In astronomy, an object's hour angle (HA) is defined as the difference between the current local sidereal time (LST) and the right ascension (α) of the object: Astrology: the study of the positions of the celestial objects relative to the Earth and how these positions affect happenings on the lives of cultures, nations and the natural environment. ... Sidereal time is time measured by the apparent diurnal motion of the vernal equinox, which is very close to, but not identical with, the motion of stars. ... Right ascension (RA; symbol α: Greek letter alpha; celestial longitude) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. ...

HA_object = LST - α_object

Thus, the object's hour angle indicates how much sidereal time has passed since the object was on the local meridian. It is also the angular distance between the object and the meridian, measured in hours (1 hour = 15 degrees). For example, if an object has an hour angle of 2.5 hours, it transited across the local meridian 2.5 hours ago, and is currently 37.5 degrees west of the meridian. Negative hour angles indicate the time until the next transit across the local meridian. Of course, an hour angle of zero means the object is currently on the local meridian. Meridian is: Meridian (astronomy): an imaginary circle perpendicular to the horizon. ... Meridian is: Meridian (astronomy): an imaginary circle perpendicular to the horizon. ...

This article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml Screenshot of KStars showing the night sky from Hanover. ... Tux, a cartoon penguin frequently featured sitting, is the official Linux mascot. ... KDE (K Desktop Environment) is a free desktop environment and development platform built with Trolltechs Qt toolkit. ...