One of the trickiest things to do as a backyard astronomer is to find the tiny, faint objects that we all enjoy looking at. Sure, we can use star maps and star hopping, or a resource like the Virtual Astronomy Club, but there is a sure-fire way to find exactly what you’re looking for every time: the celestial coordinate system.

You may have seen two odd groups of numbers when you’ve been researching what to look for. Take our article on finding the Orion Nebula as an example. In there, we say that the RA (2000.0) of this stunning object is 05h 35m 24s, and its Dec (2000.0) is -05° 27′ 00″… but what, exactly, does that mean?

Screenshot from another article giving an example of RA and Dec.
The Right Ascension and Declination coordinates for M42, the Orion Nebula

Well, RA stands for Right Ascension and Dec stands for Declination, these are the two parts of the celestial coordinate system that are enough to tell us where every single object is in the night sky.

We’ll explain what each of these are in a moment but, first, let’s make sure we all understand what we mean by a coordinate system.

What is a Coordinate System?

A coordinate system gives everything a precise location that we can use to find it quickly and definitively.

Our Earth-bound coordinate system defines every point on the planet’s surface as a combination of two measurements – latitude and longitude. By very precisely measuring these, we can pinpoint every spot on the surface of our planet.

As humans, we’re happy using addresses, like:

XYZ house name,
5th Street,
New York,
10038.

The description gets us there, but it isn’t very precise.

In the context of space, that’s like saying “The Orion Nebula is in the constellation of Orion, in the middle of his ‘sword’ which hangs underneath the middle star of his ‘belt’.” Again, it will help us find M42, but it is a definitive, unmistakable location.

On Earth, I’d use the longitude/latitude system to tell you that the coordinates of my house are 40° 43′ 50.1″ N and 73° 56′ 6.8″ W.

Because the Earth is round, we use angular coordinates to measure distance, rather than miles or kilometers. Angular coordinates are represented by three different symbols, which are:

  • ° (degrees) – there are 90 of these from the equator to the north pole and from the equator to the south pole. From the north pole to the south pole is 180°
  • ‘ (minutes) – Each degree can be split into 60 minutes
  • ” (seconds) – each minute is further split into 60 seconds (or 1/3600th of a degree). Anything less than one second is shown as a decimal, e.g. 1.5″ is one and a half seconds

So, the coordinates above tell you that my house is 40 degrees, 43 minutes, and 50.1 seconds north, and 73 degrees, 56 minutes, and 6.8 seconds west.

Ah, I can hear you ask, “North and west of what?”, and it’s a great question!

No coordinate system works without a baseline, a starting point from which all measurements are taken. The measurement on its own is useless, it has to be a measurement from something to give it meaning.

Down here on Earth, latitude is a measure of how far north or south (N or S) of the equator you are. The second measurement, longitude, measures the distance east or west (E or W) from the prime meridian.

World map showing lines of latitude and longitude
Lines of latitude (l-r) and longitude (n-s) on the World map

If my house was on the equator, it would be at 0° latitude. At the other extreme, the north and south poles have latitudes of 90°N and 90°S respectively.

Longitude is not so easy to define, so, in 1884, the world picked an arbitrary location as the standard for zero degrees longitude, also known as the prime meridian. That location is the Royal Observatory in Greenwich, England (read more about this).

Just as the equator splits the north from south, the prime meridian divides west from east.

Now we have our baselines, the equator, and prime meridian, every point on Earth’s surface can be given as a distance away from those origins.

With our firm grasp on ground-based coordinates, let’s leap off into space and see how we define the system of celestial coordinates.

The Celestial Coordinate System

In their attempts to understand the workings of the universe, Ancient Greeks looked up at the sky and believed what they saw to be the truth – the Earth was at the center of the universe with the sky looming over it like a vast, hollow dome. Stars were thought to be attached to the inside of this dome.

Impression of the celestial sphere around Earth with right ascension and declination lines
The celestial sphere surrounds the earth and has its own coordinate lines.

While we now know this to be false, the analogy of a celestial sphere proves to be extremely useful in precisely locating celestial objects.

Just like on Earth, we can combine an up/down measurement with a left/right measurement to precisely define where any object is on this sphere. Its distance from us is irrelevant.

Declination and Right Ascension Explained

To keep things as simple as possible, astronomers use what we already have on Earth and map it across to the celestial sphere. With a couple of simple thought experiments, it’s easy to see how we’ve created a coordinate system for the night sky.

