The zenith is the point in the sky directly above an observer. As the Earth rotates, different parts of the sky will move through the zenith. If a star is visible at a given location, it will eventually pass through the zenith.

The time that a star takes to pass through the zenith depends on its declination. Declination is the angle of a star above the celestial equator. Stars with a declination of 0° are located on the celestial equator. These stars will pass through the zenith twice a day, once when they are rising in the east and once when they are setting in the west. Stars with a declination of 90° are located at the poles. These stars will never pass through the zenith.

The zenith is an important point in astronomy. It is the point where the celestial equator and the meridian intersect. The meridian is the imaginary line that runs from north to south through the zenith. The meridian is used to measure the altitude and azimuth of stars.