Re: R.A. & Dec.
- From: brian@xxxxxxx (Brian Tung)
- Date: Sat, 6 Jan 2007 12:01:17 -0800 (PST)
Charlie Johnson wrote:
Hi all,
I am new to using right ascension and declination. My understanding
of how to use it is: face south, why [not] north I don't know,
subtract your latitude form 90 degrees (90-45 = 45 degrees) for me,
that is my celestial equator and my 0 degrees dec. point.
It's not north because you are in the northern hemisphere.
The celestial equator is simply the Earth's equator extended out from
the center of the Earth onto the sky. Because you are in the northern
hemisphere, the Earth's equator is south of you, so the celestial
equator is likewise south of you.
If you're at latitude L degrees, then the Earth's equator is L degrees
south of you. The corresponding thing in the sky is that the celestial
equator is L degrees south of your zenith, the spot in the sky which is
in a straight line with you and the center of the Earth. Since the
zenith is 90 degrees from the horizon, L degrees south of the zenith
means 90-L degrees up from the southern horizon.
Now, for r.a., how do I determine what r.a. is due south of me at the
celestial equator at say for example 10:00 P.M. C.S.T.? Then, wait
for a star, or other object, of a certain r.a. to pass in front of me.
Does this sound correct? If that sounds vaguely correct, then all I
need to know is how do I find my local r.a. time?
Yes, that's all right.
A rule of thumb is to take the month number plus three, double it, and
subtract 1 if you're in the first half of the month, then add your 24
hour time. Subtract 24 as needed to get the result under 24.
These days, the month number is 1, so we add 3 and double to get 8.
We're in the first half of the month, so we subtract 1 to get 7. At 10
p.m., the 24-hour clock time is 22 (10+12), and we add that to 7 to get
29. Then subtract 24 to get the answer: 5. That means that RA = 5
should be along your meridian at about 10 p.m. The answer should be
right to within about half an hour of RA.
Another option is to buy a planisphere. Highly recommended.
--
Brian Tung <brian@xxxxxxx>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
.
- References:
- R.A. & Dec.
- From: Charlie Johnson
- R.A. & Dec.
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