Simply stated, the zenith point in the sky is the point that is straight up above a specified location on the earth’s surface. Its location can be described more precisely by imagining a line originating at the center of the earth, extending up to the surface at the observer’s feet and exiting the top of the observer’s head. The point where this line touches the celestial sphere is the zenith point. It can also be defined as a 90° angle upward from any, or from every, point on the horizon.
The zenith’s location is relative to the observer’s location. So long as the observer remains in a fixed location, the zenith remains fixed, and stars of the same declination as the observer’s latitude will appear to move through the zenith as the earth rotates. If the observer moves from that location by walking north, for instance, then the zenith moves north along with the observer.
As previously alluded, the declination of the zenith is the same as the observer’s latitude. If an observer is standing of the terrestrial equator, then the observer’s zenith is on the celestial equator, which turns overhead as the celestial sphere appears to turn as a result of the earth’s rotation about its axis. Also, if an observer is standing on the terrestrial north pole, then the zenith will be at the celestial north pole, and all stars will appear to circle the zenith as the earth rotates.
– Horizon and Coordinate System. http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/lec2.html. Accessed by Jim Johnson on January 19, 2015.
– Celestial Sphere. ASTR 1230 (Majweski) Lecture Notes. http://www.astro.virginia.edu/class/majewski/astr1230/. Accessed by Jim Johnson on January 19, 2015.
–Zenith. http://www.opencourse.info/astronomy/introduction /02.motion_stars_sun/. Accessed by Jim Johnson on February 1, 2015.
© James R. Johnson, 2015.