Roger Bailey <[EMAIL PROTECTED]> writes: > I was experimenting with the shareware program "Astronomy Lab". One > calculation that this program plots is the "Moon Angular Speed" in degrees > per day. This is the lunar equation of time we have been looking for. In > minutes rather than degrees, the variation is up to 14 minutes on top of > the 48 minute average daily correction that we have been quoting. > > The moon's equation of time is the variation on that average angular speed. > The graph shows this well as the sum of two periodic cycles. The major > cycle is the monthly lunar cycle. The moon speeds up when it is closest to > the earth (perigee) and slows down when it is most distant (apogee). The > cycle ranges from about 11.8 degrees (47 min) to 14.2 degrees (57 min). A > yearly cycle is added to that giving maximum peaks of 15.4 degrees (61min) > when the full or new moon (lunation) is in phase with the lunar orbital > cycle. Arthur C. noted the connection between the lunar and solar (year) > cycles.
I don't understand why the position of the sun should have an effect on the angular velocity of the moon. Does the yearly cycle superimpose another oscillation (like making the moon run generally slower in summer than in winter) or does it modulate the amplitude of the monthly cycle (like making the moon run at a more nearly constant speed in summer than in winter)? Art Carlson
