ntt@dciem.UUCP (Mark Brader) (09/24/83)
There've been a couple of postings recently claiming that there is a "28-day lunar cycle". There is no such thing. For our purposes, we can regard the stars as fixed and stationary. Using the fixed stars as a reference frame, the moon completes one orbit around the earth in 27.322 days. That is, the earth-moon line returns to the same direction, and the moon occupies the same position in the sky (to a first approximation, anyway). If the moon passes near Regulus on December 1 at noon, it will do so again in the evening of December 28. The period of 27.322 days is called the sidereal month. But during that time, the earth has advanced in its 365.242-day orbit around the sun. Therefore the moon is not in the same phase as it was on December 1; the earth-moon line has to catch up with the earth-sun line. The time between repetitions of the same phase is 29.531 days. That is, the moon-earth-sun angle returns to the same value every 29.531 days (again, to a first approximation). This period is called the synodic month. Note that the sidereal month A, synodic month B, and year Y are related by the simple equation Y/A = Y/B + 1, which is forced by the motions. The synodic month is of course the one most easily observed, and most significant to those who go out at night where there are no lights. 29.531 days rounded to the nearest integer gives 30. 31-day months then arose when it was decided to make an exact number of months fit a year (itself rounded to, say, 365 days). Mark Brader, NTT Systems Inc., Toronto
debray@sbcs.UUCP (Saumya Debray) (10/01/83)
... as opposed, I suppose, to "false facts"?
{chortle, chortle}