Transit of Jupiter Moons Io and Ganymede
The other night I was adjusting some things on the telescope that improved the clarity of the images. The actual adjustment is called collimation and is the precise lining up of all the "lenses" in the system. To test this I was looking at Jupiter and its moons. While I looked at it I witnessed a transit of two of Jupiter's many moons.
A transit is when one of the moons crosses the face of the planet and it casts a shadow which either precedes or follows the moon across the planet depending on which direction the suns rays are coming at it. While I was looking at the planet the shadows of both Io and Ganymede could be seen! For reference Io is about 1/5 the size of the Earth and Ganymede is about 1/2 as big as the earth and Jupiter is about 300 million miles away from the earth. So we saw (Lorraine saw it, also) the shadow of something that is about a big as the US on a planet 300 million miles away!
Here are a couple of shots taken by NASA with the Hubble Space telescope or with the Galileo spacecraft when it flew by Jupiter. That is the moon Io in the pictures and you can see its shadow in the HST picture on the left. We could see Io and Ganymede before they moved between us and the planet but couldn't see them after they were over the planet since Jupiter is so bright. We could see their shadows the whole time (as in the QuickTime movie below).
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To show an animation of what we saw, I used a program called Starry Night. The movie is a time lapse of a couple of hours. The two shadows you see moving across the planet surface were clearly visible to us. Unfortunately, I missed looking at the Great Red Spot of Jupiter (I was adjusting things and not paying attention!!). You can also see the GRS move as Jupiter rotates. The following QuickTime movie is about 1.5MB in size so will take several minutes to download and start playing if you are on a dial-up modem connection. I have sold my camera and am waiting for a new one so was not able to capture this in a series of real images but someday I'll do that and make a real movie of it.
Io and Ganymede Transit Movie (1.5MB)
(When the download completes, click on the start button in the lower left to begin playback)