Monthly Archives: February 1992

PeGASus Newsletter Issue #27 – Feb. 23, 1992

Hello, everyone!! Well, as usual, not a lot has been happening. For the one clear night in weeks that we had recently (Saturday), I was tired (having gone out skiing that day) and stayed inside feeling guilty all evening! More activity can be expected in May and June when the weather improves.

When we can get the necessary barrels, we can start the mirror grinding group going, as Gerhard Bierman has kindly agreed to donate or sell some blanks that he has on hand. Yours truly will be mounting the 10″ mirror bought some time ago, making a club telescope that we be available on loan.


It was generally agreed by the members present that we would like to have David Sundberg stay on as treasurer of the society and Dave has so agreed. Welcome back Dave!


I got the application in for our next casino event which will be hopefully this summer sometime. I received acknowledgement of our application for a Science Council grant; results will be announced by late March. Even if the Science Council grant is not approved, we will still be able to proceed with the observing room part only this summer, leaving the classroom part until later.


Jupiter will be at opposition Feb 29 and will be a grand sight all month. Mercury will be visible now and in the beginning of March low in the southwest just after sunset. Try it with binoculars!! The other planets are poorly placed, or impossible to see right now.

I just received notice (via astronomical telegrams that I get) that there is a nova in Cygnus that has reached magnitude 5.2 and is at coordinates 20h 30m 32s and +52ø 37′ 53″ (2000.0). Let’s have a look for it next clear night – it’s of naked eye magnitude and well placed for us to see!

Other than that, the galaxies of the Virgo and Coma clusters are well placed for viewing later in the evening should we get a good night. Any takers?

FEATURE ARTICLE – Origin of Moon:

How did the moon form? This question has long intrigued astronomers and many theories have been proposed over the ages. Basically these theories fall into one of three general scenarios: that the moon was formed somewhere else in the solar system and was captured by the earth (intact capture, disintegrative capture), that the earth and the moon formed together (co-accretion), or that the moon was somehow ripped out of the earth after the latter was formed (earth fission, collisional ejection) and remained in orbit.

Before we can examine any of these theories, we must look at what is known about the moon, what constraints exist for these theories. First of all, the overall density of the moon (3.34 g/cm3) is somewhat less than that of the earth (4.45 g/cm3). Part of this can be accounted for by the fact that the earth has an iron core (as evidenced by its magnetic field) whereas the moon does not. Another constraint comes from the isotope ratio of oxygen. Isotopes of an element are atoms that contain the same number of protons but different number of neutrons. For example, oxygen 16 (the normal stuff we breath) has 8 protons and 8 neutrons. Other isotopes are oxygen 17 (8 protons, 9 neutrons) and oxygen 18 (8 protons, 10 neutrons) which occur in trace amounts. The precise ratio here on earth and in rocks on the moon are closely equal implying that both bodies were formed nearby.

More than a century ago, George Darwin (son of Charles Darwin, the father of evolution) proposed that the moon was ripped out of the still molten earth early in its history (fission theory). Such a theory would account for the composition that the moon has; however, the earth would have had to be rotating unreasonably fast (once every 2.6 hours) and moreover, would have had to somehow lose this rotational momentum since then. This difficulty would appear to rule out the theory as proposed.

In the 1960s, a popular theory was that the moon was formed elsewhere and captured by the earth (intact capture theory). This would account for the iron-poor nature of the moon; however, the match in oxygen abundances would make this theory unlikely. Also, the probability that a wandering alien body would be travelling at exactly the right speed and direction to be captured is extremely unlikely; furthermore, there must be a mechanism for losing excess energy.

The co-accretion model is more acceptable in that it accounts for the oxygen ratio and does not depend on some fortuitous event. However, it does not explain why the moon has so little iron.

A much better model (see Sky and Telescope for Dec 1986, p 558, also S&T for Nov 1984, p 389) is that of collisional ejection. Such a model, first proposed in the 1970s, asserts that a small planet hit the earth at an oblique angle, splashing off debris from the outer part of the earth. Because of tidal forces (friction), this material ended up in orbit about the earth, eventually coalescing to form the moon. This model accounts for the iron-poor nature of the moon (since only material from the outer layers of the earth were taken), and for the match in the oxygen abundance. Also, the angular momentum before and after are reasonable. In fact, complex simulations using super computers have predicted that the moon would have formed soon after such an impact and that the general scenario is plausible.

The above theory appears to be the front-runner. However, before rejoicing that the problem is solved, one should remember that there are some details that remain unexplained. (For example, the moon is depleted in certain elements such as chlorine, mercury, lead and zinc. Why is this?) What is needed is more information. The moon rocks that were supposed to settle the issue, regrettably came from the maria regions of the moon and are mostly basalt (results of a lava flow). They were therefore formed more recently in the history of the moon. What is needed is a “genesis rock” (similar to rocks in the Canadian Shield that date back several billion years) that would reflect conditions soon after the moon’s creation. This need is why scientists are advocating a return to the moon. One can only hope …


Wednesday, Feb 26 at 7:30 PM at the College, room 2-223. Let’s look at a videotape or some slides. Come join us!!

Bob Nelson, President