assumed that all astronomical objects move at constant speeds in circular orbits.
Al-Khwarizmi (780-850, Islamic) was the inventor of algebra. He performed detailed
calculations of the positions of the Sun, Moon, and planets, and did a number of eclipse
calculations. He constructed a table of the latitudes and longitudes of 2,402 cities and
landmarks, forming the basis of an early world map.
Omar Khayyam (1048-1131, Persian) was a great scientist, philosopher, and poet. He
compiled many astronomical tables and performed a reformation of the calendar which was
more accurate than the Julian and came close to the Gregorian. An amazing feat was his
calculation of the year to be 365.24219858156 days long, which is accurate to the sixth decimal