In article , Brian Tung wrote:

canopus56 wrote:
Yeah, that's for right ascension, not declination. A minute in right
ascension is not the same size as a minute in declination. Confusing,
ain't it?

Yes, but the term "arcminute" usually implies that 1/60 of a degree is
meant. If what is meant is 1/60 of an hour of RA, then the usual term
is "minute of RA," not "arcminute." Also, since an hour of RA is 15
degrees, a minute of RA is 1/4 of a degree, not 1/15.

1/15 hour = 1 degree
1/15 min = 1 arcminute
1/15 sec = 1 arcsecond

That's the way it should be!

I usually call a minute of RA a "minute of time". After all, 60
minutes of RA becomes one hour of RA, and an hour is a time unit
really. If you wait one sidereal hour, then the RA of your local
central meridian will increase by exactly one hour. If you wait one
sidereal minute, then the RA of your local central meridian will
increase by exactly one minute (of time, or of RA) ... etc. So the
angular units of RA (hr, min, sec) is really angularized time unit,
with the sidereal rotation rate of the Earth as the conversion factor.

One arcminute and one arcsecond are always 1/60 resp 1/3600 of a
degree, not of an hour! That's what the "arc" means: we're measuring
angles, not time. And RA hours, minutes, seconds are really time
units, although in disguise as angular units.

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