In astronomy, the zenith is simply understood as the point in the sky directly above the observer’s head. More accurately, the following definitions are equivalent:
- It is the point with an altitude of +90 degrees.
- It is the zenith of the horizon coordinate system.
- It is the intersection point between the celestial sphere and the line connecting the center of the Earth to the observer’s position on the Earth’s surface.
The point directly opposite the zenith on the celestial sphere is called the nadir.
The celestial meridian passes through the zenith, nadir, and the two celestial poles (North-South).
In the horizon coordinate system, the zenith angle is the angle between the vertical direction and the position of a celestial body, and it is complementary to the altitude angle, meaning it is the angle from the horizontal direction (the horizon).
If the zenith angle of the Sun is 0°, the Sun is at an altitude of 90° directly overhead, and we say that the Sun is at the zenith.
The shadow of a tree on the ground is shortest when the Sun is directly overhead (zenith). This phenomenon can only occur at noon on certain days in the tropics when the latitude of the location matches the declination of the Sun.
On Earth, observers located between the Tropic of Capricorn and the Tropic of Cancer (including the equator) will witness the phenomenon of the Sun reaching the zenith twice a year.
Those positioned exactly at the two tropics will only observe the Sun at the zenith once a year (on the winter solstice for the Tropic of Capricorn and on the summer solstice for the Tropic of Cancer).
Observers situated at latitudes higher than the Tropic of Cancer or lower than the Tropic of Capricorn will never witness the Sun passing through the zenith.
The location where the Sun reaches the zenith is termed the solar zenith point.
The Sun reaches the zenith at the Tropic of Cancer on June 22 (summer solstice) and at the Tropic of Capricorn on December 22 (winter solstice). At the equator, the Sun reaches the zenith twice a year, on March 20 (spring equinox) and September 23 (autumn equinox).
