The hour (symbolized hr) is a unit of time , equivalent to 3600 second s (s), 60 minute s (min), or 1/24 of a mean solar day (msd). The hour is also used by astronomers as one of two coordinates that define the positions of objects in the sky.
Informally, time is measured according to a scheme that uses two 12-hr segments per mean solar day. The first segment, a.m., begins at midnight and ends at noon. The second segment, p.m., begins at noon and ends at midnight. Time in this format is written in the form h:mm or hh:mm, where h or hh represents the hour from 1 to 12, and mm represents the minute from 00 to 59.
Astronomers, most engineers, aviators, navigators, and military people use a 24-hour time scheme that begins and ends at midnight. In this format, time is denoted in the form hhmm, where hh represents the hour from 00 to 23, and mm represents the minute from 00 to 59.
In celestial coordinates, the hour (hr) is the standard unit of right ascension (RA). Right ascension is measured from west to east along the celestial equator, an imaginary line in the heavens that lies directly over Earth's geographic equator. There are 24 hr of right ascension in a complete circle; therefore 1 hr is the equivalent of 15 angular degree s (15° or 15 deg). The point RA = 0 is the vernal equinox, the position of the Sun in the heavens as it crosses the celestial equator going from south to north. This occurs annually on March 20 or 21.