Lunar distance
As lunar distance or Lunar distance is an angular distance of the moon to bright fixed stars referred to, which lie in the vicinity of its orbit in the sky.
By measuring lunar distances to determine the local time , geographical longitude could be calculated indirectly on boat trips from the Middle Ages ( longitude determination , longitude problem ). Today there are much more convenient procedures for this.
Astronomical background
Since the moon needs 27.321 days for its orbit in the fixed star sky ( sidereal month ), it covers an average of 13 ° per day and approximately 0.5 ° every hour. This 0.5 ° corresponds approximately to its own apparent diameter , so the moon moves relative to the fixed stars (not: relative to the horizon) every hour a little more than a full moon width in its orbit to the east . Therefore, during each orbit around the earth ( lunar month ) it has a certain distance from each fixed star exactly once (if the order or west-east sequence is observed).
Procedure for determining length
From the measurement of the distance between the moon and a certain fixed star, the time of the observation can be determined by comparing it with the data of the lunar orbit , provided that the approximate location and the cycle of the current synodic month are known . If the local sidereal time ( sidereal time : based on the position of the starry sky) is determined at the same time by methods of astrogeodesy , the geographical longitude can be determined from the difference between sidereal time and local time . This procedure was first described and carried out by Tobias Mayer at the beginning of the 16th century .
The lunar orbit, d. H. the course of the moon under the fixed stars has been calculated using lunar tables since the early Middle Ages ; Kepler was able to do this more precisely than 0.1 ° around 1600. From 1763 to 1925 these tables were tabulated in the astronomical and nautical yearbooks ; to this day they are calculated in relation to Greenwich Mean Time (GMT).
Individual evidence
- ^ Franz Adrian Dreier: angle measuring instruments. From the 16th to the early 19th century. Staatliche Museen Preußischer Kulturbesitz, Berlin 1979 ( catalogs of the Kunstgewerbemuseum Berlin 13, ZDB -ID 1168417-3 ).