iOptron 8401 manual Celestial sphere is an imaginary sphere of infinite radius

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Celestial sphere is an imaginary sphere of infinite radius.

NOTE:

You need two numbers to define a position on the celestial sphere.

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To understand the celestial coordinate systems there are several concepts that should be clarified.

The Celestial sphere is an imaginary sphere of infinite radius concentric with the earth on which all celestial bodies are assumed to be projected. Celestial coordinates are used to define a point on the celestial sphere. A great circle, a.k.a., orthodrome, is the intersection of a sphere and a plane through its center. For the celestial sphere, a great circle is the intersection of a plane through the observer (on the earth) and the celestial sphere. Celestial pole is either of the two points of intersection of the celestial sphere and the extended axis of the earth. There are two celestial poles--the north celestial pole and the south celestial pole. The Zenith is the point of the celestial sphere vertically overhead. The Nadir is the point on the celestial sphere vertically below the observer, or 180 degrees from the zenith. A horizon is a great circle on the celestial sphere midway between the zenith and nadir. Celestial meridian is a great circle of the celestial sphere through the celestial poles and the zenith. Celestial equator is the intersection of the extended plane of the equator and the celestial sphere. It is the primary great circle of the celestial sphere in the equatorial system, everywhere 90-degree from the celestial poles.

We will talk about two different kinds of celestial coordinate systems. One is the altazimuth system. And the other is the equatorial system. The major difference between them is the referencing great circle. In altazimuth it is the celestial horizon, while in equatorial it is the celestial equator.

To define a position on the celestial sphere, we need two angles. In the altazimuth system (FIG. D1) these two angles are altitude (A) and azimuth (Z). Imagine a vertical plane perpendicular to the observer’s horizon that passes through the observer and the star. The intersection of the vertical plane and the observer’s plane of horizon defines the azimuth. It is measured from the south (or the north) to the intersection (in the direction of motion of the star, in degrees, 0°~360°). In GoToNovaTM azimuth is measured from the north. On the vertical plane, altitude is measured from the intersection to the direction of the star (also in degrees, - 90°~90°).

In the equatorial system (FIG. D2 and D3), hour angle (t) is measured on the equator from the point of intersection of the celestial equator and the local meridian in the direction of motion of the star. The value of hour angle is measured in hours, minutes and seconds instead of degrees.

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