Chuck,

You have to remember that the moon travels around the Earth in an ellipse, and that the Earth
travels around the sun in an ellipse as well. Also remember that according to the laws of motion,
the Earth speeds up when it is closer to the sun, and the moon speeds up when it is closer to the
Earth. Also, since the lunar cycle's length is based relative and not sidereal motion, the two can
combine to greatly shorten or lengthen the cycle length. It all averages out, of course - but see
this link:

http://www.obliquity.com/astro/lunarmonth.pdf


HTH,
Bernie
MS Excel MVP


"Chuck" wrote in message ...
On Wed, 25 Mar 2009 19:57:21 -0500, Chuck wrote:

Is there an equation that will calculate the phases of the moon?

Chuck
Bernie,

If you are still watching the thread, I found a listing of Blue Moons (second
full moon in a month)

Subtracting the first date/time value from the second should give the time for
one lunar cycle. The list has 9 Blue Moons. The longest cycle was 29.73750000
days (10/1/2020 21:05 to 10/31/2020 14:47) GT. The shortest cycle was
29.29583333 days (8/1/2023 18:29 to 8/31/2023 1:35). The difference in cycle
time is 10 hrs 36 mins. That is a huge cycle time variation. Can this
possibly be? No two months had the same cycle time. Makes me wonder about the
accuracy of the source data.

From "Pictorial Astronomy" by Alter, Cleminshaw, and Philips 1966 synodic month
= 29d, 12h, 44m, 2.8s. (29.53058796 days).

Chuck