An object’s crossing of the meridian, at the highest point of its path; the best time to observe it. For an object south of the zenith, its altitude then is 90° − latitude + declination. From London (51.5°), the Orion Nebula (−5.4°) culminates at about 33°.
ToolsWhat to see tonight?, Celestial fix
Related terms
- 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.
- 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.
- Declination
The celestial “latitude”: an object’s angle north (+) or south (−) of the celestial equator, from +90° to −90°. Vega is at +38.8°, Sirius at −16.7°. From a given latitude, declination tells you whether an object is always up, never up, or rises and sets.
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