Lower culmination
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The lower culmination is the time and place in the sky where an object reaches its lowest altitude, with respect to some observer on the earth.
Background
The culmination of an astronomical object is the time when and place where passes across the meridian of an observer on earth. The crossing can either be overhead, or imagined to be below the observer's feet.
When and where the event happens depends on an observers point of view, so observers in different places will report the event at a different time, and see the culmination at a different altitude. The exception is that if they happen to be lined up east-west, or aligned north south, two observers will agree on at least one of the numbers.
An astronomical object always reaches its highest and lowest altitude when it crosses the meridian; the lowest altitude is called the lower culmination. For most stars this is below the horizon.
The rotation of the earth gives astronomical objects the appearance of moving across the sky during the course of the night. Objects in the sky, such as stars and planets, rise in the east and set in the west.
The meridian is an imaginary line rising perpendicular to the horizon, directly south of the observer, crossing directly overhead (zenith), then rejoining the horizon directly north of the observer.
During a sidereal day, an astronomical object will cross the meridian twice: once at its highest point as seen from the earth, and once at its lowest point. Usually the lowest point will be below the horizon, the exception is for objects close enough to the celestial north pole that the observer sees them above the horizon full time. So for circum-polar stars, both culminations are reported.
Altitude at Upper Culmination
A simple formula for the altitude of an object in degrees at its upper culmination is equal to (90 - l - d), where l is the observer's latitude, and d is the object's declination.
See also
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