Spherical astronomy studies extraterrestrial bodies visible from Earth, using the celestial sphere to describe their positions. It was important to ancient cultures and used for navigation. The telescope allowed for new discoveries, but measurements were still based on spherical astronomy until the 20th century. Today, it is still used for observations made from Earth’s surface.
Spherical astronomy is the oldest form of astronomy, the study of the stars and the cosmos. It concerns those extraterrestrial bodies that can be observed from the earth’s surface. These include the constellations and other familiar features of the night sky, such as the North Star. In spherical astronomy, the night sky is seen as an imaginary structure called a celestial sphere that surrounds 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 about 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 have been positioned to align with astronomical models. Long before the invention of the compass, sailors used the stars for navigation.
All of these were early applications of spherical astronomy. The astronomers based their calculations on the approximately 3,000 stars and planetary bodies visible to the naked eye at any given time of night. As science advanced slowly in the Middle Ages, some astronomers detected unknown moons and planets by observing the fluctuations in the orbits of the visible ones. The perfection of the telescope in the 1600s made it possible to observe many of these bodies 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 was because observations had yet to be made from the surface of the Earth. It wasn’t until the 20th century that astronomers realized that all the stars and constellations move as the universe expands. Those that appear to be stationary are simply moving very slowly. Although based on astronomical observations, this new view of the universe forced astronomy to radically alter its star map.
Today, orbiting telescopes, space probes and radio telescopes provide a much more complete picture of the universe than can be observed from the earth’s surface. Objects at the ends of the universe have been observed, as well as planets and extrasolar systems that would be impossible to see from inside the Earth’s atmosphere. However, spherical astronomy is alive and well. It is used whenever an astronomer is scanning the night sky with a telescope. The resulting measurements are based on the celestial sphere, just as they have been for centuries.
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