Astronomy is concerned with the positions, motions, distances, and physical conditions and with the origins and evolution of the celestial bodies which were mentioned previously.

It is divided into such areas as astrophysics, celestial mechanics, and cosmology and it is one of the oldest recorded sciences with observational records from ancient Babylonia, China, Egypt, and Mexico.

Remarkable recent extensions of the powers of astronomy to explore the universe is in the use of rockets, satellites, space stations, and space probes; while the launching of the Hubble Space Telescope into permanent orbit in 1990 enabled the detection of celestial phenomena seven times more distant than by any earth-based telescope.

The development of astronomy can be divided into four primary periods

1. Ancient astronomy, dating from the first significant contributions of the earliest civilizations to the Almagest of Ptolemy.
2. Medieval astronomy, from the decline of Alexandrian culture to the Renaissance.
3. Modern astronomy, from the Copernican revolution to the present time.
4. The new astronomy of astrophysics, primarily a product of the 20th century.

Astronomy is the science of the celestial bodies; such as, the sun, the moon, and the planets; the stars and galaxies; and all of the other objects in the universe.

Astronomy is concerned with their positions, motions, distances, and physical conditions; as well as, with their origins and evolutions.

Astronomy is considered to be the oldest recorded science because there are records which can be examined from ancient Babylonia, China, Egypt, and Mexico.

As far as we can determine, the Greeks were the first "true astronomers" because they deduced that the earth was a sphere and they attempted to measure its size. A summary of Greek astronomy can be found in Ptolemy of Alexandria's Almagest which contains nearly all that is known of the astronomical observations and theories of the ancients.

The Almagest is also described as being a text on astronomy written by Ptolemy in the second century A.D. setting out his view of the universe with the earth at its center surrounded by spheres.

Ptolemy’s model of an earth-centered universe influenced astronomical thought for over 1,300 years.

In 1543, the Polish astronomer Copernicus demonstrated that the sun, not the earth is the center of our planetary system; and the Italian scientist, Galileo, was the first to use a telescope for astronomical study, in 1609-1610.

The 17th century saw several momentous developments; such as, Johannes Kepler's discovery of the principles of planetary motion, Galileo's application of the telescope to astronomical observation, and Isaac Newton's formulation of the laws of motion and gravitation.

The most remarkable recent extension of the powers of astronomy to explore the universe is in the use of rockets, satellites, space stations, and space probes; while the launching of the Hubble Space Telescope into permanent orbit in 1990 has enabled the detection of celestial phenomena seven times farther than by any earth-based telescope.

1. A field Guide to the Stars and Planets by Donald H. Menzel and Jay M. Pasachoff; Houghton Mifflin Company, Boston; 1983.
2. Astronomy, The World Book Encyclopedia of Science; Volume 1; World Book, Inc., Chicago, Illinois; 2000.
3. National Geographic Picture Atlas of Our Universe by Roy A. Gallant; Published by the National Geographic Society; Washington, D.C.; 1994.
4. Stars and Planets by Jay M. Pasachoff and Donald H. Menzel; Houhton Mifflin Company; New York; 1992.
5. The Astronomical Almanac; U.S. Government Printing Office; Washington, D.C.; 1944.
6. The Cambridge Illustrated History of Astronomy edited by Michael Hoskin; Cambridge University Press; Cambridge, U.K.; 1997.
7. The International Encyclopedia of Astronomy Edited by Patrick Moore; Orion Books; New York; 1987.

This is in contrast to extragalactic astronomy, which is the study of everything outside our galaxy, including all other galaxies.

It includes the study of Kuiper belt objects, dwarf planets, Titan, other icy satellites, extra-solar planetary systems (from brown dwarfs to stellar disks), and the occasional inner solar system body.

Radar contact with the moon was first made in 1945 and with Venus in 1961. The travel time to radio reflections allows the distances of objects to be determined accurately.

Analysis of the reflected beam reveals the rotation period and allows the object's surface to be mapped. The rotation periods of Venus and Mercury were first determined by radar. Radar maps of Venus were obtained first by earth-based radar and subsequently by orbiting space probes.

In addition to celestial bodies that radiate visible light, the universe contains many objects that emit radiation of various non-visible wavelengths; such as, radio waves.

Some astronomical objects emit very little light or even none at all; however, they may radiate relatively large amounts of energy at non-visible wavelengths.

Radio observations of celestial objects can be made from the earth's surface during the day and at night, both in cloudy and clear weather.

The point 180° opposite the zenith, directly underfoot, is the nadir and the astronomical zenith is defined by gravity; that is, by sighting up a plumb line.

If the line were not deflected by such local irregularities in the earth’s mass as mountains, it would point to the geographic zenith.

Because the earth rotates and is not a perfect sphere, the geocentric zenith is slightly different from the geographic zenith except at the Equator and the poles.

Geocentric zenith is the intersection with the celestial sphere of a straight line drawn through the observer’s position from the geometric center of the earth.