What we’re describing below is known as the equatorial coordinate system. There are other celestial coordinate systems, such as the galactic coordinate system, which uses the plane of the Milky Way as a base, and the ecliptic coordinate system that uses the path of the sun, but they’re beyond our scope (haha) for today.

A Guide to Right Ascension

If you were to stand at the north pole and look directly overhead, you see the north celestial pole (NCP), which is almost perfectly marked by the star Polaris in Ursa Minor. Similarly, if we find ourselves at the south pole, our zenith (overhead point) is the south celestial pole (SCP).

On Earth, as we just saw, the lines that join the north and south poles are known as lines of longitude. In space, on the inside of our imaginary celestial sphere, they are known as lines of right ascension. They are the green lines shown in the image below.

These lines are not split into degrees, but hours. And, since there are 24 hours in a day, i.e. one full rotation of the Earth on its axis, there are 24 lines of right ascension. Historically, the celestial sphere was thought to rotate around the Earth (rather than Earth itself rotating) with a full sweep of 360 degrees completed every 24 hours.

The other main difference between right ascension and longitude is that RA is only ever positive, i.e. we count upwards from zero to 24 – there is no east/west right ascension.

While each hour is 1/24th of 360°, so 15°, we do still have minutes and seconds to subdivide them. However, we use the symbols ‘m’ and ‘s’ instead of ‘ and “.

We don’t have a Greenwich prime meridian in space, so we need a different baseline to take our measurements from. Instead, we use the line joining the north and south celestial poles that runs through the center of the sun at the moment it crosses the celestial equator from south to north.

This instant is better known as the vernal equinox and, in the northern hemisphere on Earth, this is the first day of spring.

This was initially known as the First Point of Aries since 0 hours of RA was present in Aries nearly 2,000 years ago. The Earth’s axis with respect to the universe has shifted slowly since then, and zero hours of RA has since crept into the stars of Pisces, where it now resides.

This star chart shows the north celestial pole (near top, center) and lines of right ascension originating from it. Two of these are highlighted green, those at 00h and 22h. Lines of declination circle the celestial sphere from top to bottom and two of them are highlighted orange, those at +00° (the celestial equator) and -30°. You can click this edited SkySafari 6 image for a full-screen version.

A Guide to Declination

For right ascension, we imagined that we were standing at the Earth’s poles. This time, place the imaginary version of you at the equator. Look straight up and you’ll be staring at the celestial equator.

This is a ring of space that goes all the way around our planet’s equator, and it’s also the zero-degree line of declination. Move ten degrees north from the equator and, if you look straight up again, you’ll be looking at +10° of declination.

Conversely, if you traveled 10 degrees south from Earth’s equator and looked toward the zenith, you’d be looking at part of the -10° declination line from the 10° south latitude line. I.e., declination is an equivalent measurement to latitude.

In other words, lines of declination run parallel to the celestial equator which, in turn, mirrors the position of Earth’s equator.

Any declination measurement south of the celestial equator is given a negative sign, those north of it have a positive sign. At each of the poles, the declination measurement is 90°, but it’s +90° at the north celestial pole and -90° at the south celestial pole.

Just like the lines of longitude on Earth, declination is measured in degrees, minutes and seconds, using the same symbols, i.e. °, ‘, “.

On the star chart above, you can see where we’ve picked out a couple of lines of declination in orange.

The diagram below brings all of this together, so you can see how declination and right ascension are used in combination to give a precise location for any star or deep sky object.

Celestial sphere around Earth with celestial equator and 0h right ascension lines shown.
Shows Celestial Equator (green) and 0h Right Ascension (red) (Source)

Interestingly, we spoke of the vernal equinox in the description of right ascension, yet it is declination that tells us when the solstices and equinoxes happen.

The moments when the sun is as far north or south of the celestial equator (0° declination) as it gets, are the summer and winter solstices, respectively.

Similarly, the spring and vernal equinoxes both happen when the sun crosses the celestial equator. And that’s to be expected, equinox = equal day and night, and that happens when the sun is on the celestial equator.

Bringing it All Together

Let’s skip back to the very beginning of this article where we looked at the coordinates of M42, the Orion Nebula: RA (2000.0) 05h 35m 24s and Dec (2000.0) -05° 27′ 00″.

We now know that this is telling us exactly where to find the nebula.

We need to look for the line that is five hours, 35 minutes and 24 seconds of declination. When we’ve found that, we travel south of the celestial equator for 5 degrees and 27 minutes exactly, and we’ll land on the stunning nebula.

