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To add a few points:
- The zenith = The overhead point on the celestial sphere for any observer; it is always 90 degrees from the horizon.
- The meridian = The arc that goes through the north point on the horizon, zenith, and south point on the horizon. The meridian is what separates the morning and the afternoon positions of the sun. a. m. = ante (before) meridian and p.m. = post (after) meridian.
According to Professor Nick Strobel,
The positions of the zenith and meridian with respect to the stars will change as the celestial sphere rotates and if the observer changes locations on the Earth, but those reference marks do not change with respect to the observer's horizon.
Depending upon the time of the year, someone on the equator can see the noon sun due north or due south. However, for people in the Northern Hemisphere, the sun at noon is due south, while for people in the Southern Hemisphere the sun at noon is due north.
- The celestial equator = An imaginary circle that goes around the sky directly above the equator. All the stars rotate in a path parallel to the equator that is always 90 degrees from the North and South Poles.
In astronomy the celestial sphere refers to an imaginary sphere with the Earth at the center of the sphere. Though the sky extends infinitely in all directions stars are assumed to lie on the surface of the celestial sphere. From different locations on the surface of the Earth the visibility of stars changes. The zenith refers to a point directly above the observer, which is defined by the line from the center of the Earth , and perpendicular to the horizon.
If the observer is at the Earth's equator, the celestial equator passes though the zenith. The north celestial pole moves closer to the zenith as the observer moves towards the North and the two coincide at the North pole. Similarly as the observer moves South, the south celestial pole moves closer to the zenith and the two coincide at the south pole.
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