Re: RA question

canopus56 wrote:

On Mar 3, 2:01 pm, Eric <N...@xxxxxxxxxxx> wrote:
If an object is at RA x:yy for example, how do i know where it is in the
sky? I cant figure out how to use RA to determine a point in my view of
the sky.

Your current local sidereal ("cy-dear-e-el") time (LST) can be found
at this handy USNO web applet:

http://tycho.usno.navy.mil/sidereal.html

LST means the right ascension that is currently transiting your south
meridian. You can also follow LST with a planisphere. Orient the
planisphere to the current time. The RA (that corresponds to the
current LST) can be read off the dial plate along the noon-midnight
line.

I found spending a couple of days with my plainisphere, following the
rotation of celestial dome throughout an entire day - including
daylight - helped me to wrap my head around the concept of LST. The
analogy mentioned in this thread about sidereal time as a watch that
runs 3 minutes and 56 seconds fast is a good one.

Why 3:56 minutes fast? If the Earth were just hanging motionless in
space rotating on its axis, the solar day would have a nice equal 24
hours periods. But the Earth also revolves around the Sun. Because
the Earth also is moving along a tangent to its solar orbit, the Sun
gets to back to local solar noon 3:56 minutes "faster". The 3:56
minutes represents the linear travel of the Earth in its orbit around
the Sun.

Enjoy - Canopus56
I think i have it now, I just looked up my LST (19:34 local time) and it was
6:18, so from that i gather that Rigel with a RA of 5:15 will pass through a
line drawn from south to north (the meridian) in about 10hrs and 44
minutes? 19:34 -> 6:18 == 10Hrs 44minutes (sounds too long to me by
recollection of rigels position here)
Thanks
Eric
.

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