Solving Lunar Distances
This diagram illustrates how the moon appears to move through the stars. It shows that there is a difference between the measured altitude of the moon and its actual altitude, due to atmospheric refraction. Locations of the moon are shown at 3 and 6 hours, to show its actual position as given in the Nautical Almanac, and that its observed position must be interpolated between the given positions. From: Man is Not Lost; A Record of Two Hundred Years of Navigation with the Nautical Almanac, 1767-1967, p. 7.