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Astronomical terms used to measure direction

#### Declination and right ascension

In astronomy, [**declination** is] the angular disance of a body north or
south of the **celestial equator**. Declination and **right ascension**,
an east-west coordinate, together define the position of an object in the sky.
North declination is considered positive and south, negative. Thus, +90
degrees declination marks the north **celestial pole** [near the North
Star], and -90 degrees the south celestial pole.

#### Altitude and azimuth

In astronomy, gunnery, navigation, and other fields, [altitude and azimuth are]
two coordinates describing the position of an object above the Earth.
**Altitude** in this sense is expressed as angular elevation (up to 90
degrees) above the horizon.
**Azimuth**, in astronomical measurement,
is the number of degrees clockwise from due south (usually) to the object's
vertical circle (i.e. a great circle through the object and the zenith). For
nonastronomical purposes, azimuth (or bearing) is generally measured clockwise
from due north.

#### Zenith and nadir

[The **zenith** is the] point on the celestial sphere directly above
an observer on the Earth. The point 180 degrees opposite
the zenith, direclty underfoot, is the **nadir**.
**Astronomical zenith** is defined by gravity, i.e. by sighting up a plumb
line.
If the line were not deflected by such local
irregularities in the Earth's mass as
mountains, it would point to the
**geographic zenith**.
Because the Earth rotates and is not a perfect sphere, the
geocentric zenith is slightly different from
the geographic zenith except at the Equator and the poles. **Geocentric
zenith** is the intersection with the celestial sphere of a straight line
drawn through the observer's position from the geometric center of the Earth.

#### Source

Encylopeadia Britannica, 15th edition, 1986.