Caption: The Sun's path on the sky on a day of an equinox for a location/observer in the Northern Hemisphere. The setup is basically that for illustrating the horizontal coordinate system (see Celestial sphere file: horizontal_coordinates.html).

The celestial sphere ITSELF is the tilted sphere in the image.

Features:

1. The celestial equator and the horizon are both great circles on the celestial sphere (the tilted sphere in the image) and their intersection points actually define due east and due west.

   To understand this, note that on the horizon plane (i.e., on the ground), the due east-due west line is perpendicular to the due north-due south line (i.e., the projection onto the horizon plane of the celestial axis). Thus the due east-due west line is perpendicular to the celestial axis and thus intersects the celestial equator.

   The image, in fact, illustrates how the cardinal directions NSEW are actually defined.

2. On an equinox day, the Sun is on the celestial equator, and so rises due east and sets due west.

3. The Sun is carried around by the daily rotation of the celestial sphere.

4. Diagrammatically, it is clear that equinox days have exactly 12 hours of daytime and 12 hours of nighttime for any location on Earth.

5. When the Sun is above the celestial equator in the Northern Hemisphere, it is carried about on a small circle that is still perpendicular to the celestial axis (the NCP axis), but higher up. It is above the horizon longer than 12 hours.

6. When the Sun is below the celestial equator Northern Hemisphere, it is carried about on a small circle that is still perpendicular to the celestial axis (the NCP axis), but is lower down. It is above the horizon less than 12 hours.