on certain days in the
tropics, where the trees'
latitude and the Sun's
declination are equal. The term
zenith sometimes means the
highest point, way, or level reached by a
celestial body on its daily apparent path around a given point of observation. This sense of the word is often used to describe the
position of the Sun ("The sun reached its zenith..."), but to an astronomer, the
Sun does not have its own zenith and is at the zenith only if it is directly overhead. In a scientific context, the zenith is the direction of reference for measuring the
zenith angle (or
zenith angular distance), the angle between a direction of interest (e.g. a star) and the local zenith - that is, the complement of the
altitude angle (or
elevation angle). The Sun reaches the observer's zenith when it is 90° above the horizon, and this only happens between the
Tropic of Cancer and the
Tropic of Capricorn. The point where this occurs is known as the
subsolar point. In
Islamic astronomy, the passing of the Sun over the zenith of
Mecca becomes the basis of the
qibla observation by shadows twice a year on 27/28 May and 15/16 July. At a given location during the course of a day, the Sun reaches not only its zenith but also its
nadir, at the
antipode of that location 12 hours from
solar noon. In
astronomy, the altitude in the horizontal coordinate system and the zenith angle are
complementary angles, with the horizon perpendicular to the zenith. The astronomical
meridian is also determined by the zenith, and is defined as a circle on the
celestial sphere that passes through the zenith, nadir, and the
celestial poles. A
zenith telescope is a type of telescope designed to point straight up at or near the zenith, and used for precision measurement of star positions, to simplify telescope construction, or both. The
NASA Orbital Debris Observatory and the
Large Zenith Telescope are both zenith telescopes, since the use of
liquid mirrors meant these telescopes could only point straight up. On the
International Space Station,
zenith and
nadir are used instead of
up and
down, referring to directions within and around the station, relative to the earth.
Zenith star Zenith stars (also "star on top", "overhead star", "latitude star") are stars whose declination equals the latitude of the observers location, and hence at some time in the day or night pass
culminate (pass) through the zenith. When at the zenith the right ascension of the star equals the local sidereal time at your location. In
celestial navigation this allows
latitude to be determined, since the declination of the star equals the latitude of the observer. If the current time at Greenwich is known at the time of the observation, the observers
longitude can also be determined from the
right ascension of the star. Hence "Zenith stars" lie on or near the circle of declination equal to the latitude of the observer ("zenith circle"). Zenith stars are not to be confused with "steering stars" of a
sidereal compass rose of a
sidereal compass. ==See also==