The local sky appears to an observer as a vast dome arching overhead, serving as the immediate frame of reference for celestial mapping. To effectively describe where an object is, astronomers created a system that uses the viewer’s immediate surroundings as the measuring tape. This framework allows any observer to pinpoint an object’s location at a specific moment in time using angular measurements.
Establishing the Reference Points
An observer must first define several fixed reference points relative to their position. The most fundamental reference is the horizon, the boundary where the sky and the Earth’s surface appear to meet. This circular line divides the celestial sphere into the visible upper hemisphere and the unseen lower hemisphere.
Another point of reference is the zenith, the single point on the celestial sphere located directly overhead the observer. The zenith is the highest point in the local sky and is always 90 degrees above the horizon. Conversely, the point directly beneath the observer is called the nadir.
The third reference feature is the meridian, an imaginary line that runs across the local sky from north to south. This line passes through the north point on the horizon, the zenith, and the south point. The meridian marks the moment when a celestial object reaches its maximum altitude, or highest point in the sky, for that day.
The Horizontal Coordinate System
The reference points defined by the observer form the basis of the Horizontal Coordinate System, also called the Altitude-Azimuth (Alt-Az) system. This system uses two coordinates to specify an object’s position on the celestial dome.
Altitude
Altitude specifies the object’s vertical position above the horizon. It is an angular distance measured upward from the horizon plane toward the zenith. The horizon is 0 degrees altitude, and the zenith is 90 degrees altitude. An object halfway up the sky would have an altitude of 45 degrees.
Azimuth
Azimuth specifies the object’s horizontal direction along the horizon. It is measured as the angle clockwise from true North, which is defined as 0 degrees. The angle increases as one moves eastward around the horizon.
Continuing clockwise, East is 90 degrees, South is 180 degrees, and West is 270 degrees. For example, a star at 45 degrees Altitude and 135 degrees Azimuth would be found halfway up the sky in the southeast direction. The Alt-Az system is intuitive because it directly relates to the observer’s immediate physical surroundings.
Understanding Coordinate Changes
While the Horizontal Coordinate System is simple and direct, it has a significant limitation: the coordinates of any celestial object are constantly changing. The system is fixed to the observer’s local horizon, not to the distant stars. As the Earth rotates, the apparent position of every object in the sky shifts relative to the horizon.
For example, a star at an altitude of 30 degrees and an azimuth of 90 degrees (due East) will move across the sky and eventually set in the west. Its Alt-Az coordinates change continuously over the course of the night. This rotation means that the coordinates are valid only for a specific place and a precise moment in time.
If two people in different cities observe the same star simultaneously, they will measure different altitude and azimuth values. This dependence on the observer’s location and the time of observation makes the Horizontal Coordinate System unsuitable for long-term astronomical catalogs or for communicating positions between distant observers. The system effectively describes the sky as it appears now and here.