Hour Angle:
The angle between an observer's meridian and the hour circle on which some celestial body lies. This angle is conventionally expressed in units of time (hours, minutes, and seconds), which gives the time elapsed since the celestial body's last transit at the observer's meridian (for a positive hour angle), or the time unit the next transit (for a negative hour angle).
Source:
http://scienceworld.wolfram.com/astr...HourAngle.html
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