Lunar distance (navigation)

As Moon distance or Lunar distance, the angular distance of the Moon is referred to bright fixed stars that lie near its path in the sky.

Since the moon for his outstanding at the fixed stars takes 27.321 days ( the sidereal month), he lays back an average of 13 ° per day and every hour approximately 0.5 °. This 0.5 ° correspond approximately to his own apparent diameter. The moon moves accordingly under the stars in each hour a little more than a full moon in its orbit width further east. Therefore he has during each round about the earth ( lunar month ) for every fixed star exactly once a certain distance ( in compliance with the order or west-east sequence ). From the measurement of the distance between the moon and a certain fixed stars can therefore be determined from the data of the lunar orbit, the time of observation, if one knows the approximate location and the cycle of the current synodic month. If at the same sidereal time (as of the sky ) is determined by methods of Astro geodesy, can determine the longitude from the difference between sidereal time and local time.

The course of the moon among the fixed stars can be calculated since the early Middle Ages with so-called lunar tables. Kepler did this in 1600 already more accurate than 0.1 °. Today, these panels are in relation to Greenwich Mean Time (GMT ) is calculated and tabulated in the astronomical and nautical almanacs from 1763 to 1925. By measuring lunar distances, the longitude could then be calculated on cruises from the Middle Ages.