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Log, Apr 28, 2006

15 people. Ernie and some other folks were out in the parking area with telescopes but I never managed to get out there because we had folks in the Dome the whole time. Even before it got dark, I managed to get a 1.9% crescent Moon in the telescope. Normally I would not even attempt such a view but the Moon was located at a particularly good (read safe) angle to the north east of the Sun causing it to set after and above the Sun. This meant that we could focus on the Moon without any chance of dangerous stray light from the Sun. It was an interesting sliver but quite unstable in the rapidly cooling air. The air dropped from 56 degrees to 42 degrees within an hour of sundown and dropped another 6 degrees by midnight. It was unusually cold for the last weekend of April.

Mars was a wiggly blob most of the time but kids love to see it even thought there was little to see. At less than 5 arc seconds across it made a very small target. Later in the evening, Ganymede at 1.6 arc seconds was large enough to see as a disk but just barely. Jupiter at 45 arc seconds had a visible surface area more than 80 times that of Mars. Saturn was just visible at the edge of the Dome's shutter at sundown but M42 was already setting by the time Betelgeuse was visible.

I don't know exactly why but I spent a good deal of time greeting the sky that will dominate the summer. We spent a good deal of time comparing globular clusters - M15 (lovely), M92 (nice) and M13 (absolutely stunning). We looked at some of the famous summer multiple stars: Epsilon Lyrae (the famous double-double) and Albireo rose before midnight and we got a decent look at them. I have to admit that Albireo had less than perfect color. The topaz yellow star look suspiciously red and the sapphire blue star was definitely green but then again the stars weren't that far off the horizon and the sky shine from Providence and Warwick interfered in that direction.

A couple of fellow who were quite interested in astronomy came later than usual, and I spent a good deal of time helping then get used to the sky. One had a telescope (which I hope he will bring the next time) but couldn't find some things. I showed him how to find certain objects. For example, he knew that M13 was in Hercules, but didn't know exactly how to find the keystone that forms Hercules' chest. [The trick is to locate Vega, Arcturus and Alkaid which are all prominent stars. Hercules is between Vega and Arcturus with Alkaid indicating which side of the line to look.]

We got into quite a discussion on how the rings of Saturn were formed. I described how a moon which wonders too close to a large planet (inside it Roche limit) tries to orbit the object. The center of the moon travels at the correct orbital velocity. The outer edge of the moon goes too fast (because more distant object needs to travel slower). The inner edge of the moon goes to slow (opposite reasoning). When the moon close enough to a large planet, dust, gravel, snow and eventually rock and what have you are moving so fast (or so slow) that they exceed the orbital velocity of the small moon's gravity and they "fly off" forming a ring. A few diagrams showing the moon far off and near by showed how the diameter of the moon and the proximity to the planet allowed the distant moon to survive while the close in moon was destroyed by the planet's tidal gravity forces.

The sky grew better and better over the evening as the air settled down but eventually it was midnight and we had to wrap up. It was nice to have a clear nearly moonless night. Well next week we'll have a bit more than 58% illuminated Moon sitting squarely in Leo where it will degrade viewing no end. Sigh.

-Les Coleman

Leslie Coleman
Author:
Leslie Coleman
Entry Date:
Apr 28, 2006
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Leslie Coleman's Log
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