Declination and Right Ascension

We saw in Unit 1 how we use latitude and longitude to locate a point on the surface of the Earth. How do we locate a point in the sky, e.g. a star. The problem is that as the Earth rotates and orbits around the Sun the position of the stars relative to the Earth is constantly changing. They do not change, however, on the Celestial Sphere.

Imagine that the stars are fixed to a huge hollow sphere, much larger than the Earth. Imagine also that this sphere rotates around the earth, its axis of rotation being a line through the celestial poles, points directly above the Earth's poles. The celestial equator is a circle on this sphere directly above the Earth's equator.

If you were at the North pole the celestial sphere would appear to rotate directly above your head. The pole star would be at your zenith, the point directly above your head. At a latitude of 55N (e.g. Middlesbrough) the pole star is 55⁰ above the horizon, i.e. at your latitude. All the stars would still appear to rotate about the pole star.

Declination

The declination of a star is its angle above the celestial equator. (remember the latitude of a point on Earth is its angle above the Earth's equator)

The pole star therefore has a declination of 90 degrees. The declination of any other star is therefore 90⁰ - the angle it makes with the pole star. The grey circles above are 10⁰ apart. A star 25⁰ from the pole star would have a declination of 90 - 25 = 65⁰. Basically the declination of a star is equivalent to its latitude on the celestial sphere if the pole star has a latitude of 90⁰ and the celestial equator zero.

Notice that some of the stars on the animation above are visible all the time. They are said to be circumpolar. At a latitude of 55⁰ stars with a declination equal to or greater than 90 - 55 = 35 are visible all the time. Stars with a declination less than this set in the west and rise in the east.

Right Ascension

When we defined longitude we had to say where our zero of longitude was, then all other longitudes would be measured as an angle relative to this. Greenwich was chosen as the zero meridian. We now need to choose a celestial Greenwich so that we define the equivalent reference meridian on the celestial sphere.

There is a point on the celestial equator called the First Point of Aries. (it is actually in Pisces now but that's not important) It is one on the two points where the ecliptic crosses the celestial equator. Just think of it now as a point on the celestial equator.

The point where a star is highest in the sky is called its culmination. Imagine the point in time when the first point of Aries, the white point above, culminates and imagine a line joining this point, the zenith and the pole star. If we were to imagine a line between any star and the pole star then this would make a certain angle with the first line we imagined. The right ascension of a star is this angle. It is not measured in degrees however but in hours and minutes which is actually more convenient. The right ascension of a star is the time it takes to culminate after the culmination of the First Point of Aries. The star on the diagram above has an RA of about 3 hours (as the angle is about 45⁰).

Lets say you want to find an object in the sky that has a RA of 4h 26mins. The first point in Aries isn't particularly easy to find. This isn't a big problem, just locate a star that you recognise and know its RA, e.g. Betelgeuse in Orion has an RA of 5h 55mins. This means that the star you want to find culminates 1h 29mins before Betelgeuse. It will be about 22 degrees anticlockwise of Betelgeuse (as each hour of RA is 15 degrees). So imagine a line between the pole star and Betelgeuse. Now imagine another line that goes through the pole star but at an angle of 22 degrees anticlockwise to the first. Your star will be on that line somewhere.

So we now have a coordinate system to define the position of the stars which is independent of ones location on Earth and its rotation. To get used to using it you should practise with a star chart.