In the coming months, the buzz surrounding the August 21 total solar eclipse will reach a fever pitch. It will culminate in a cosmic spectacle that will captivate millions across the United States from Oregon to South Carolina. What would this event look like if we viewed it from the Moon? Satellite images showing the Moon’s shadow crossing Earth’s surface during previous solar eclipses give us a pretty good idea. But did you know that you can view an eclipse shadow in person, and in real time, by observing what astronomers refer to as a shadow transit of one of Jupiter’s moons?
In their orbital waltz, Jupiter’s four large satellites — Io, Europa, Ganymede, and Callisto in order of their orbital distances — frequently pass between the big planet and the Sun. When they do, they cast shadows that appear as small black dots traversing Jupiter’s cloud tops. Because of the moons’ relatively short orbital periods (1.8, 3.6, 7.2, and 16.7 days, respectively), shadow transits occur on an almost nightly basis. Callisto is the only one whose shadow can miss the planet. The outermost moon began a transit hiatus last year that will continue through late 2019. On occasion, two (and rarely three) moons simultaneously cast their shadows on the gas giant.
This month and next, a series of such double shadow transits are in the offing. The events below come from the Royal Astronomical Society of Canada’s Observer’s Handbook 2017, which provides the date and Universal Time for the moment when the second shadow first touches the cloud tops — the official beginning of the double transit. I’ve dropped those events not visible from North America and converted Universal Time to Eastern Daylight Time.
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