Absolute Beginners No. 5: Right ascension and declination
2016 March 22
As you’ve probably realised, there are a fantastic number of objects out there beyond the atmosphere of Earth. The bright stars of the night sky are easy to find, and from a human perspective they appear to reside permanently in constellations which remain unchanged century after century. Other objects like planets or comets for example, move rapidly through the sky, and keeping track of them is important if we wish to establish their orbits properly and observe them frequently.
Similarly, if a new asteroid or nova is spotted, we are going to need a precise coordinate system to pinpoint its location so we can alert other astronomers who will want find it quickly – a description ‘somewhere in Leo’ isn’t really going to help! What we need is a universal coordinate system that we can all use and understand. Astronomers have devised such a system which is based on two quantities called Right Ascension (RA) and Declination (Dec). This celestial coordinate system of the sky is easy to understand and is rather similar to latitude and longitude used for locating landmarks here on Earth.
If you have a telescope equipped with setting circles, you will see that the RA circle is similar to a clock in that it runs from 1 hour all the way through to 24 hours (although this is usually denoted 0h on most RA circles). It is often further subdivided into 15 minute intervals. The declination circle runs from 0 degrees to 90 degrees both to the left of the 0 degree mark and to the right of it.
Everything in the sky, from the Sun to the faintest galaxy is assigned an RA and Dec coordinate, and once you understand how this system works, you will be able to use the setting circles on your telescope to track down objects. Instead of star hopping to find that elusive galaxy, you will be able to dial up its coordinates and find it in no time at all!
The celestial sphere
In order to understand how RA and Dec actually work, we shall need to refer to a model which, unfortunately, has no physical bearing in reality, but is nonetheless a very useful tool. We shall imagine that the entire sky is a vast globe surrounding the Earth. All the objects in the sky (including the Sun and the Moon) sit on this globe, which rotates around the Earth once every 24 hours. We call this imaginary globe the celestial sphere and we can divide it into two parts: the north celestial sphere above the celestial equator and the southern celestial sphere below it (see Figure 1).
From this point of view, the Sun appears to move through the constellations of the zodiac over the course of a year, spending on average about one month in each constellation. So in July (from the UK perspective) the Sun is located high in the sky in the constellation of Gemini. Conversely in December it resides in the constellation of Scorpius and is very low down.
We can plot the path of the Sun as it passes through the constellations on the celestial sphere. We call this path the ecliptic, and if we plot the motions of the Moon and planets as well, we find that they never stray too far from the ecliptic.
Of the two coordinates, declination is the easier to understand, and is equivalent to latitude on the Earth. Declination is measured in degrees (°) minutes (‘) and seconds (“). There are 60 seconds to a minute, and 60 minutes to one degree. The celestial equator is located at 0°, and everything above this is said to be in the north of the celestial sphere and is given a positive value of declination. Conversely, everything south of the equator is given a negative value.
For example, the brilliant star Vega in Lyra has a declination of +38° 47′ 1.3″ and is located well north of the celestial equator (it is almost overhead in the UK at summertime). However, the bright star Canopus has a declination of –52° 41′ 44.4″ and is far south of the celestial equator, so it cannot be seen at all from the UK.
The north celestial pole is located at +90°, and the corresponding southern celestial pole is at declination -90°. The north pole star ‘Polaris’ has a declination of +89° 15′ 50.8″ so it is not quite located at the north celestial pole.
With the ‘up and down’ direction covered by the declination coordinate, we now need something which takes account of the ‘left to right’ direction, and this is fulfilled by right ascension. For historical reasons we choose to measure right ascension in the same units as time, namely hours, minutes and seconds, so as you would expect, there are 60 seconds to a minute, and 60 minutes to one hour of RA.
I have given an illustration of RA in Figure 3. As you can see the RA axis runs along the celestial equator. We start at 0 hours, and moving right (eastwards when facing north) the value of RA increases to 1 hour, then 2 hours and so on. After we have moved along by 12 hours, we have covered half of the celestial equator and if we continue moving east for another 12 hours, we arrive back at 0 hours. A question now arises: where should we put 0 hours on the sphere? In the case of declination it is clear that putting the equator at 0 degrees makes a lot of sense, but where on the celestial equator would be a sensible place to put 0 hours?
