h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I started an astronomy evening class last night and the lecturer gave an "explanation" of right ascension and declination which, frankly was not brilliant. I would have pushed the matter but he clearly didn't understand it too well himself...or didn't communicate very well.

I understand how the position of an object is descibed using this method once the reference points have been determined; that is simple enough but can anyone tell me how the reference points (the solar equator(?) and zero point of right ascension) are determined?

Help greatly apppreciated!

This is a good question. Both are defined by the movement of the Earth. You have a rotation axis which defines the equator, and the moving sun that crosses the equator at the spring point which is the zero point of rectascenson.

However both is constantly changing albeit very slowly. So astronomers use extrapolated standard epochs that refer to the position of equator and ecliptic at a specific date (e.g. 1.1.2000).

On the other hand there cannot be a really neutral coordinate system in this universe, so everything can only be an estimate. But if it's sufficiently accurate, it's fine.

Hi Stephen,

One way to think of all of this is to imagine the Earth rotating around inside another sphere, far out in space, which does not move.

The big sphere and small sphere (Earth) share two common points - the North Pole and the South Pole. If you imagine a thin wire being drawn from the north pole of the big sphere to the south pole of the big sphere, positioning the Earth at the mid-point, and getting the Earth to rotate around this wire just as it rotates normally, you might see the relationship better.

So you can fix the North pole on this big sphere, the celestial sphere. It is the +90 degree point. You can also fix the South Pole on the -90 degree point. Midway between both is a narrow ring that is
equidistant from both points - the Celestial equator. This ring is right overhead all points on the Earth's equator, and all points on this equator are at the 0 degree point.

Declination, then, is simply a measure of the position of a star above or below the celestial equator. If a star has a +45 degree declination, then it is half-way between the North Pole and the Equator. If it is +80 degrees it is close to the North Pole, and if it is +10 degrees, it is just north of the celestial equator. If it is a minus amount, it is somewhere closer to the South Pole.

With Right Ascension, the problem is a bit more difficult, because there is no fixed point on the celestial sphere that works well for this. Its a bit like the longitude problem - why should the 0 degree point go through Greenwich, London? No reason, really other than it was agreed by an international convention. As I understand it, a similar convention set a similar arbitrary point - it is known as the First Point of Aries, and represents a point on the celestial equator. It isn't even a very interesting point in the sky!

Right ascention goes from 180 degrees east to 180 degrees west of the First Point of Aries, with the First Point of Aries (and all points north and south of it) at 0 degrees.

The whole thing is very much like longtitude and latitude on Earth, except that the Earth rotates, and the celestial sphere does not.

I hope this helps!

Woodpigeon

http://liftoff.msfc.nasa.gov/academy/universe/radec.html

Thanks very much for this folks, things are now much clearer!

I thought that Right Ascension was measured in hours and minutes rather than degrees but that is a small quibble..I can handle either!

I can't quite get my head around the business of the sun crossing the equator at the Spring Point (21 March?) but that's because I don't yet have a clear picture of exactly how the Sun and Earth move relative to one another...it will come.

Thanks for taking the trouble to explain!

Stephen

Hi Steven,

Yes, you are right, RA is measured in hours and minutes. This has a very close corellation between degrees though. 12hrs is 180 degrees from Aries (the other side of the sky), 6 hours is 90 degrees, and 3 hours is 45 degrees. Each "hour" is equal to 15 degrees, just the same as time zones on earth. Minutes and seconds etc are just subdivisions of hours.

As for the sun crossing the equator, an experiment that you can try might help to clarify it for you. Get a football and an apple. The football represents the sun, and the apple the Earth. Now incline the apple so that it is at an angle to the vertical and then move the apple in a circular "orbit" around the football. Keep the apple inclined in the same position as you orbit it around the football.

You will see that at certain times, the "south pole" of the apple will face the "sun", and at other times the "north pole" of the apple will face the sun. At the half-way points in the orbit (the equinoxes), both the north pole and the south pole will be at the same distance from the sun. You will then notice that when the south pole is facing the sun, that the closest point between the earth and the sun is below the Earth's equator; and that when the north pole is facing the sun the closest point between both objects is above the Earth's equator. And guess what, at the equinox points the closest point is a point on the equator itself.

Woodpigeon