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Log, Aug 21, 2009

68 people. We had a large crowd this Friday and why not? The sky really behaved itself in a most delightful fashion. The Moon didn't rise until after midnight, the air while damp wasn't cloudy, and the contrast was high allowing naked eye detection of details of the Milky Way, the Great Spiral Galaxy in Andromeda, M4 and numerous swirls and pockets in Sagittarius. We spent a great deal of time looking at three objects, Albireo, M4 and Jupiter with less time spent on a view of M31 and M32 in the same eyepiece.

We stayed open until the witching hour. The telescope was shut down and there were still folks in and about the observatory. We had two special visitors that we haven't seen enough of in recent years - our past FDO directors Bill Penhallow [who founded the observatory] and Art Guarino who was the director between Bill and Les Coleman.

About the only negative aspect of the night was the high humidity which caused everything below say 20 degrees elevation to take on distortions in color and even shape. As we drove out of the park I noted that the Moon was coming up but the moisture cause it to look far too much like a real archery bow with a shift near the middle of the crescent (the hand hold). It was a layer of heavy moisture that acted like a lens distorting the shape quite noticeably. Yet in the scan t twenty minutes from the observatory until I reached home, the Moon had risen far enough to stop being a reddish archery bow to the more pedestrian 38% crescent which the software assured me was the case.

One special event should be noted about Jupiter's moons. We had a "partial eclipse" [more technically, an occlusion] of Europa by Io that was watched by literally dozens of people. Describing this event in true three dimensions is rather confusing, so I'll try to simplify what happened by treating Europa as stationary and describing Io's path relative to Europa. As we had the eyepieces set up, Io and Europa to the right of Jupiter. Io was nearing its right most orbital point.

What occurred prior to Jupiter rising high enough to be seen:

4:30: Io is left of Europa, moving right.

5:30: Io "grazes" Europa, passing above it.

6:50: Io reaches its rightmost orbital position and begins to double back.

8:00: Io passes over Europa reaching its maximum partial eclipse.

What occurred after Jupiter cleared the horizon and trees at 8:20:

8:20: We could see the two moons clearly. They looked like a single point to most eyes but a few sharp eyed folk could detect that they were more like a short line segment than a single point of light.

9:00: The two moons were separated by a gap wider than either moon but not wide enough to be seen as two points.

9:30: The gap widens to 6 arc seconds allowing everyone to see the two moons as distinct objects.

-Les Coleman

Leslie Coleman
Leslie Coleman
Entry Date:
Aug 21, 2009
Published Under:
Leslie Coleman's Log
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