Right Ascension & Declination: Celestial Coordinates for Beginners

Once you hold the golden keys of right ascension and declination, finding your way around the sky is almost as easy as finding your way around town. 

Gettin' griddy

The sky is draped in an invisible coordinate grid that will help you easily find any object you wish, the same as you would locate a city on a map. 
Stellarium

Sooner or later every novice skywatcher runs into the terms RA and Dec., short for right ascension and declination. When I got started in astronomy at age 11, they were a little scary. Then I realized they were just numbers like latitude and longitude, except applied to the stars.

Latitude & Longitude

To find anyplace on the globe of Earth, you only need to know its latitude, the distance in degrees north or south of the equator, and longitude, the distance in degrees east or west of the prime meridian. The prime meridian is an imaginary line that runs through the Royal Observatory in Greenwich, United Kingdom, and extends to the North and South Poles. It defines the the zero (0°) longitude line just as the equator defines the 0° latitude line.

Each city has a unique latitude and longitude. Take Tuscaloosa, Alabama, for example, which is located at latitude +33.2° north, longitude 87.6° west. Or Wonglepong, Australia, situated along that continent's east coast at –27.0° south, 153.2° east. A negative sign in front of the latitude indicates south and a positive sign north. Every location, whether it be a city, airport, or even your own home or apartment building lies somewhere on the worldwide coordinate grid (below), its location fixed by two numbers.

Global grid

Earth is shown covered in an imaginary grid of latitude lines (measured from 0° to 90° north and south of the equator) and longitudes lines (measured from 0° to 180° east and west of the prime meridian). You'll occasionally see the East and West longitude suffixes replaced by a negative sign for the western hemisphere and a positive sign for the eastern hemisphere.
CC0 1.0

Each degree of latitude is equal to about 111 kilometers on Earth's surface. For precision we break down degrees into either fractions of a degree or divisions called minutes and seconds of arc. There are 60 minutes in one degree and 60 seconds in one minute. Tuscaloosa's precise location is +33° 12′ (minutes) 35″ (seconds) north, 87° 34′ 9″ west. You can convert fractions of degrees into minutes and seconds here.

Cities by numbers

Every city on the planet has a unique latitude and longitude, its "spot" on the grid. Detroit, Michigan, is shown here.
CC 3.0 / Wikipedia

Right Ascension & Declination

Like cities, every object in the sky has two numbers that fix its location called right ascension and declination, more generally referred to as the object's celestial coordinates. Declination corresponds to latitude and right ascension to longitude. There are no roads in the sky, so knowing an object's coordinates is crucial to finding it in your telescope.

Let's use our imaginations and picture the latitude–longitude grid on the planet as the surface of a flexible, transparent soccer ball. If you could pump the ball up into a gigantic sphere centered on the Earth, you'd look up and see lines of latitude and longitude imprinted on the sky. The equator, which marks the 0° latitude line, now circles the sky as the celestial equator, while the north and south celestial poles hover over either end of the planet's polar axes.

Viewed from Earth's equator, the celestial equator begins at the eastern horizon, passes directly overhead and drops down to the western horizon. Since we're inside a sphere, it would continue around the backside of the Earth as well.

Celestial coordinates defined - right ascension and declination

Here's Earth inside the big soccer ball. Declination (green) is measured in degrees north and south of the celestial equator. Right ascension, akin to longitude, is measured east from the equinox. The red circle is the Sun's apparent path around the sky, which defines the ecliptic.
Tom Ruen / CC BY-SA 3.0

From mid-latitudes, the celestial equator stands midway between the horizon and overhead point, while from the poles the celestial equator encircles the horizon. Anything north of the celestial equator has a northerly declination, marked with a positive sign. Anything south of the equator has a negative declination written with a negative sign. For instance, Vega's declination is +38° 47′ 1″, while Alpha Centauri's is –60° 50′ 2″. One star is north of the celestial equator and the other south. Can you guess the declination of the north celestial pole? If you said +90°, you're already getting the hang of this.

