Seattle Astronomical Society The Webfooted Astronomer << Previous Page Next Page >>

The Webfooted Astronomer - December 1999


Minutes: Astronomer Tales, the Moon, and an Eclipse

by Leslie Irizarry

The November SAS meeting began with visitors and new members standing up to introduce themselves.

The Space Needle wants to put a huge spotlight on top of the Needle to celebrate the millennium. Karl Schroeder suggested that concerned members meet with the Landmarks Preservation Board and express their thoughts about this. Notify Karl if you want to be involved or if you know someone who does.

Jerry West announced that there's a reservation form in the newsletter. Reservations must be in to Jerry no later than the January 25. You will have a choice of food, which is also be on the application.

Election of new officers: All of the present nominees for office stood before the SAS, and a vote was taken. Most were in favor; one opposed.

Tales of an Amateur Astronomer
George Best introduced the first speaker, Nick Hallett, who talked about tales of amateur astronomy. Nick talked about observing the transit of Mercury across the sun—it was a circular B-B. In l986, a satellite intersected Vega. He observed the movements of the Pallas asteroid by drawing the surrounding stars and Pallas each night. This technique works for comets, novae, and supernovae.

He also described his experience with law enforcement. On one occasion an officer, who had approached him with lights, thought he was poaching a deer. On another occasion, he was wrapped in a sleeping bag with only his head protruding, the telescope above his eye, and a police officer came up with a car and idled for a long time. He eventually left without a word. In Russia, he observed MIR, and then Salut 7. The next night, he observed MIR, and Salut 7 catching up. Unfortunately, the night they docked, clouds obscured the view. From his observing notebook, he recorded Comet Halley’s movements on Dec. 8, 10, 11, 15, and 24 of l985 through the constellations Pisces and Aquarius. He concludes that it is easy to catch up with moving objects such as asteroids and comets.

Lunar Formation
The next speaker was Pat Lewis who spoke on how the moon was formed. There is a distinctiveness about the moon. It is unique in our solar system because it is large in comparison to its primary. It is more than 1% of the mass of Earth.

The moon is useful because it stabilizes the axis of the Earth. The Earth is an oblate spheroid—like a balloon pressed on opposite sides. Because the Earth has an axial tilt, the attraction of the moon to the nearest point is stronger than the farthest point. The moon follows the ecliptic, and rises above and below the equatorial plane like a sine wave. So alternately for two weeks, the moon tugs it one way and then the other way. It pulls the South Pole, and then the North Pole. Uranus' axis of rotation is parallel to the rotation/revolution around the sun. It takes 84 years to go around. Some of the time the North Pole is towards the sun, then the equator is towards the sun, and then the South Pole is towards the sun. There are long periods of time that there is no sunlight on parts of the planet. Mars is more like Earth—there is a similar axial tilt. A computer model of Mars, however, indicates that the axial tilt may have varied in the past from 0% - 60%. Mars does not have a large moon. The other planets in the solar system tug on Mars, and it does not have the protection of a large moon.

On Earth we have tidal bulges. There are two low and two high tides per day. Water has a frictional effect on the Earth. The Earth turns faster than the moon, so the tidal bulge gets ahead of the moon. The tidal bulge’s pull on the moon tends to cause the moon to shift farther from the Earth due to angular momentum. This distance is about 2 cm. per year. The pull of the moon is slowing the Earth down a little. The sun is 400 times bigger than the moon; the sun is 400 times farther away from the moon. In a solar eclipse, the moon can block out the photosphere, but leaves visible the chromosphere and corona. This is unique in the solar system. There is no other place that the moon subtends the same angular space in the sky as the sun. This is circumstantial evidence for an intelligent design.

A few million years in the past (when humans were not yet here!) the moon was so close as to blot out the sun. A few million years in the future, the moon will be so far away that there will only be annular eclipses. There were three leading theories as to the formation of the moon: the Earth, moon and sister planets formed at the same time. The problem with this theory is that the moon and Earth are not exactly of the same material.

The Fission Theory states that the Earth was spinning fast, and centrifugal force threw off a part of the Earth, forming the moon. The problem is one of celestial mechanics. There is no mechanism that would have caused the Earth to slow down to its current rate of spin. The Capture Theory is theoretically difficult. A body the size of the moon approaching the Earth would be moving too fast to be captured by the Earth.

After sending men to the moon and procuring rocks, it was determined that all three theories were wrong. "The best explanation is observational error. The moon does not exist!" exclaimed one scientist. In 1984 at the Origin of the Moon conference, a fourth idea was proposed. The Giant Impact Theory was modeled on a computer, and stated that a glancing blow from an object caused ejectae from the Earth's mantle as well as from the impacting body's mantle to form a disc of particles around the Earth. Pieces of the disc accreted and formed the satellite Moon. The material had to accrete outside the Roche Limit. (2.9 times the radius of a body.) (If something orbits inside the Roche limit, it cannot accrete.) The iron core of the impacting body also merged with the iron core of the Earth. This model explains why the moon is so light--there is not a big iron core.

Approximately two years ago computer simulations were done to determine the size of the impacting body. The scientists created many simulations varying different conditions: different size impactor, different forces, and so forth. The new information derived from computer models found that an impactor the size of Mars wouldn't work because it would form a moon, but not one the size of our moon. The impactor had to be the greater than two times the size of Mars in order to produce a moon as large as ours. Tom and Carol Dempsey gave an intriguing slide show of the solar eclipse as seen in Turkey.

Top of Page