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Hi All, Novice requires Dec R/A advice


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Wow, thanks guys I get the concept. Thinking of the earth inside a big celestial globe or bubble really helped me to visualise RA and Dec.

Next question :D

OK, given that RA and Dec are fixed during one night, If a given object say Sirius is at RA: x and Dec: y, Do I need to do some kind of calculation depending on time of day to find it in the sky (To compensate for the rotation of the earth?).

I must sound like such an airhead. Can you believe I used to be able to do differential calculus!

Peace and Clear Skies.

Becky.

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Your OK with the Dec. bit, as that doesn't change, but each time you start observing you can either do a shed load of hard sums, or point your 'scope at something and set the RA setting circle to where that star is kept. This is then your reference point and everything else will be calibrated from your setup. As everything is relative to each other, one star is all you need to set it up.

Kaptain Klevtsov

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Do I need to do some kind of calculation depending on time of day to find it in the sky (To compensate for the rotation of the earth?).

Yes! Imagine ever bigger rings around Polaris. That will correspond to different Dec coordinates. +90 is Polaris, +80 is 10 degrees away from Polaris, etc.

With the RA, it's trickier. I just looked up that

Delta Orionis has an RA of05h 32m
Epsilon Orionis has an RA of05h 36m

Delta is the right-most (western) of the Orion's belt stars and Epsilon is the middle one. They are four minutes apart, so I have to wait 4 minutes for the trailing one to cross the meridian after the first one has. I know that Orion will be up in the southern sky this evening so I know that I will see things with roughly that RA and however many hours later I am willing to wait! Something with RA 11h 32m, say, will cross the southern meridian 6 hours later. Of course, some objects with very high declination (close to Polaris) never set or rise at all: they are circumpolar. What is circumpolar in your location may not be for someone a few hundred miles south of you!

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