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A Coordinate System to Find Your Way

Equatorial System
Ecliptic Guides
Galactic Guides

There are two types of coordinates used by amateur astronomers: altitude-azimuth (alt-az) and equatorial. Professional astronomers use equatorial, ecliptic and sometimes galactic guides.


Altitude-Azimuth is the simplest to define: this system is based on the observer. Altitude is up and down while azimuth is across the horizon. Zero and 180 is your horizon, and 90 is straight above - called Zenith. This system is difficult to use in Astronomy for two reasons: the celestial sphere does not rotate this way, and the coordinates of stars, planets, and galaxies are given based on the equatorial system.

If you were to give coordinates based on your location of an object you see in the sky, an observer in another country (or even another state) would not know which object to which you are referring.

Back to Top  | Image Credit: Starry Night v. 4.5

Equatorial System:

The equatorial system is based on Earth's rotation (the sky does not really rotate, the Earth does). The axis of rotation for the equatorial system is the North and South Poles. Measurements are made by Right-Ascension (left and right - longitude) and Declination (up and down - latitude). The celestial equator is the imaginary plane that marks 0 declination. Right-Ascension is measured in hours, minutes, and seconds with its 0 point (0h 0m 0s) to be the imaginary line connecting the North and South Celestial Poles crossing the Celestial Equator.

The coordinates of celestial objects given in the equatorial system are universal. Computerized telescopes and star charts use this method to plot and track objects in the night sky. This system is also very convenient when trying to find an object during the daylight hours.

The point in which the ecliptic plane meets the celestial equator is called the Vernal Equinox.

The equatorial coordinate system is also known as the Celestial Coordinate System. Other terms to remember are NCP (North Celestial Pole) and SCP (South Celestial Pole).

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Ecliptic Guides:

This coordinate system is based on the Sun. The North and South Poles are the points above and below the Sun, and the Equator is the extended plane of the Sun's equator. Zero meridian is the Vernal Equinox. The ecliptic plane is the orbit of Earth about the Sun.

This coordinate system is useful when plotting the positions of the other planets as well as asteroids and comets.

Back to Top | Image Credit: Starry Night v. 4.5

Galactic Guides:

The galactic coordinate system is based on the ecliptic, except the North and South Poles are the points above and below the center of our galaxy, the Milky Way. This is the axis of rotation. The equatorial plane corresponds to the disk plane of our galaxy. Zero meridian is the point in which the imaginary line connecting the galactic north and south poles crosses the galactic equator.

This system is rarely used by amateurs and professionals. Only Astronomers studying galactic dynamics and rotation will find this coordinate system useful.

Back to Top | Image Credit: Starry Night v. 4.5

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