Equatorial Coordinate System - Right Acension & Declination

6.8 - Understand the use of the equatorial coordinate system (right ascension and declination)

6.10 - Understand how the observer’s latitude can be used to link the equatorial and horizon coordinates of an object for the observer’s meridian

When we consider the position of stars, some are seasonal, some are circumpolar, some we never get to see from our location. We need a reference to say where the position of a star is so we use Right Ascension and Declination.

Declination is the latitude of a star in the sky measured from the Celestial Equator.

If the star is given a declination of +45° then you would see it from the equator at 45° in the sky.

Declination measured in degrees (°), arc minutes (‘) and arc seconds (“) and is abbreviated as dec (δ).

Right Ascension is the equivalent of longitude in space.

We need a marker that everyone on Earth can use. On Earth we measure longitude from and have defined a prime meridian from the late 19th Century but an older reference position was known previously. We use a position called the First Point of Aries.

The Sun appears to cross the celestial equator along the ecliptic at the vernal equinox. This position is called the First Point of Aries (but actually takes place in Pisces). It changes every year because of the slight wobble in Earth's axis.

Right Ascension is measured in hours (h), minutes (m) and seconds (s) and is  abbreviated as RA or α.

If a star lies on a line between where the Sun crosses the Celestial Equator in spring and north (or south) then it will be said to lie at 0h 0s 0m° RA. If it lies 90° to the east* it would be located at 6h RA (360/90 = 4. 24/4 = 6).

Most stars have their right ascension and declination published. These are usually abbreviated to RA and DEC. Star maps and planispheres feature gridlines for the observer to follow. In a star map you would see a star listed in this format: Arcturus: RA: 14 15 40 DEC: +19 10 57. Due to the effects of precession the Right Ascension coordinates of objects are updated in star catalogues.

With my telescope set to polar alignment I can tilt the tripod vertically so it sits aligned to the declination of a star. I can then move it horizontally to its right ascension.

See the Celestial Calculations page to understand locations and timings of observing objects.

* East rather than West as in the hour angle.


Mix & Match

Explain the terms Right Ascension and Declination.


Right Ascension
How far an object is along the celestial equator from the first point of Aries.

How far north and south an object is from the Celestial Equator