The early morning hours of June 26th will be the only opportunity this year to see a dual shadow transit on Jupiter. Over the course of 2 hours and 17 minutes the moons Callisto and Io will cast their shadows on Jupiter at the same time. This time-lapse from the planetarium program Stellarium shows what the event will look like through a telescope.
Callisto, with a diameter of 2,995 miles, orbits Jupiter at a distance of 1,170,000 miles while Io, with a diameter of 2,274 miles, orbits at just 262,000 miles. Because of this difference the shadow from Callisto appears larger and more diffuse than the shadow from Io. On the other hand, the shadow from Io moves faster than the shadow from Callisto.
Here are the key times for this event:
00:07 – Jupiter rises above the horizon with Callisto’s shadow already in transit
01:04 – Io’s shadow begins transit
02:10 – Io begins transit
02:25 – Io’s shadow catches up with Callisto’s shadow
03:22 – Io’s shadow completes transit
04:21 – Callisto’s shadow completes transit
04:27 – Io completes transit
With this event starting just 11° above east-southeastern horizon, the best viewing locations will be in or near FDR State Park. Inside the park the best views will be from the park office and Dowdell’s Knob (if open). Just outside the park the best view will be from the parking lot across from the Callaway Gardens Country Store. All of these locations are detailed in my Rocket Launch Viewing Guide.