Astronomical Engines, I Spy With My Little Eye

A Celestial Coordinate System


A somewhat challenging coordinate system.

Greetings, everyone.

Today I want to talk about how celestial objects are mapped and located in the sky by astronomers and telescope wielding enthusiasts. It is really not all that complicated, it is much like terrestrial coordinates except mapped out to the sky onto what is called the celestial sphere. I’ll try to keep this as short as I can, but there are several terms that will need explaining. Yes, I’m going to require that you read again. Ah, I can hear the groans already.

Orientation of Astronomical Coordinates

Orientation of the galactic, ecliptic and equatorial coordinate systems, projected on the celestial sphere, showing the galactic equator (black), north galactic pole (NGP), the ecliptic (orange), north ecliptic pole (NEP), the celestial equator (blue) and north celestial pole (NCP). Sun and earth not shown to scale but to indicate sun's orbital direction around the galactic center and earth's orbital direction around the sun.

Celestial Coordinate Systems

There are several coordinate systems used  to map spherical coordinates onto the celestial sphere, but we’re only going to concern ourselves with the most widely used system, the equatorial coordinate system.

 The Celestial Sphere

The celestial sphere is essentially regarding the sky as a sphere that is concentric to the earth and rotating upon the same axis, and extending outward as far as is required. All objects in the sky can be considered as projected onto the celestial sphere. Projected up from the Earth’s equator and poles are the celestial equator and the celestial poles.

The celestial sphere

The Equatorial Coordinate System

The equatorial coordinate system allows an observer on the Earth to describe the apparent location in the sky of celestial objects in terms that are similar to the way object locations are described on Earth. On the Earth you would use the longitude and latitude of an object, while on the celestial sphere you use right ascension and declination to describe a location.


Elements of the equatorial coordinate system

Left: A star is at culmination on an observer's meridian (HA = 0 h), then RA = LST. Right: Now the vernal equinox point is at culmination on the meridian m (LST = 0 h) (Positive angles: RA, counterclockwise; HA and LST, clockwise)

Right Ascension

Right ascension (RA) is like terrestrial longitude only projected onto the celestial sphere. This next part is a little tricky, so I’m going to quote from Wiki:

Both right ascension and longitude measure an angle that increases toward the east as measured from a zero point on an equator. For longitude, the zero point is the Prime Meridian on the geographic equator; for right ascension, the zero point is known as the first point of Aries, which is the place in the sky where the Sun crosses the celestial equator at the March equinox.

First point of Aries? What the hell is that? It sounds like something from an astrology reading. (Actually, the term is used in astrology, but it’s different). To understand the term, you need to understand what an equinox is.  An equinox occurs twice a year, in the spring and the fall, at the times when the tilt of the Earth’s axis points neither towards nor away from the sun, and the sun lies in the same plane as the Earth’s equator, and thus the same plane as the celestial equator. The time this happens in the spring, rather than the day, determines the first point of Aries. Got all that? In addition, because the Earth wobbles and the constellations move, the first point of Aries now lies in Pisces. Because the point moves slowly over time, astronomers always refer to the epoch when referring to a set of equatorial coordinates, and make adjustments if comparing to observations from a different epoch.

Click on this link to show a simple animation that may help you visualize this stuff.

Right ascension is usually measured in hours, minutes, and seconds, with 24 hours being a complete circle. It can be measured in degrees, with 1 hour equivalent to 15 degrees of arc, but it’s considered to be tacky.


Declination is like terrestrial latitude, only projected onto the celestial sphere. Declination is measured in degrees north and south of the celestial equator,  north of the equator yields positive declinations, and south of the equator yields negative declinations. The celestial equator has a declination of 0o, the north pole has a declination of +90o, and the south pole has a declination of -90o.

 An example:

The right ascension (R.A.) and declination (dec) of an object on the celestial sphere specify its position uniquely, just as the latitude and longitude of an object on the Earth’s surface define a unique location. Thus, for example, the star Sirius has celestial coordinates 6 hr 45 min R.A. and -16 degrees 43 minutes declination, as illustrated in the following figure.

Right ascension and declination for Sirius

So how are you doing with all of this? The example pulls everything together pretty well, but it takes some time to get used to thinking in celestial coordinates. I’ll do my best to answer questions in comments.

You can also send me email at

A late update:

I found a great free software program called Stellarium in my rummaging. It’s basically a planetarium for your computer, and it’s really slick. You can turn on and off the equatorial grid lines so that you can see in the sky what I was talking about in the article. Plus, it has a huge star catalog and tons of viewing options. Well worth downloading if you’re interested in astronomy at all. You can download it here.



The University of Tennessee has a nice page on the celestial coordinate system. The equatorial elements picture and example picture and text are from that article. Used with gratitude.

The map of Discworld and the Great A’tuin courtesy of frankwm1’s Photobucket media



  • Well done! You took me back to my rockets class in college. Although, Dr. Madler took an entire hour to explain this. Probably because we had a bad habit of interrupting him with questions.

