The Horizon
Jason Harris
The Horizon is the line that separates Earth from Sky. More precisely, it is the line that divides all of the directions you can possibly look into two categories: those which intersect the Earth, and those which do not. At many locations, the Horizon is obscured by trees, buildings, mountains, etc. However, if you are on a ship at sea, the Horizon is strikingly apparent.
The horizon is the Fundamental Plane of the Horizontal Coordinate System. In other words, it is the locus of points which have an Altitude of zero degrees.
Hour Angle
Jason Harris
As explained in the Sidereal Time article, the Right Ascension of an object indicates the Sidereal Time at which it will transit across your Local Meridian. An object’s Hour Angle is defined as the difference between the current Local Sidereal Time and the Right Ascension of the object:
HAobj = LST - RAobj
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.
The Local Meridian
Jason Harris
The Local Meridian is an imaginary Great Circle on the Celestial Sphere that is perpendicular to the local Horizon. It passes through the North point on the Horizon, through the Celestial Pole, up to the Zenith, and through the South point on the Horizon.
Because it is fixed to the local Horizon, stars will appear to drift past the Local Meridian as the Earth spins. You can use an object’s Right Ascension and the Local Sidereal Time to determine when it will cross your Local Meridian (see Hour Angle).
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