Quote:
Originally Posted by morls
Thanks Malcolm.
So am I right to think in RA we have hours, minutes and seconds, up to 23 hours, 59 minutes, 59 Seconds, and these do relate to time? Is Greenwich 0hr 0' 0"?
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Not quite
RA (or more correctly AR - Ascension Recta - see here:
http://books.google.com.au/books?id=...0recta&f=false) is a (sideral) time, elapsed between culmination moment of Ascending node (intersection point of Ecliptic and Celestial equator) and the culmination moment of the object in question.
Or, if we divide the celestial equatorial circle by 24, and each section we call "hour", then if we divide each "hour" by 60 "minutes" and so on, then AR is distance between celestial meridians of Ascending node and the meridian of a said object.
Quote:
Originally Posted by morls
In DEC, is it between +90º (N) and -90º (S)? And each degree is also divided into arc minutes and arc seconds, but in this case they have nothing to do with time, but are just subdivisions of a degree of angle?
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Quite right
!
That is why we call those units "arc minutes" and arc seconds", sometimes even omitting word "arc"