This feature is inspired by the questions we have received over the years from interested readers. If you have a question about an astronomical topic, please forward it to us.

John Brennan asks:

In the Fall 2000 issue in the article about the petroglyphs, you stated that there were 30 days in a lunar month. Thirteen moons x 28 days = 364 days/sun cycle. Thirteen moons x 30 days = 390 days? Please clarify for me.

Dr. Bruce Weaver responds:

The motions of the Moon are amazingly complicated. The various periods vary from daily to thousands of years. Because the Moon is not exactly spherical, the gravitational forces due to the Earth and Sun are not symmetric, and every property of the lunar orbit varies over time. One symptom of these variations is the fact that there is no easy way to predict lunar and solar eclipses accurately.

A great deal of this complexity is reflected in some megalithic calculators of lunar and solar geocentric motions such as Stonehenge. While these have not been lost to science, they have, sadly, been lost to our general education system.

The length of the synodic month, which is measured from new moon to new moon, is 29 days, 12 hours and 44 minutes; or about 29 ½ days. Folks without fractions, like those who built Stonehenge, approximated this by alternating 29 and 30 day months. The average (29 ½) is an excellent approximation to the true synodic month.

Note that a synodic month depends on the appearance of the Moon illuminated by the Sun. It takes the Moon only 27 ½ days to return to the same place in the sky as measured against the stars. This is a sidereal month. It takes two more days for the Moon to get to the same position with respect to the Earth-Sun alignment because the Earth has moved about 8% further along its orbit (and everything we're talking about goes counterclockwise as viewed from the north).

The time it takes the Earth to go around the Sun (defined as the time it takes the Sun to return to the same place in the sky) is 365.242190 . . . days. The number of synodic months in a year is about 12.368267. . . . There is no reason to expect that the time it takes the Earth to revolve about the Sun should be some multiple of the time it takes the Moon to revolve about the Earth.

The calendar tries to solve the problem that the revolution of the Earth about the Sun is not an integer number of days and, thus, requires complicated rules to approximate the fractional parts of a day that are left over every year. The months, as we know them, are modeled after lunations, but the concept of the lengths of months--never very accurate with respect to either the number of integer days or days in a year--have been ravaged by custom, religion, and politics. For example, July has 31 days because Julius Caesar didn't want his month to have fewer days than the one named after Caesar Augustus (August).

I hope this helps.

THANKS TO OUR GENEROUS DONORS!!

Thanks to Donors: In December 2000, MIRA received a $4,000 unrestricted grant from The Kenneth & Gabrielle Adelman Fund.

We've also received a $4,000 donation from Dr. Bruce and Sandra Weaver for the Weaver Student Observatory, and several generous anonymous donations.

The Fund for Astrophysical Research, Inc. has awarded MIRA a Theodore Dunham, Jr. Grant for Research in Astronomy that is earmarked for the purchase of a site licence for software to control and reduce data from the CCDs that are an essential element in much of MIRA's research.

And most recently, the Ralph Knox Foundation has contributed an additional $5,500 towards maintenance of the MIRA shops.

Thanks again to all of these generous individuals and foundations for funding MIRA's educational and research endeavors.

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Last updated 3/18/02 DMC