P a g e | 1

Equatorial Coordinates The equatorial coordinate system is the most frequently used of the coordinate systems in conventional astronomy. The equatorial coordinate system uses the meridian through the first point of Aries, the point where the ecliptic crosses the celestial horizon.

Equatorial Celestial Coordinates

North Celestial Pole

♈ 1st Point of Aries

Arbitrary Star

Celestial Equator

♈ α Right Ascension

Declination

H Hour Angle

A rectangular coordinate system can also be employed in order to identify an object's position in space.

Rectangular Equatorial Coordinates

Right Ascension

Declination

Distance (from )

Physics

2 | P a g e

Horizontal Coordinates For a celestial sphere centred on an observer, , the reference direction being the vertical as defined by gravity at the observer's location. The point immediately above the observer is the zenith, the point below, the nadir. Each is a pole of the celestial horizon. The hemisphere above the celestial horizon is visible.

Horizontal Celestial Coordinates

Zenith

North Cardinal Point

South Cardinal Point

Arbitrary Star

Observer's Meridian

Observer's Horizon

North Celestial Pole

A Azimuth

Co-altitude

Altitude

Geographic Latitude

Coordinate Transformations

Altitude

Azimuth

Declination

Hour Angle

Geographic Latitude

Circumpolar Stars

A circumpolar star is one which never sets for an observer. The qualification for a circumpolar star is that:

P a g e | 3

Ecliptic Coordinates The Earth's axis of rotation is tilted compared to the plane of its orbit. The ecliptic is the great circle which is formed by the line the Sun traces out as it moves through the sky.

Ecliptic Celestial Coordinates

North Celestial Pole

♈ 1st Point of Aries

Arbitrary Star

North Ecliptic Pole

Ecliptic

♈ λ Longitude

β Latitude

ϵ Obliquity of the ecliptic

Rectangular ecliptic coordinates are often used in the calculation of ephemerides.

Rectangular Ecliptic Coordinates

Latitude

Longitude

Distance (from )

Ecliptic coordinates can be transformed from equatorial coordinates.

Coordinate Transformations

Latitude

Longitude

Right Ascension

Declination

Obliquity of the ecliptic

Physics

4 | P a g e

Galactic Coordinates The galactic system of coordinates is based on the plane of the galaxy, which provides the galactic equator.

Ecliptic Celestial Coordinates

North Celestial Pole

North Galactic Pole

PA of Galactic Centre

Arbitrary Star

Galactic Equator

Longitude

Latitude

To convert between galactic and equatorial coordinates three quantities must be known: the right ascension and declination of the galactic north pole, and the position angle of the galactic centre.

Coordinate Transformations

( ) ( )

( ) ( )

Latitude

Longitude

Right Ascension

Declination

Supergalactic Coordinates

The supergalactic coordinate system relates to the supergalactic plane - the plane of the local universe. It therefore has similar principles to the galactic system of coordinates.

Coordinate Transformations

( ) ( )

( ) ( )

Latitude

Longitude

Right Ascension

Declination

P a g e | 5