Right ascension and declination in astronomy are like longitude and latitude on Earth. Together, they give coordinates identifying the location of stars, planets, satellites, and other objects in the sky.
Right Ascension
Right ascension corresponds to longitude. It is abbreviated as RA or lowercase Greek letter alpha (α). Usually, right ascension has units of hours, minutes, and seconds. For example, a constellation with a right ascension of 3 hours 4 minutes and 38 seconds is written as 3h 5m 38s.
These units have equivalents in degrees. So, the Earth turns 360° in 24 hours, 180° in 12 hours, or 15° in one hour.
Right ascension moves east around the celestial equator from 0, which is the location of the Sun on the first day of spring in the northern hemisphere (vernal equinox in March). Another way to look at it is the 24 hours increase moving clockwise around the globe as viewed from the North Pole.
The celestial equator, in turn, is a great imaginary circle extending outward from the Earth’s equator. So, the celestial equator is the same as a flat horizon for a viewer at the equator. The celestial equator angles across the sky for viewers in the mid latitudes in either hemisphere.
Declination
Declination is the celestial equivalent to latitude. It is abbreviated as DEC or lowercase Greek letter delta (δ).
Declination moves up (+ or no symbol) or down (-) relative to the celestial equator. It uses units of degrees (°), minutes (‘), and seconds (“), which are not to be confused with temperature degrees or the symbols for feet and inches.
- An object on the celestial equator has a declination of 0°.
- The north celestial pole has a declination of +90°. Polaris or the North Star is within 1° of the north celestial pole.
- The south celestial pole has a declination of -90°.
- An object directly overhead has a declination within 0.01 degrees of the observer’s latitude. The slight difference is because astronomical latitude is slightly different from geodetic latitude and because the Earth is an ellipse rather than a perfect sphere.
Using Right Ascension and Declination
A right ascension and declination measurement tells you the position of an object in the sky.
For example, an object with RA 3h 5m 38s is slightly more than three hours east of where the Sun rises on the vernal equinox. A RA of 12 hours is to the west. A right ascension of 22 hours (22h) goes almost all the way around the circle back toward east (as defined by the equinox).
Right ascension tells you what direction you need to look (moving east), while declination tells you how far up (or down) and object is from the equator. Assuming you don’t live on the equator, you need to know your latitude. An object with DEC 48 50′ 2″ is directly overhead at 48 degrees north latitude. A viewer at 38 degrees north latitude sees this object 10 degrees lower (48 – 38 = 10). At 60 degrees north latitude, the object is below the the celestial equator (48 – 60 = -12). A viewer in the southern hemisphere at 25 degrees south latitude sees this object around 73 degrees above the celestial equator [48 – (-25) = 48 + 25 = 73].
Once you understand right ascension and declination you can find any object in the sky. If you’re still confused, that’s okay, too. Fortunately, many apps and telescopes do all the math for you, so all you do is enter in the right ascension and declination!
References
- Chaisson, E.; McMillan, S. (2016) Astronomy: A Beginner’s Guide to the Universe (8th ed.). Pearson. ISBN 9780134241210.
- Lathrop, John (1821). A Compendious Treatise on the Use of Globes and Maps. Wells and Lilly and J.W. Burditt, Boston.
- Moulton, Forest Ray (1916). An Introduction to Astronomy. Macmillan Co., New York.
- U.S. Naval Observatory Nautical Almanac Office (1992). Seidelmann, P. Kenneth (ed.). Explanatory Supplement to the Astronomical Almanac. University Science Books, Mill Valley, CA. ISBN 0-935702-68-7.