Understand star co-ordinates and star charts

Navigate your way around the night sky, understanding how starcharts and co-ordinates work


When those early pioneers of science, the ancient Greeks, looked up at the night sky, they described it simply as it was seen. They imagined that the Earth was fixed at the centre of things, with the Solar System orbiting around us.

Beyond was the night sky, which they imagined as a sphere with the stars projected on the inside. This became known as the celestial sphere. It was the starting point for mapping the stars. 

Even though we now know much, much more about our planet’s place in space today, we’ve held onto the celestial sphere because it’s such a useful way of thinking about the sky. Indeed, it even makes it possible to put all that starry-sky vastness onto a flat sheet of paper and make a starchart, with which you can find your way around the sky.

And just as latitude and longitude are used to find locations on the globe of Earth, the same idea works up there with the stars. The celestial sphere also has a grid system, which is simply a projection of latitude and longitude up onto this imaginary sphere.

The only difference with this celestial grid is that it has different names. Latitude – the lines that run around the Earth parallel to the Equator and give the north-south location – is called declination, while longitude – the lines that run up and down through the north and south poles and give the east-west location – is called right ascension.

Getting your bearings
Declination, often shortened to dec., is quite simple to get your head around because it uses the same units as latitude, degrees (°) and minutes (’), along with the smaller arcsecond (”). The ‘arc’ here is used to make it clear that this is a measurement of distance and not of time.

There are 60 minutes in one degree and, not surprisingly, 60 arcseconds in one (arc)minute. Degrees of declination start from the zero marker on the starry version of our equator, known as the celestial equator.

If you move into the northern sky hemisphere you’ll find a ‘+’ symbol is used before the figures, whereas you’d put a ‘–’ symbol first to show that the degrees and minutes are located in the southern hemisphere.

Right ascension, or RA for short, is slightly more complicated because it uses hours (h), minutes (m) and seconds (s) as units. One hour of RA is the same as 15º of longitude – that’s movement left or right. Time is used as the unit here instead of angles because it represents the Earth’s rotation: in one hour the sky turns 15°. You’ll see this all adds up perfectly for one day and night, or 24 hours worth of rotation: we have 24 hours x 15 degrees, which equals 360° and one complete rotation of the Earth.

Here’s how we’d write the position of the brightest star in The Plough, Dubhe: RA 11h 04m 15s, Dec. +61° 42’ 03”. You’ll notice that RA comes first (it always does) and that there’s a ‘+’ sign in front of the Dec. number, showing that the position of the star is in the northern hemisphere. With the position of a star written like this, you’ll be able to find it on a chart (see photo below).

Star magnitudes
Of course, when you find Dubhe on your chart you won’t see it shining brightly. Instead, starcharts show the brightness of stars (their magnitude) by having bigger dots for brighter stars.
We know that all stars are the same-sized single points of light in the real night sky, but
it’s impossible to show their brightness any other way on a printed page.

Also, don’t forget that starcharts will show north as up and east to the left, instead of to the right. This is different from normal maps because the starchart is showing you the sky from below, while a normal map shows you the land from above. To align the starchart and begin using it, hold it up over your head and you’ll see that the directions fall into place.

From alpha to omega
As we’ve seen, the beginnings of astronomy in the west were in evidence during ancient Greek times, from the 5th century BC. In homage to these early Greek astronomers, German astronomer Johann Bayer published a starchart called Uranometria in 1603, in which he labelled the brightest stars of a constellation with Greek letters for the first time.

This stuck, and it has become the standard way of referring to stars in constellations. Usually, but not always, the brightest star in each constellation was designated as Alpha, the next-brightest as Beta, then Gamma, all the way to Omega. The Greek symbols for these letters can be seen below. 

Ursa Major is a good example: the main star, Dubhe, is not the brightest, but Bayer labelled it Alpha anyway, so it became known as Alpha (α) Ursae Majoris. You’ll notice the last two words are spelt differently. This is because they are the Latin possessive for the name – they mean ‘belonging to Ursa Major’.

All constellations have a Latin possessive, with some sounding grand indeed, such as Geminorum, which means ‘belonging to Gemini’.

This article appeared the October 2008 issue of Sky at Night Magazine

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