In another example, Alpha Centauri, our closest stellar neighbor, is located in the southern celestial hemisphere and so has a negative declination of –60° 50′ 2″ (minus 60 degrees, 50 minutes and 2 seconds).

But hang on a moment… what’s this bit all about: (2000.0)?

That is the final piece of the puzzle to make sure we’re all talking about the same coordinates, and it has to do with a feature of Earth’s movement through space known as precession.

The Effect of Precession on Celestial Coordinates

While you needn’t worry about these celestial coordinates changing day-to-day, it is helpful to know that these numbers haven’t always been this way.

Earth wobbles slightly under the influence of the sun’s and moon’s gravitational forces. These wobbles are collectively known as precession, and the impact on the celestial sphere is the change in the position of the vernal equinox.

We noted above that it has moved over millennia, and it is still moving to this day at almost an arcminute (1/60th of a degree) west every year.

This would mean a slow shift in the celestial grid and its coordinates so, to avoid annual confusion, we only reset the coordinate system every fifty years. Current star maps are based on the J2000.0 epoch, i.e. the north and south celestial poles as they were at midday UT on 01 January 2000.

In real terms, especially for us backyard astronomers, the impact of this reset is negligible. When the 2000 epoch reset 1950 coordinates, the whole grid had moved less than 1° along the ecliptic.

The next grid reset will be to the position of the vernal equinox at midday UT on 01 January 2050.

The movement of the north celestial pole against background stars due to precession
Effect of precession on north celestial pole (orange line). Right now, at +2000, the NCP is very close to the star Polaris (source)

How to Use the Celestial Coordinate System for Astronomy

Now that you have learned the basics of celestial coordinates, it is time to look up and start spotting! Learning declination and right ascension is the key to locating any and all of the celestial objects on your observation list.

Wondering if you need to memorize the coordinates of your favorite objects? No, of course not!

Sky maps, also known as star charts, contain all the information that you need. They help us fit the immense vastness of the universe into a flat sheet.

We recommend Sky & Telescope’s Pocket Sky Atlas or a planisphere but software like Stellarium (free) and Sky Safari will provide what you need.

Using Declination and Right Ascension, An Example

Imagine you wanted to find the star Aldebaran. Its coordinates are listed as RA 04h 35m 54.32s, Dec +16° 30’ 26.1’.

It depends on the scope you have available as to what happens next.

If you have a goto model, enter the coordinates into your controller, and the motors will slew the scope point directly at the star.

If you don’t have that luxury but do have an equatorial mount, then you can still use the coordinates. You need the equatorial mount to be ‘polar aligned,’ i.e., that your scope’s pivoting axis is the same as Earth’s. once properly aligned, use the setting circles on the mount to find Aldebaran.

While you can’t use celestial coordinates with an altazimuth mount directly, you can use them with a star atlas or software to pinpoint precisely where you need to look for the star. Use your aligned finderscope to pinpoint the exact object.

Shows the right ascension and declination coordinates for Aldebaran
The J2000.0 epoch R.A. and Dec. shown for Aldebaran (source)

Celestial Coordinates Video Explainer

Here’s a video I created to show you more about how the celestial coordinate system works and how we use it for backyard astronomy.

Summary

Coordinate systems are designed to give a universal, precise, and unambiguous way of defining where to locate an object.

The celestial coordinate system assumes that all objects in the night sky are the same distance from Earth and ‘pinned’ to the inside of a great celestial sphere. Modeling that way means we only need to define how far up/down and left/right an object is from a given baseline to find it easily.

The up/down (north/south) direction on the celestial sphere is known as declination. It is measured in degrees from the celestial equator which runs parallel to Earth’s equator. North measurements run from 0° to 90°, while south run from 0° to -90°.

The left/right (east/west) measurement is called right ascension (R.A.) and is measured in hours from a zero-hour line which is perpendicular to the point where the sun crosses the celestial equator at the vernal equinox. There are no negative R.A. measurements; they only run from 0h 0m 0.0s to 23h 59m 59.99s.

Since the vernal equinox is constantly traveling west, we reset the ‘epoch’ of celestial coordinates every fifty years. The current epoch is J2000.0.

You can use these coordinates to find any object using star maps, polar aligned or goto telescopes.

There are not many people in this day and age who still love to look up at the night sky. By putting in the effort to understand celestial coordinates and star charts, you already belong to the rare class, and we appreciate your efforts!


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