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Jupiter Moons Io-Ganymede Occultation and Eclipse

Posted: 6 March 2015

Open: Thursday, 5 March 2015, 2036 MST
Temperature: 52°F
Session: 788
Conditions: Clear, breezy

Opening was delayed as I attended a meeting of the Oracle Dark Skies Committee.

First, I synced the observatory clock to WWV so that I would have accurate time stamps in the HD video recordings audio track for the night's Jupiter Moons Mutual Events.

2045 MST: viewed the just past Full Moon, 83X. Bright. A slight terminator was visible. This handheld iPhone 5s afocal photo was taken at 2047 MST:


2052 MST: viewed Jupiter, 83X. Three moons were visible: Io, Ganymede, and Callisto. Began setting up for imaging. Mounted the D7000 DSLR at prime focus + 2X PowerMate. Did a focus test on the star Regulus using the Bahtinov Mask; locked the telescope focus. At 2107 MST took this composite image (1/160sec and 1/20sec, ISO 1600) showing the three moons:


This composite image taken at 2141 MST shows Io and Ganymede getting closer together:


Wind had increased and seeing had deteriorated by this time, hence the blurring of the moons.

Io would begin occulting Ganymede at 215028 MST, decreasing Ganymede's magnitude by about a third, with the occultation ending at 215807 MST. I turned on the shortwave radio so that WWV time signals would be recorded on the audio track and began HD video recording, 1/30sec, ISO 5000, one minute before the occultation was to begin. I continued recording through the entire occultation, ending recording one minute after the occultation had ended. This following three (cropped) images are from three frames from the video recording; before the occultation, mid-occultation, and after the occultation. They show the changing positions of the moons.

Io occults Ganymede


Occultation over

The next Mutual Event would be the shadow of Io eclipsing Ganymede. It would begin at 233459 MST, decreasing Ganymede's brightness by nearly a full magnitude, ending at 234557 MST. I started HD video recording, with WWV in the background, 1/30sec, ISO 5000, one minute before the start of the eclipse, ending recording one minute after the eclipse was over. These three (cropped) video frame images show the change in Ganymede's brightness at mid-eclipse.

Io eclipses Ganymede


Eclipse over

There are more Mutual Events of Jupiter's moons this month. This night was the first time I've ever observed this Mutual Events. I hope to image more of them. See the March 2015 issue of Sky & Telescope, page 53, for more information about these Mutual Events.

Close: Friday, 6 March 2015, 0001 MST
Temperature: 47°F

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