Sky navigation made easy

Here's a slice of sky bisected by the celestial equator with hours of right ascension (RA) labeled along the top and declination (Dec.) at left. The coordinates of the bright stars Betelgeuse and Sirius are also shown.
Stellarium

While we use a physical location on Earth as our reference for longitude, what reference do we use for right ascension? Where is the 0° mark in the sky separating east from west? Astronomers use the spot the Sun arrives at on the first day of spring, called the vernal equinox. Presently, it's located in the constellation of Pisces, the Fish. The sky can be treated as a clock, since it wheels by as Earth rotates, so the zero point of right ascension is called "0h" for "zero hours." Unlike longitude, right ascension is measured in just one direction — east. Because there are 24 hours in a day, each hour of right ascension measured along the equator equals 1/24th of a circle (360° divided by 24) or 15°. That's a little more than one-half the width of the W-shaped constellation Cassiopeia.

Rockin' Around the Clock

This view shows the north celestial pole (NCP) and polar regions. Declinations are labeled every 10° and the hours of right ascension are shown around the circle. By convention, 0h is used instead of 24h
Stellarium

In keeping with right ascension's time theme, hours are subdivided into minutes and seconds, and are even written out as minutes (m) and seconds (s). Let take the North Star for example. Polaris is located at RA 2h 41m 39s, Dec. +89° 15′ 51″. Because the stars circle about the sky every 24 hours, right ascension or RA ranges from 0h to 24h. The star 29 Piscium, located immediately east of the equinox point is very close to 0h with an RA of 0h 01m 49s, while its neighbor, Omega (ω) Piscium, located just west of the equinox point, has an RA of 23h 59m 19s.

Unlike Earth coordinates, celestial coordinates change due to the slow wobble of Earth's axis called precession. Precession causes the equinox points to drift westward at a rate of 50.3 arcseconds annually. As the equinox shifts, it drags the coordinate grid with it. That's why star catalogs and software programs have to be updated regularly to the latest "epoch." This is done every 50 years. Most catalogs and software currently use Epoch J2000.0 coordinates (for the year 2000). The next major update will happen in 2050.

Learning RA and Dec. provides you with a golden key to unlock the position of any object in the night sky. Before computer software effortlessly plotted the paths of newly discovered comets and fast-moving asteroids, I couldn't wait to get my hands on their coordinates. I'd hand-plot the positions on a paper star atlas, then swing my scope to the spot, and thrill when I found it on my own.

RA and Dec. also come in super-handy if you have a Go To telescope and a new comet or nova is discovered. Just input its coordinates, hit enter, and you're there. If you hear of a new comet or fast-moving asteroid, a quick check of its changing coordinates will tell you not only where it is but where it's headed, so you can plan the best time to see it.

I made friends with right ascension and declination long ago. Knowing I could drive anywhere on its invisible roadways helped me, and it'll help you too become more familiar with the night sky.

10 thoughts on “Right Ascension & Declination: Celestial Coordinates for Beginners

  1. RodRod

    Good review and I enjoy RA and Dec. Much of my observing is with a Telrad. You can convert RA and Dec. by coordinate transformation (Jean Meeus Astronomical Algorithms) to your geographic location and find the altitude and azimuth of the target for viewing along with transit time, etc. I hear this works very well on *round* planet Earth. It does for me 🙂

    1. stewshou

      Bob King said:
      I’d hand-plot the positions on a paper star atlas, then swing my scope to the spot, and thrill when I found it on my own.
      I don’t know how to take this step with my Dobsonian. Do I need some add on protractor etc? How do I find the vernal eqinox? How do I go to a specific RA Dec without a system to show RA Dec?
      Not much sky watching this winter.
      Stew Shouldice

      1. Bob KingBob King Post author

        Hi Stew,
        I also use a Dob and have for many years. You don’t need to know where the vernal equinox is if you’re hand-plotting. You get the specific RA and Dec. of what it is you want to look at. If it’s a bright star, then that’s easy, you look to see if that star is up in the sky at the time you want to view it. You can do that by using a planisphere (star-wheel) or a free software planetarium program like Stellarium. If you’re looking for a fainter object, you’ll need a star atlas. Star atlases will always shows right ascension along the bottom and declination along the side. You locate a specific object by interpolating between those numbers. If you’re looking for a new comet or variable NOT on an atlas, then you hand-plot its RA and DEC. right on the chart using a straight-edge or protractor, what have you. Larger atlases come with their own separate fine-gradation grids you can place over the page to really nail the position down. Does this help?

