Equatorial Co-ordinates

[flapjackboy]

Im confused about the equatorial co-ordinate system...I've looked around but can only find bits and pieces, so Im hoping someone can help. My understanding of this system is that there are 2 axis (right ascension/decliation), with declination depending on how high/low something is from the celestial equator. The right ascension is in a time format, with 1 hour equaling 15 degrees. I hope thats right....im not sure how to find an object in the sky using this format though...im thinking by relation to polaris but im not sure. Help would be greatly appreciated. Thanks.

[Kaptain Klevtsov]

You need to think of being inside a big ball with all the stars drawn on and lines just like on a terrestrial globe like you had at school. The north pole equivalent is (very close to) Polaris, so that's the starting point. Dec. is measured as the angle from Polaris down to 90 degrees where you get to the celestial equator. Then keep going until you get to the south celestial pole, which you can never see from the UK.

Then you get the lines going the other way. These are calibrated in hours as all the stuff up there floats past as the earth rotates. As an hour passes, everything turns one hour in RA, which makes it convenient to measure stuff. You line up two sticks on one thing, start your timer and when the second thing comes into line you can measure the distance in hours of RA. The zero line is, like the Greenwich zero line, purely arbitrary.

HTH

Captain Chaos

[The Warthog]

If the Earth were transparent, and you were standing in the centre, you could see London by pointing a telescope at London's latitude and longitude. If you then wanted to see Toronto (God knows why!) you could adjust the latitude and longitude of the telescope mount, and see Toronto.

The sky is measured the same way, but we call 'latitude,' 'Declination', and 'longitude,' 'Right Ascension.' You are at the centre of this coordinate system.

When you want to find something that you cannot directly see, if you polar-align your telescope you can go to a star near the target, whose RA and Dec you know, from looking at the star chart. Check that the declination on the dial is correct, and turn the RA dial so it shows the correct value for the star. Then move the scope to the RA and Declination of your target. This method works well, particularly if you don't have to move your scope too far.

If the declination is a little off, take this into account when moving to the target.

[flapjackboy]

Thanks Both your explanations have helped hugely - imagining im in a ball/center of the Earth helps visualise it a lot. Ive written down some co-ordinates for M42, M43 and 109P for tonight and will give it a try.

[Chubster]

Pretty much what both CC and Warthog have said....but something else to read when its cloudy

http://www.astronomy.org/astronomy-survival/coord.html

Chub