The circle where the sky seems to meet the ground, at 0° altitude. The astronomical horizon is a perfect plane; the real one is hidden by trees, hills and buildings. Near the horizon, thick air and light pollution degrade the view.
ToolsCelestial fix, What to see tonight?
Related terms
- Altitude
The angle between an object and the horizon, from 0° (on the horizon) to 90° (at the zenith); also called elevation. Objects are usually well placed above 30°. With azimuth it locates an object in your local sky.
- Atmospheric refraction
The atmosphere bends starlight and makes objects look higher than they are: about 0.5° on the horizon, barely 1′ at 45° altitude. When the Sun seems to touch the horizon it has in fact already set. Celestial navigation corrects for it.
- Airmass
The thickness of atmosphere the light passes through, as a multiple of the amount at the zenith (airmass 1). It is 2 at 30° altitude and nearly 6 at 10°: the image there is dimmer, redder and more turbulent. Hence the value of observing objects near culmination.
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