Table of Contents
- What is the Celestial Sphere?
- North Celestial Pole
- South Celestial Pole
- Celestial Sphere : Related Terminology
- Lesson Summary
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.
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.
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.
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.
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.
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.
Midway between the north celestial and south celestial poles is the celestial equator. In fact, the celestial equator is an imaginary projection of Earth's equator into space. It is an important point on the celestial sphere because it intersects the Earth's horizon directly east and west, no matter where a person stands on the Earth. The celestial equator is also important because it marks zero degrees of declination, the point at which other objects' declination north or south are derived.
The celestial meridian is the circle on the celestial sphere that passes through the celestial poles and zenith. It is a great circle that arcs directly overhead, passes through the north and south poles on the horizon, and continues, albeit invisibly, through 360 degrees. The point directly overhead along the arc of the celestial meridian is the zenith.
The points on the celestial sphere, the zenith and nadir in astronomy, are related to the concept of the meridian. The meridian arcs overhead from pole to pole; the point directly overhead on the arc is the zenith. The arc continues invisibly through a full circle, 360 degrees, so that the point beneath the feet of an observer is directly opposite the zenith, a point called the nadir. Together, the zenith and nadir help astronomers describe the positions of celestial objects.
If an observer were to, for example, travel north around the earth, the north celestial pole would move closer to the zenith until, at the north pole, the celestial equator would lie on the horizon. If an observer were to stand instead at the Earth's equator, they would see the celestial equator passing through the zenith.
Another point on the celestial sphere is the celestial horizon, a plane passing through the Earth's center perpendicular to the zenith-nadir axis, or the boundary between the Earth and the sky. In other words, the celestial horizon is a great circle on the celestial sphere that passes through a point halfway between the zenith and nadir. The celestial horizon is useful in separating the celestial sphere into two hemispheres. This way, an observer from any point on the Earth can see only half of the entire sky. The other half is therefore hidden below the celestial horizon. Astronomers refer to the celestial horizon in measurements of the Earth relative to the sky.
The celestial sphere is an imaginary sphere where all the objects in the heavens are projected. Although it is based on a geocentric (Earth-centered) system, the celestial sphere is useful for pinpointing objects in space as well as defining Earth-based positions relative to the sky. The celestial sphere, like the coordinates on maps, uses a celestial coordinate system. Declination is the celestial equivalent of latitude; right ascension is the celestial equivalent of longitude.
Several points on the celestial sphere are important in astronomy. The north celestial pole and south celestial pole are imaginary extensions of the Earth's axis into space. The north celestial pole is a point directly above the Earth's North Pole. The celestial equator is an imaginary projection of the Earth's equator into space. The celestial meridian is the circle on the celestial sphere that passes through the celestial poles and zenith, the point directly overhead for an observer. Directly opposite the zenith is the nadir, the point directly beneath an observer's feet. The celestial horizon is the boundary between earth and sky.
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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.
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.
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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