16 Celestial Coordinates – A Summary

The celestial sphere is a tool that astronomers use to map the sky. To navigate the map, we need coordinates.  Maps of the Earth have longitude and latitude; the celestial sphere has right ascension (RA) and declination (Dec).

To make sense of longitude and latitude, you first need to locate the North and South Poles.  The axis of the Earth – that imaginary line around which Earth rotate – extends out of the Earth at its two poles. Lines of longitude wrap around the earth from north to south, meeting at the North and South poles.  The equator wraps around the globe, forming a great circle that is always equidistant from the two poles – cutting the Earth into a northern hemisphere and a southern hemisphere. Lines of latitude run parallel to the equator; unlike lines of longitude, they never meet each other.

To make sense of right ascension and declination, we need to identify the North and South Celestial Poles and the Celestial Equator. If you stand on the Earth’s North Pole, the North Celestial Pole (NCP) will be directly above you at all times – at your zenith.  If you stand on the Earth’s South Pole, the South Celestial Pole (SCP) will be at your zenith.  If you stand on the Earth’s equator, the celestial equator will arc across the sky, from due east to due west, passing through your zenith along the way.

Image by the Lunar and Planetary Institute

Declination is the easier of the two celestial coordinates to understand, it measures the angle north or south of the celestial equator.  The celestial equator has a declination of zero, the NCP has a declination of 90 degrees, and the SCP has a declination of -90 degrees.  Degrees of declination therefore correspond to degrees of a circle, with 180 degrees from the SCP to the NCP and a full 360 if you return to the SCP.

To get a better sense for a degree of arc, the Moon’s angular diameter is about half of a degree.  Almost everything else in the sky has an angular size much, much smaller than a degree. To accurately chart stars, nebula, galaxies, and other astronomical objects, we need to be more precise than a degree.  This is where arcminutes and arcseconds come in.  If you divide one degree into 60 equal parts, you get an arcminute (the angular diameter of the Moon is more precisely 31 arcminutes).  Divide one arcminute by 60 and you get an arcsecond. Locations in the sky are typically given with an accuracy of an arcsecond or less.

Right Ascension is not based on a circle; it’s based on the Earth’s rotation around its axis.  The celestial sphere is divided into 24 hours of RA, with each hour divided into 60 minutes, and each minute divided into 60 seconds.  It’s important to note that a second of RA does not correspond to an arcsecond.  Instead, it corresponds to the angle that the Earth turns through in one second.  In a full day, the Earth rotates 360 degrees and through 24 hours of RA, which means that 1 hour of RA is 15 degrees (so one minute of RA is a quarter of a degree, or 15 arc minutes… and 1 second of RA is 15 arcseconds).  Adding to the unit converting fun, RA and Dec are sometimes expressed in decimal form instead of in degrees, arcmin, and arcsec (for Dec) or hours, minutes, and seconds (for RA).

In this map of Ursa Minor (also called the little dipper), you can see lines of constant RA and Dec.  The lines of RA meet at the NCP, which is extremely close to the bright star Polaris, and then fan out around the image. The circles are lines of constant Dec, marked every 10 degree from 90 degrees at the pole out to 60 degrees towards the bottom of the image.

The constellation Sagittarius is in the southern half of the sky, so the declinations are all negative.  Lines of constant RA are labelled at the top and bottom of the map.  You might notice that RA decreases from left to right, unlike what you might expect.  This happens because we look up at the sky, but down at this map.