Jason Harris
The Celestial Equator is an imaginary great circle on the celestial sphere. The celestial equator is the fundamental plane of the Equatorial Coordinate System, so it is defined as the locus of points with Declination of zero degrees. It is also the projection of the Earth’s equator onto the sky.
The Celestial Equator and the Ecliptic are set at an angle of 23.5 degrees in the sky. The points where they intersect are the Vernal and Autumnal Equinoxes.
The Celestial Poles
Jason Harris
The sky appears to drift overhead from east to west, completing a full circuit around the sky in 24 (Sidereal) hours. This phenomenon is due to the spinning of the Earth on its axis. The Earth’s spin axis intersects the Celestial Sphere at two points. These points are the Celestial Poles. As the Earth spins; they remain fixed in the sky, and all other points seem to rotate around them. The celestial poles are also the poles of the Equatorial Coordinate System, meaning they have Declinations of +90 degrees and -90 degrees (for the North and South celestial poles, respectively).
The North Celestial Pole currently has nearly the same coordinates as the bright star Polaris (which is Latin for “Pole Star”). This makes Polaris useful for navigation: not only is it always above the North point of the horizon, but its Altitude angle is always (nearly) equal to the observer’s Geographic Latitude (however, Polaris can only be seen from locations in the Northern hemisphere).
The fact that Polaris is near the pole is purely a coincidence. In fact, because of Precession, Polaris is only near the pole for a small fraction of the time.
Tip
Exercises:
Use the
Find Objectwindow (Ctrl+F) to locate Polaris. Notice that its Declination is almost (but not exactly) +90 degrees. Compare the Altitude reading when focused on Polaris to your location’s geographic latitude. They are always within one degree of each other. They are not exactly the same because Polaris isn’t exactly at the Pole. (you can point exactly at the pole by switching to Equatorial coordinates, and pressing the up-arrow key until the sky stops scrolling).Use the
Time Stepspinbox in the toolbar to accelerate time to a step of 100 seconds. You can see the entire sky appears to rotate around Polaris, while Polaris itself remains nearly stationary.We said that the celestial pole is the pole of the Equatorial coordinate system. What do you think is the pole of the horizontal (Altitude/Azimuth) coordinate system? (The Zenith).
The Celestial Sphere
Jason Harris
The celestial sphere is an imaginary sphere of gigantic radius, centered on the Earth. All objects which can be seen in the sky can be thought of as lying on the surface of this sphere.
Of course, we know that the objects in the sky are not on the surface of a sphere centered on the Earth, so why bother with such a construct? Everything we see in the sky is so very far away, that their distances are impossible to gauge just by looking at them. Since their distances are indeterminate, you only need to know the direction toward the object to locate it in the sky. In this sense, the celestial sphere model is a very practical model for mapping the sky.
The directions toward various objects in the sky can be quantified by constructing a Celestial Coordinate System.
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