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18 January 2003 (Sunday) The 2002 Chesley Bonestell Lecture: "The Discovery of the Tenth Planet (or is it just another Pluto?)" Dr. Michael E Brown of Caltech, Co-Discoverer, will lecture on the discovery of Quaoar on the out4er edges of the Solar System. 7:30pm MPC Lecture Forum 102.
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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. |
How close to synchronized are the periods of rotation and revolution of the Moon? Is there a plausible explanation for this phenomenon (other than random chance and divine intervention)?
They are essentially the same. That is, about half of the Moon is never seen from Earth. This is often called "the dark side of the Moon" but, of course, it sees as much of the Sun as the side that faces the Earth.
This apparent coincidence has been caused by tidal interactions with the Earth. The Moon and Earth raise tides (differential gravitational forces) in each other. This occurs in the solid material as well as in the Earth's oceans. This constant deformation causes heating; energy for this heating comes from the rotation rates of the two bodies. It slows the rotation rates of both the Earth and the Moon. The first effect was to slow the rotation of the Moon until its rotation and revolution rates were the same.
This tidal friction continues, but, since there is no lunar rotation (with respect to Earth) left, all of the energy comes from revolution, which results in a slow increase in the distance between the Earth and the Moon (about 3.7 cm per year). Eventually, the orbital period of the Moon and the length of the day on Earth will be the same.
We actually see a bit more than 50% of the surface of the Moon due to the phenomenon known as libration. First, because the equator and the orbital plane of the Moon are inclined at about 7 degrees so, depending which part of the orbit the Moon is in we see up to an extra 7 degrees of latitude. This is called libration in latitude.
A second effect is called libration in longitude and results from the fact that the lunar orbit is not exactly round so the Moon moves at slightly different speeds as it goes about its orbit; however, the rotation rate is quite constant, so we see up to an extra 8 degrees of longitude. All in all, we get to see about 59% of the surface of the Moon.
Thanks for the great question!
Dear Friends,
As we conclude with this issue the 25th year of publication of the Newsletter, it is fitting that we reflect for a moment on MIRA's past achievements and plans for the future.
With the modernization of the 36-inch telescope virtually complete, MIRA's research effort is fully underway. The research projects that use direct imaging-on gravitational lenses and star formation regions-have been on line for a year or more. In recent months, the spectrograph and its two CCD cameras and video guiding system have been put into full service. This instrument, now more powerful than ever, is being used both by MIRA astronomers and by researchers from other institutions.
Among our many educational activities, we had a bumper crop of high school interns this past summer, more than we have ever had in the past. A newly invigorated docent team, coordinated by our new Administrator, Leslie Tau-Holzhausen, is bringing astronomy to the young people of Monterey County, both in the schools and on our Marina campus.
MIRA's warmest thanks and congratulations go to our volunteers and docents, whose only reward is the satisfaction they derive from furthering astronomical research and education. Without them, MIRA could not function.
Nor could it function without the generous financial support given by the Friends. Not only do benefactors step forward time after time to fund some important new instrument or program, but hundreds of members of our organization, through their memberships, support our day-to-day operations.
We hope that you will include MIRA in your end-of-year giving. If there is a particular project you would like to support, please contact Leslie Tau-Holzhausen or Dr. Bruce Weaver; they will be happy to help with any special gift needs.
The Editor
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Last updated 12/14/02 DMC