Important Points on the Celestial Sphere

Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

The imaginary celestial sphere surrounding Earth is a reference tool that allows for the locations of objects in the sky to be located. Learn about the important points on the celestial sphere, including the horizon, zenith, nadir, equator, the North and South poles, and the meridian. Updated: 10/28/2021

The Celestial Sphere

If you could just imagine for a second being transported inside of a snow globe, what would it look like? I don't think it'd look any different to you than if you were to walk outside right now and look at the sphere-ish looking thing that surrounds you, the sky. Well, you only see half of a sphere, but you know another half exists down below you. People in Brazil and Australia also have a snow-globe or dome-shaped sky as much as anyone in Europe or the U.S.

The imaginary sphere that surrounds the Earth is called the celestial sphere, and it's a useful tool used to reference real and imaginary points on the sky.

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  • 0:01 The Celestial Sphere
  • 0:37 Horizon, Zenith, & Nadir
  • 1:47 The Celestial Equator,…
  • 3:00 Lesson Summary
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Horizon, Zenith, and Nadir

As I just recently said, at any location on our planet, you'll only see half of the celestial sphere. We'll follow an image on-screen for an observer in North America to explain the reference points on the celestial sphere that are commonly used.

The celestial sphere
Diagram of the celestial sphere points

The half of the celestial sphere you see is bounded by the horizon, the boundary between Earth and the sky. This celestial horizon is best seen when you have an unobstructed view of the sky, such as in a vast, flat, and open plain or when you're in the middle of the ocean. As you stand outside, if you look straight up, you'll be looking at the zenith, the point in the sky directly overhead. Do not get the zenith confused with looking north. Looking straight up is not the same thing as looking north.

Conversely, if you could look through the Earth, right underneath where you stand, you'd be looking towards the nadir, the point on the celestial sphere directly opposite the zenith. Again, this is not the same thing as saying you are looking south. As seen on the screen, the nadir and the cardinal direction of south are not the same points.

The Celestial Equator, Poles, & Meridian

The point on the celestial sphere directly above the Earth's North Pole is known as the north celestial pole, and not surprisingly, the point on the celestial sphere directly below the Earth's South Pole is the south celestial pole.

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