One element of the astronomical coordinate system on the sky, which can be though of as longitude on the earth projected onto the sky. Right ascension is usually denoted by the lower-case Greek letter alpha and is measured eastward in hours, minutes, and seconds of time from the vernal equinox. There are 24 hours of right ascension, though the 24-hour line is always taken as 0 hours. More rarely, one sometimes sees right ascension in degrees, in which case there are 360 degrees of right ascension to make a complete circuit of the sky. When specifying a comet's location on the sky, one must state the right ascension and declination (with equinox), along with date and time (since a comet moves with respect to the background stars). For examples of how right ascension (R.A.) and declination (Decl.) appear on a star atlas, see the Millennium Atlas (also here and here). Note that, at the celestial equator, there are 15 arc seconds in one second of R.A. (often stated as "one second of time"); as one moves away from the celestial equator, one must multiply this factor of 15 by an additional factor (cosine of the declination), because the lines of right ascension get closer and closer as one nears the celestial poles, to get straight-line distances between two celestial objects that are close to each other (for long distances across the celestial sphere, a more complex formula is used). Thus, when the R.A. is given in h, m, and s, it is usually given in seconds of time to one more significant digit than is the Decl. in arcsec (i.e., if R.A. is given to 0s.01, the Decl. should be only given to 0".1, though this significant-figures requirement disappears as one approaches the celestial poles).