Celestial Sphere in Astronomy

Lisa Baird, Artem Cheprasov
  • Author
    Lisa Baird

    Lisa Baird is a writer who teaches writing. After receiving her doctorate in Rhetoric and Composition from Texas Christian University, she has been developing writing curriculum for over twenty years.

  • Instructor
    Artem Cheprasov

    Artem has a doctor of veterinary medicine degree.

Understand what the celestial sphere is. Know what zenith and nadir are. Learn what the celestial sphere is used for and discover what the south celestial pole is. Updated: 01/15/2022

Table of Contents


What is the Celestial Sphere?

The celestial sphere definition in astronomy is an imaginary sphere surrounding the Earth. Another way of imagining the celestial sphere is to picture the Earth inside a transparent, celestial dome on which the Sun, Moon, planets, and stars are fixed. In this conception of the celestial sphere, the Earth is at the center of the universe. In truth, the Sun is at the center of the solar system, though the concept of the celestial sphere is useful for plotting the locations of stars and planets. Just as a map shows the latitude and longitude of places on the Earth, the celestial sphere shows the locations and positions of celestial objects like stars and galaxies.

An 18th century celestial sphere on display in the Brooklyn Museum

A bronze globe of the celestial sphere

The concept of an imaginary sphere encircling the Earth came from Aristotle's cosmology. To account for the movement of the heavens, he believed the stars and planets were affixed to crystal spheres that moved around the Earth. Since the Earth seemed stationary, it seemed reasonable to claim that the heavens were moving rather than the Earth. Aristotle's conception of the celestial sphere was undisputed for centuries. However, Aristarchus of Samos (310-230 BCE), another Greek philosopher, used mathematics to show that the Earth, rotating around its axis, instead revolved around the Sun. Aristarchus also believed the stars were very large and very far away. Unfortunately, the works of Aristarchus have been lost to time.

Illustration of the celestial sphere, an imaginary sphere encircling the Earth

Picture of the imaginary celestial sphere

Use of Celestial Sphere

Although the celestial sphere is based on the erroneous assumption that the Earth is the center of the universe, the system of plotting celestial latitude and longitude is useful for referencing real and imaginary points in the sky. Like a map or globe of the Earth that shows the location of a place by its latitude and longitude, the celestial sphere has celestial coordinates similar to latitude and longitude. These coordinates are projected into the sky. Declination is the equivalent of latitude; Right Ascension is the equivalent of longitude. Declination is measured in degrees while hours, minutes, and seconds are used as measurements of right ascension. For example, Polaris, the North Star, has a declination of +89 degrees, 15 arc minutes and a right ascension of 2 hours, 41 minutes.

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North Celestial Pole

Several important points, also imaginary, are associated with the celestial sphere. One of them is the north celestial pole, an imaginary extension of the Earth's North Pole into space. It is a point on the celestial sphere directly above the Earth's North Pole. At night, the stars seem to turn around the north celestial pole. A camera pointed at the North Star records trails of starlight as the Earth rotates on its axis around the pole. Constellations closest to the north celestial pole such as the Big Dipper (Ursa Major) are circumpolar, meaning they never set but are always visible near the pole.

In astrophotography, the north celestial pole is the most important consideration since a telescope mount must be accurately aligned to that point to produce the spectacular images of deep space objects that require hours of exposure. Since the Earth is rotating, an equatorial mount is used to track a deep space object through the night.

Streaks of starlight demonstrate how some constellations move around the north celestial pole

Streaks of starlight demonstrate circumpolar constellations moving around the north celestial pole

South Celestial Pole

The other significant point on the celestial sphere is the south celestial pole. Like its counterpart in the north, the south celestial pole is the point on the celestial sphere directly above Earth's South Pole. In the northern hemisphere, Polaris, or the North Star, is within one degree of the Earth's north celestial pole, so navigation using the North Star is easy. The southern hemisphere, however, lacks a bright pole star for navigation and astronomy. Instead, the bright constellation, the Southern Cross, or Crux, is used as a polar reference.

Celestial Sphere : Related Terminology

Other significant points on the celestial sphere include the celestial equator, the celestial meridian, zenith and nadir, and the celestial horizon. It is important to distinguish these points because they are terms used to describe events and objects in astronomy. These points on the celestial pole provide fixed references, though 'fixed' is a relative term since the Earth is moving. The International Celestial Reference System (ICRS) is used for more precise astronomical positioning.

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Frequently Asked Questions

Where is the north celestial pole?

The north celestial pole is an imaginary projection of the Earth's axis into space. It is located directly above the Earth's North Pole. While it is imaginary, it nevertheless is effective in celestial navigation.

What is the celestial sphere concept?

The celestial sphere is an imaginary sphere on which are projected objects in space. It is used to locate the positions of real and imaginary celestial objects. Although the concept is based on an Earth-centered system, it is effective for plotting celestial coordinates of objects in space.

What is the celestial sphere used for?

The celestial sphere is an imaginary sphere encircling the Earth. It is used for pinpointing celestial objects, real and imaginary. Like a globe, the celestial sphere has corresponding coordinates for celestial objects.

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