Orwell Astronomical Society (Ipswich)

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Shadow Transits of Ganymede And Callisto, 04 October 2021


On 04 October 2021, a rare event occurred: a double shadow transit involving Ganymede and Callisto. (The event was forecast in the "Sky at Night" magazine for October 2021.)

The shadow of Callisto was already on Jupiter when that of Ganymede joined at 18:53 UT. Unfortunately, at this time it was cloudy. Due to a commitment elsewhere, I was not able to attempt to observe again until around 20:15 UT. By this time, there were still banks of cloud to the south, but the sky as a whole was much clearer. I was able to follow the motion of the shadows of both satellites until that of Callisto was no longer visible on the planetary disk. I finished observing at around 21:06 UT, although the predicted time of the event was 21:22 UT.

Jupiter showed an apparent diameter of 46 arcsec and was at an altitude of around 23° throughout the event. At this altitude, my telescope does not clear the lower edge of the observatory aperture: this is not normally an issue as I usually observe objects at higher altitudes. My planetary camera is rather ancient (reviewed in 2008) with a resolution of around 0.5 arcsec but this did not compromise resolution as seeing could best be described as "wobbly".


Nigel Evans