  2. Anthony BarreiroAnthony Barreiro

    Bob, this explanation would fit nicely in the first chapter of your next book!

    Lately I’ve been figuring out how to use the northern circumpolar stars and the Sun’s approximate right ascension to tell time at night.

    Here are the bright circumpolar stars that happen to lie close to an exact hour of right ascension:
    Beta Cassiopeiae 0 h
    Gamma Cas 1 h
    Epsilon Cas 2 h
    Alpha and Beta Ursa Majoris (Dubhe and Merak, the pointer stars) 11 h
    Gamma UMa 12 h
    Epsilon UMa 13 h
    Beta Ursa Minoris (Kochab, the brightest star in the bowl of the little dipper) 15 h
    Gamma Draconis (Eltanin, the brightest star in the head of the dragon) 18 h

    Put Polaris at the middle of your imaginary clock face, find whichever of these stars are visible, and interpolate the rest of the clock. Now use the seasons or the Sun’s current astrological sign* to figure out the Sun’s approximate right ascension.

    The Sun is at 0 hours RA at the March equinox, 6 hours at the June solstice, 12 hours at the September equinox, and 18 hours at the December solstice. Figure out how far we are between an equinox and a solstice and interpolate. Astrological signs are easier, because every astrological sign is about two hours of right ascension wide, starting with 0 h at the first point of Aries, 2 h at the start of Taurus, etc. (the signs follow ecliptic rather than equatorial coordinates, so they don’t match up exactly, but close enough for this purpose).

    The next step is to deduce the angle between the Sun and a convenient circumpolar star, and finally to figure out where the Sun’s right ascension is on your circumpolar clock. Imagine the Sun moving below the horizon from your western horizon toward the eastern horizon, and that allows you to “see” where the Sun is at any given moment from sunset to midnight and from midnight to sunrise.

    The Sun is currently about one week into the sign of Pisces, or three weeks before the March equinox, so the Sun’s right ascension would be about 22 hours 30 minutes (according to theskylive.com, the Sun’s current RA is 22 h 38 m). In other words, the Sun is about 30 minutes west of being opposite Dubhe and Merak. So if it’s nighttime and Dubhe and Merak are east of the meridian, the Sun is west of the meridian, i.e. it’s before midnight. When Dubhe and Merak are transiting the meridian it’s 30 minutes after midnight, and when Dubhe and Merak are west of the meridian the Sun is east of the meridian, i.e. it’s after midnight. By estimating the angle between the pointer stars and the meridian (and adding the equation of time), I find I can estimate the time to within 15 or 30 minutes.

    Anyway, it’s a bit of fun that needs neither optical aid nor an especially dark sky.

    * No further endorsement of the validity of astrology is hereby expressed or implied.

    1. Anthony BarreiroAnthony Barreiro

      I made a mistake: in order to get the most accurate estimate of clock time, you need to *subtract* (not add) the equation of time to your local solar time. The current equation of time is (-12 minutes 45 seconds), so subtracting a negative number means that clock time is about 13 minutes later than local solar time.

      You also need to correct for your longitude east or west of your standard time zone’s central meridian. For each degree east of your central meridian your clock time is four minutes earlier than mean solar time; for each degree west of your central meridian your clock time is four minutes later than mean solar time. Here in San Francisco we are 2 degrees 26 arcminutes west of the central meridian, which means our clock time is an additional ten minutes later than mean solar time.

      It’s easy to go down a rabbit hole with timekeeping. But it’s fun.

      Also, the genitive cases of Ursa Major and Ursa Minor are Ursae Majoris and Ursae Minoris.

  3. Luvlivemusic

    Hey Bob!
    great article! But I’m not a young man anymore and it’s a bit tough to get some things through my feeble brain. Am I correct in understanding the the difference between finding Cassiopeia and Ursa Major is just the RA? As I read this they both lie at 60 degrees of Dec. but one is at 1 hour RA and the other at 12 hour of RA. Am I on the right track here? Sorry to be so daft.
    Michael

All comments must follow the Sky & Telescope Terms of Use and will be moderated prior to posting. Please be civil in your comments. Sky & Telescope reserves the right to use the comments we receive, in whole or in part, and to use the commenter’s username, in any medium. See also the Terms of Use and Privacy Policy.

COMMENT

This site uses Akismet to reduce spam. Learn how your comment data is processed.