Imaginary Sphere around the Earth
The Celestial Sphere
The celestial sphere is an imaginary sphere concentric with the Earth. This sphere is used by astronomers to navigate through the night sky. 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 allows to map the position of heavenly bodies in the sky, in the same way that cities or countries are mapped on earth using the geographic coordinate system. The celestial coordinate system helps locate the positions of stars, planets, the moon, and other objects.
Declination and Right Ascension
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.
On the Earth´s map, the lines that run parallel to the equator are used to measure angular distance (latitude) to the north or south of the equator. On the celestial sphere, astronomers use a measure known as declination, in the same way to measure angular distances above or below the celestial equator.
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)
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 includes declination and right ascension lines covered until here.
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