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Spherical astronomy is the most ancient form of astronomy, the study of stars and the cosmos. It is concerned with those extraterrestrial bodies that can be observed from the Earth’s surface. These include the constellations and other familiar fixtures of the night sky, such as the North Star. In spherical astronomy, the night sky is viewed as an imaginary structure called the celestial sphere that circles the Earth. All stars, planets, and constellations can be described by their positions on the celestial sphere.
For most of human history, all that was known of the cosmos was what could be observed in the sky. Even primitive cultures soon realized that some extraterrestrial bodies remained stationary, while others, including the sun and moon, changed position depending on the time of year. Astronomy was important to the religion and culture of many ancient civilizations. Structures such as Stonehenge and the pyramids of Egypt and Central America were positioned to line up with astronomical patterns. Long before the invention of the compass, sailors used stars for navigation.
All of these were early applications of spherical astronomy. Astronomers based their calculations on the roughly 3,000 stars and planetary bodies that are visible to the naked eye at any given time of night. As the science slowly advanced in the Middle Ages, some astronomers detected undiscovered moons and planets by observing fluctuations in the orbits of visible ones. The perfection of the telescope in the 1600s allowed many of these bodies to be observed for the first time. It also provided discoveries that were shocking to established science and religion, such as Galileo’s observation that the Earth orbited the sun rather than the other way around.
Even after the advent of the telescope, all new measurements were based on those of spherical astronomy. This is because observations still had to be made from the surface of the Earth. It was not until the 20th century that astronomers understood that all stars and constellations are moving as the universe expands. Those that seem stationary are merely moving very slowly. While it had been based on astronomical observations, this new view of the universe forced astronomy to radically alter its stellar map.
In the present day, orbiting telescopes, space probes, and radio telescopes provide a far more complete picture of the universe than can be observed from the Earth’s surface. Objects at the far end of the universe have been observed, as well as extrasolar planets and systems that would be impossible to see from within the Earth’s atmosphere. Nevertheless, spherical astronomy is alive and well. It is utilized every time an astronomer searches the night sky with a telescope. The resulting measurements are based on the celestial sphere, just as they have been for centuries.