celestial coordinates vocab

Celestial Sphere

An imaginary sphere surrounding Earth to which all celestial objects are considered to be attached.

Celestial Equator

The projection of Earth's equator onto the celestial sphere.

Ecliptic

The apparent path of the Sun across the celestial sphere over the course of a year.

Right Ascension (RA)

The celestial equivalent of longitude, measured eastward along the celestial equator in hours, minutes, and seconds (0h to 24h).

Declination (Dec)

The celestial equivalent of latitude, measured north or south of the celestial equator in degrees (−90° to +90°).

Hour Angle

The angular distance between an object and the local meridian, measured westward, in time units.

Local Sidereal Time (LST)

The right ascension that is currently on the observer's meridian.

Vernal Equinox (γ point)

The reference point for RA = 0h; where the Sun crosses the celestial equator moving northward in March.

Azimuth

The angle around the horizon from due north (0°) clockwise to the projection of the object on the horizon.

Altitude

The angle of a celestial object above the horizon, ranging from 0° to 90°.

Zenith

The point on the celestial sphere directly overhead the observer (altitude = 90°).

Nadir

The point directly opposite the zenith, below the observer.

Meridian

The great circle that passes through the zenith and the celestial poles; it divides the sky into eastern and western halves.

Sidereal Day

The time it takes for the Earth to rotate 360° relative to the stars: approximately 23h 56m.

Equinox

When the Sun crosses the celestial equator, resulting in equal day and night. Happens in March and September.

Solstice

Points where the Sun is at its maximum declination, occurring in June (summer) and December (winter).

Circumpolar Star

A star that never sets below the horizon, due to its declination and observer's latitude.

Celestial Poles

The extension of Earth's axis onto the celestial sphere—North and South Celestial Poles.

Precession

The slow wobble of Earth's axis, causing the celestial poles and equinoxes to shift over time (~26,000 years).