    • The Professor

      Thanks. I've been working on this for a couple of weeks, trying to figure out how to get the ideas across succinctly.

  • A friend of mine teaches a college-level introductory astronomy class. A few years ago he took a group of students out at night to set up a small telescope for some practical experience at locating various objects. As they were walking along, one student suddenly cried out "What the hell is that?!?" while pointing at the Moon. My friend and the rest of the class turned and looked, seeing nothing unusual. Finally my friend asked "What do you mean?" The student said "The Moon!" My friend asked "What about it?" The student said "What happened to it?"

    After an extended, confusing exchange, it turns out the student had been frightened by the appearance of the Moon, as it was not full. Having no concept of phases, the student wanted to know what had happened to the rest of it….

    I only half-believed this story until the day I found myself leading a field trip to Mt. St. Helens. As we were driving towards the mountain, I told the students to look towards a break in the trees where I knew we would get our first glimpse of it. One student pointed and yelled, word for word, "What the hell is that?!?" We all looked at the mountain, seeing nothing wrong. As with my friend, I finally asked "What do you mean?" The student replied "That, that MOUNTAIN!" I explained it was Mt. St. Helens, our destination and the subject of the pre-trip lecture (with its profusely illustrated slide show). It turns out the student had always been under the impression that nothing whatsoever was left of it after the 1980 eruption except a big hole in the ground.

    Now I will believe any instructional story I'm told.

    • Number_Six

      I have a forty-something friend who is smart, witty, very observant of human behaviour, and an excellent entrepreneur. One day I was walking with her and her husband when she suddenly exclaimed, "What the hell is that?!" Her husband and I both jumped and started looking for an explosion or the second coming of Elvis when it hit us both: she was pointing at a jet contrail. When we told her it was an airliner, she asked if it was on fire…

    • The Professor

      Amazing. Your friend should have told her that Galactus must be nearby and he's obviously peckish. She would have probably understood that. I hear stories like yours and the one by Number_Six (I am number four, be seeing you.) I wonder what is becoming of our society where people are so unaware of such basic things. It makes me think of H.G. Wells 'The Time machine' and the denizens he found occupying the Earth in the far future.

    • coupeZ600

      One of the coolest things I've ever seen is the New Moon. Only two or three that I can remember, but each time I was driving straight into the setting/rising Sun and thought, "WTF is that?"

    • OA5599

      When I was in college, I had a tendency to ask, early in the semester and in the presence of my professors, "What the hell is that?" about objects significantly related to my coursework. I knew their years in academia made them gullible enough to believe any instructional story they were told, and after I convinced themI needed an explanation regarding "verb", "equals sign", "the Constitution" or "a woven basket", I would, in their eyes, make a stunning academic improvement over the course of the semester, attributable (they believed) to their brilliant instruction. It never failed to get me a high position on the class curve.

      • skitter

        I did the opposite with Greek letters, five minutes before the final.
        Wait… that squiggly is the same as that squiggly?

    • While not quite as absurd, we were on a college level bird watching trip (yes there is such a thing). There was a sea lion in the tide water of the nearby river. One of the guys started shouting that he had just seen a manatee! He was from Florida after all…

  • skitter

    Okay, I think I've got this:

    From my spot on earth, I take the h/m/s to the time-appropriate First Point of Aries, then subtract the h/m/s of the Right Ascension.
    Next, I take the d/m/s to the celestial equator, then add the d/m/s of the Declination Angle.

    And it seems to need an extra step or three for any Earth Latitude over (90-angle to celestial equator).

    • The Professor

      Yes, you've got it. The high latitudes are a special case when using coordinates from an earlier epoch and amount to declinations of greater than 90 degrees because of precession and nutation when applied to the current epoch. I haven't tried dealing with those, so I'm not much help there.

  • The Professor

    Has anyone played with Stellarium yet? What do you think of it?

    • Number_Six

      Working on it…will it be more enterformative than Google Sky?

    • Number_Six

      It's awesome. I clicked on a satellite and it indicated that it should be visible; went outside and saw the satellite. It's also amazing to zoom in on Jupiter and watch the dance its moons are doing while the whole mess blazes through space.

      • The Professor

        Isn't it cool? I think that it's in a completely different league than Google Sky. The ocular view is pretty neat. There are more star files and landscape files, too.

        • Number_Six

          Being able to click on a star and get its details is incredible, as is the moving satellite view, complete with distance from your viewing point!

    • Number_Six

      Why are some star descriptions in different shades or colours? Some of them are tan, some are blue. Is it something to do with the kind of light they give off?

      • tonyola

        Stars are classified by their spectral color, and the first letter in the star catalog name reflects the spectral class. In general, hotter stars are blue-white or white while cooler stars tend from yellow to red. The text color reflects their classification.

        • Number_Six


  • Nice post!