![]() This means that the angles provided by the horizontal coordinate system apply only to a specific location at a specific time. ![]() What's more, most celestial objects move across the sky, so their coordinates change as time goes by, even if you stay put. As the horizon's limits – and, therefore, the portion of the sky you see – depends on your location, an object's altitude and azimuth angles shift as you move to a different spot on the Earth's surface. The horizontal coordinate system owes its name to the fact that it is based upon the observer's horizon. However, some astronomical conventions still follow the traditional method of using true south as a starting point for azimuth angles. Note: Since true north is the reference direction most commonly used, all azimuths displayed on refer to true north. Illustration 3: The azimuth refers to the object's cardinal direction.Įxample: If Venus is at an altitude of 45°, with an azimuth of 270°, as seen from your location, this means that you will find the planet in a western direction at an elevation exactly half way between the horizon and the zenith. This means, for example, that an azimuth of 180° means due south. If true north is used as reference, it is represented by an azimuth of 0°, and angle values increase towards the east. The azimuth is the angle between the spot where that line crosses the horizon and the reference direction. Imagine a vertical line connecting the object with the horizon. It is specified as the horizontal angle the object makes with a reference direction, such as true north (see illustration 3).
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