The point in the sky directly overhead, at 90° altitude. That is where you look through the least air, so images are at their sharpest and brightest. Its opposite, below your feet, is the nadir.
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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.
- Meridian
The imaginary line from north to south through the zenith. When an object crosses it, it reaches its highest point: its culmination, the best time to observe it. The Sun crosses it at solar noon, around 1 pm in winter and 2 pm in summer in France, close to noon or 1 pm in the UK.
- 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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