More shallow sky on Saturday night

by David Kingsley

The Jupiter mutual satellite event that Bob Jardine mentioned earlier was interesting to watch at the eyepiece. A little before nine tonight, the shadow of Ganymede was due to pass partially over the face of Io. We are now pretty far past the opposition of Jupiter, so the two moons themselves were not particularly close to one another during the eclipse. When I first set up my Starmaster 7 inch Dob at the end of the driveway, all four moons of Jupiter were laid out in a line, with Callisto on the preceeding site of Jupiter, and Ganymede, Io and Europa all well separated from one another on the following side. A little before 9 pm, Io clearly began to fade in magnitude. In the somewhat blurry seeing, this could only be detected by a change in brightness, not in apparent size of Io. However, the three other moons made great comparison points for watching the brightness change. At the peak of the eclipse, Io had faded until it looked as dark as Callisto, and obviously fainter than Ganymede or Europa. Then at about 5 minutes after 9, Io's brightness began to rise again, and was quickly back up to a brightness between Ganymede and Europa, (and clearly brighter than Callisto). The mutual events of Jupiter's moons take place in groups every 6 years. I had enjoyed watching the moons themselves overlap in mutual occultation events earlier this year. However, this was the first mutual eclipse I have seen, and will probably be the last for awhile, because Jupiter is getting pretty low now for good observing.

When the eclipse was over, I turned to the first quarter moon and whiled away a happy hour and a half or so exploring all the interesting detail around the terminator. Much of this has already been well described by Marek and Rich, so I will just add my own two favorite lunar highlights of the night. First, on the south side of the moon, the crater Moretus put on a show that anybody who likes double stars would have appreciated. Most of the crater was still completely submerged in blackness. However, two points of the central peak were just high enough to catch the rising sun, producing what looked like a very close, perfectly matched double star floating in the middle of the crater. The sight reminded me of the double star Porrima in Virgo that Rich mentioned in his report: two points, similar colors and magnitudes, nearly touching, and a beautiful juxtaposition of light and dark.

At the opposite end of the terminator, near the north of the moon, the sunrise over Plato was spectacular. When I first looked around 9:25 or so, I noticed that there were two thin shafts of light extending as parallel sunrise rays across the floor of the crater. The larger one shown right across two of the small craterlets on the floor of Plato, plucking them out of the darkness. By 9:45, a third sunrise ray was evident, making a third flashlight beam across the otherwise dark floor. Each shaft of light varied in thickness and position. Although each began near the center of the crater, and extended in a shaft of light to the western wall, it was clear that each was positioned in line with a gap or notch in the eastern wall of Plato. Sun shining through such gaps can produce dramatic rays near sunrise and sunset (see http://www.lunar-occultations.com/rlo/rays/rays.htm for other known lunar rays and times to observe them at various locations in the moon). I have seen many lunar rays before, but three sunrise rays all at once in a crater like Plato made a grand sight. I sketched a few stages as the rays gradually broadened and joined as the sun rose higher and higher over the eastern wall. The fun ended when the marine layer arrived in Palo Alto around 10:45 pm or so. Still, two hours of great views, with a total set up time and travel time of only about 5 minutes. Those two hours were the most relaxing part of my day, and I might not have made it out at all if Bob had not posted the heads up on Jupiter earlier this evening.