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.
ToolsCelestial fix
Related terms
- Culmination
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°.
- Zenith
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.
- Hour angle
The time since an object crossed the meridian, in hours: hour angle = local sidereal time − right ascension. It is 0h on the meridian and grows towards the west; an object at 2h crossed the meridian two hours ago. Polar scopes and equatorial mounts rely on it.
- Meridian flip
On a German equatorial mount, the tube can hit the tripod once the object crosses the meridian. The mount then swings the tube over to the other side to keep tracking. In imaging the frame turns by 180° and the object must be re-centred.
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