The Celestial Sphere
The Celestial Sphere
The celestial sphere is an imaginary sphere concentric to Earth. The sphere is used by astronomers to navigate through the night sky, permitting to specify the apparent positions of stars, planets and other objects. The coordinates used in the celestial sphere correspond to the geographic coordinate system used on Earth. All objects in the sky can be thought of as projected upward from the Earth´s equator, and the North, and South poles.
The celestial coordinate system assumes that the earth is at the center of the sphere whose radius extends to infinity. Three dimensional coordinates are used to mark the positions of the objects in the sky. In spite of their distances from the earth, all objects; stars, planets, seem to be at a fixed distance within the sphere. The eastward rotation of the earth produces an apparent westward rotation of the map of heavenly bodies.
In the celestial sphere, the point of the sky directly above the Earth´s North Pole corresponds with the celestial north pole, likewise, the Earth´s South Pole corresponds with the south celestial pole, and the Earth´s equator, with the celestial equator.
Declination is a measure that is comparable to latitude on an earth´s map and it´s used to point locations north or south of the equator. Those north of the equator have positive declination while those south have negative declinations. They are measured in degrees, minutes and seconds.
Anywhere from 0 degrees at the equator to 90° north or south of the equator.
Right ascension is the angular distance used to measure locations eastward the celestial equator starting at the vernal equinox. It uses hours, minutes and seconds. When used along with declination, it is used to indicate the astronomical coordinates of an object in the celestial sphere.
On the Earth´s globe, the lines that run vertically (line of longitude) from pole to pole are used to denote angular distance east or west of the Prime Meridian (0°) located in Greenwich England. On the celestial sphere, in the same way, right ascention (R. A.) is used to to measure distances east or west of a point of reference (0) which is the position the Sun is in the sky on the Vernal Equinox.
Right ascension is measured in hours minutes, and seconds from west to east in the celestial sphere at a zero point. Every 24 hours the Earth rotates on its axis 360°. 360 divided by 24 is equal to 15. Astronomers divide the celestial sphere in 24 R.A. lines, so that each line corresponds to 15° in the sky.
Celestial Coordinates (Orion)
Locating Orion in the Celestial Sphere
The Orion constellation (The Hunter) is one of the most recognisable in the night sky which can be seen at the Northern latitudes throughout the winter season. It is located at (5h, 30m R.A.), and since it runs on the line of the equator, it is at declination 0.
To practice locating objects in the sky, you can get a star chart analog (planisphere) which is an instrument that helps recognize stars and constelations. It makes use of declination and right ascension lines covered in this hub.
© 2012 Jose Juan Gutierrez