In Figure 1, we can see that the ecliptic meets the celestial equator in two places – the spring equinox and the autumn equinox. Astronomers put 0h of right ascension at the spring equinox. When this was first defined, the point of intersection between the ecliptic and the celestial equator was found to lie on the western edge of the constellation of Aries and so it was called the First Point of Aries.
We can now assign a value of RA and Dec to any fixed object in the sky, and its position will be uniquely established on the celestial sphere. If, for example, you look up the RA for Vega, you will find it listed as 18h 36m 56.3s. Its declination is +38° 47′ 1.3″. (The coordinates for planets, comets and other solar system objects change as they move in their orbits, so to find one of these objects with your setting circles you need to know its RA and Dec for the time you will be observing it.)
By choosing to measure RA in hours, we have incorporated into our coordinate system a useful way to time the position of any object in the continually rotating sky. If you look up the RA of Betelgeuse, you will find it listed as 05h 55m 10.3s. Now, if Betelgeuse is on your central meridian at 20:00UT tonight (and therefore at its highest point in the sky from your location), this means that Vega with RA of 18h 36m 56.3s will be on your central meridian 12 hours 41 minutes and 46 seconds later.
There is an additional effect we have to take into account for RA, and this is the phenomenon of precession. At the moment, the north pole star is ‘Polaris’ in Ursa Minor. However, 5000 years ago the pole star for our ancestors in the Bronze Age would have been the star Thuban in Draco. The gravitational pull of Sun and Moon causes the Earth to wobble on its axis – this phenomenon is called precession, as a result of which the direction of the north celestial pole changes slowly over time; consequently so does the constellation in which the spring equinox lies. This is why the first point of Aries is no longer in Aries, but is instead now to be found in the constellation of Pisces!
Keeping track of this is simple; every 50 years astronomers simply update the values of the RA of all celestial objects to take account of this gradual drift. For this reason, you will see RA values quoted with an epoch (a reference date) like ‘J1950’ or ‘J2000’. The ‘J1950’ values mean an RA value from the year 1950, and so today we use RA values from the J2000 epoch (values determined in the year 2000). In 2050 the system will be updated once more. Some advanced programs like WINJUPOS and Stellarium that can need very precise timing quote RA and Dec in terms of both J2000 or the date and time you enter into the system, but in practical terms, there is little difference between them at the moment.
Using RA and Dec
At the start of this article, I said that RA and Dec would save you from a tedious amount of ‘star hopping’ – so this is how you use them. Go out on a clear night, and set up your telescope. Unless this is of the ‘goto’ variety, you will need to polar align it in order to use your setting circles (for a ‘goto’ telescope, setting circles are not necessary, and you should refer to the telescope manual for information on how to find objects). First, point your telescope at Polaris, and when it is in the centre of your field of view, set the declination circle to 90°. Unless you change your location, there should be no need to do this again.
Next move the telescope tube to a star you can easily recognise – we will use Betelgeuse as an example. As we have seen, the RA of Betelgeuse is 05h 55m (this is accurate enough for the setting circles), so keeping your telescope still, turn the RA circle so that it reads 05 hours 55 minutes.
You can now use your setting circles to find objects in the night sky: simply manoeuvre the telescope (without touching the circles!) so that the RA and Dec dials read the values of your desired object. So, if you now wish to view the galaxy M81, turn your telescope until your RA dial is at 09 hours 55 minutes, and the Dec dial reads 69° (again, this is accurate enough for your circles). You should find the galaxy in your eyepiece field of view, or at the very least in that of your finder.
If your telescope is not clock driven then you will have to set the RA dial using a bright star each time you want to move to a new object. If it is driven, then your telescope is keeping sidereal time and rotating at the same rate as the Earth. This means the RA dial will be turning at the same rate and is now set until you switch off your drive.
Armed with the knowledge of how RA and Dec works, you never need to star hop again; simply dial up the coordinates of the target that you want to view!
Paul G. Abel
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