Jovian transits

Jupiter has been on the rise earlier and earlier each evening lately. Look for it peeking above the eastern horizon after 8:00 p.m. (EDT) tonight - but don't confuse it for the reddish-orange star Aldebaran right next to it!

With binoculars or a telescope you can see the four Galilean moons, those moons observed by Galileo when he first turned a telescope toward the stars, as they continue their eternal march around the king of planets. While these moons all orbit along the same general plane, and this plane is mostly oriented along our line of sight, catching any one of these moons in the act of being anywhere but off to the side is challenging. However, on November 7th we get just such an opportunity as the volcanic moon Io ("eye-oh") can be seen transiting its host planet that evening. This event actually reoccurs on November 30, in case you miss it on the 7th. During these two transits, the shadow of Io appears first against the disk of Jupiter because we have not yet passed Jupiter in our orbit around the Sun:

The shadow being cast by Io extends straight back away from the Sun, but because we are viewing the alignment from an angle we see the shadow cross in front of Jupiter first, then Io appears soon after. The shadow makes its first appearance at 10:11 pm EST (NOTE: daylight savings time ends this weekend, so when you're viewing the transit you will be on standard time). Io the crosses against the jovian disk about a half hour later. In case it's cloudy that night, you can see the same event happen again on Nov 30, with the shadow's first appearance at 10:22 pm EST, with Io appearing only about 5 minutes later. This difference in appearance delays reflects the fact that our viewing angle is shrinking as Jupiter approaches opposition (actually, it's due to our movement, not Jupiter's). If the transit occurred exactly at opposition, both the shadow and Io would appear at the same time.  Viewed from Earth with a modest telescope, Io will be very difficult to see - so look for the shadow instead.

If it's cloudy both nights, then check out this nice NASA image of a transit as viewed by the Hubble Space Telescope and pretend you're watching it happen live.

I for one will be hoping it is clear that night so I can watch it from the SUNY Oneonta observatory with my students, who will (hopefully) be enjoying an outdoor lab that night.

Happy viewing!