Celestial Equator: Definition & Use

Instructor: Josh Corbat

Josh has taught Earth Science and Physical Science at the High School level and holds a Master of Education degree from UNC-Chapel Hill.

Ancient people used to believe all the stars in the night sky were fixed in place on giant glass spheres. They even invented ways to map these spheres, and we still use some of these methods today. This lesson will discuss the celestial equator and how astronomers use it.

Introduction and Definition of Celestial Sphere

Ancient scientists had nowhere near the number of tools available to them that modern astronomers do. We often make fun of ancient scientists for the things they believed. We need to keep in mind, though, that ancient scientists made some amazing discoveries using only very basic tools and their own observational skills. One field in particular that relied heavily on pure observation is astronomy. Ancient astronomers relied on their eyes to study the night sky. We can forgive them, then, if some of the things they thought turned out to be wrong.

One thing that ancient astronomers believed to be true was that all the stars in the night sky were fixed in place on giant glass spheres. They believed that many of these glass spheres encased the Earth and rotated together. To them, this explained the distance of the stars and the fact that they moved in predictable patterns through the night sky. Although this idea was proved false once scientists concluded that the Earth is spinning on its axis, some of the basic parts of the idea remained useful.

Even modern scientists recognize a concept called the celestial sphere. Essentially, the celestial sphere is the sky we can see beyond our atmosphere. You have to think like an ancient scientist a little bit to fully understand this. Imagine that all of the stars were fixed on one giant glass sphere. That's the concept behind the celestial sphere. In order to map this sphere, both ancient and modern scientists used reference points. One particularly useful reference point is the celestial equator.

The celestial equator is an imaginary line that extends out from the Earth's equator into the celestial sphere. It divides the celestial sphere in half (just like the real equator does with Earth) and is used to map stars and other celestial objects.

A diagram of the celestial equator. Notice that the celestial equator extends directly out from the Earth
Celestial equator

How the Celestial Equator Was Used

Earth's equator is used as a reference line to locate places on the planet. You can easily determine your location by giving your latitude (referenced from the Equator) and longitude (referenced from the Prime Meridian; like the equator, but vertical). The celestial equator is used the same way. To locate objects in the night sky (stars or planets, for example), measurements can be taken from the celestial equator and other reference lines to pinpoint exactly where the objects are located. For example, the White House in Washington, D.C., is located at 38.9 North latitude, 77.0 West longitude. If you punched that into a GPS unit or app, it would pinpoint its location.

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