June 6, 2020

Astro-Event of the Week: September 2nd-8th 2008; A Double Shadow Transit!

The Jovian system of moons is a place where things happen. Shadow transits are probably the most dramatic event to be seen in this “mini solar system.” It’s definitely cool to watch the inky black dot shadow of a moon sliding silently above the cloud tops of Jupiter.

On September 7th, 2008, viewers positioned on the East Coast of North America are in for a rare treat; a double shadow transit. Jupiter will be positioned due south, and the action will already be in progress at local sunset. This is a telescopic event; feel free to crank up the magnification on this one! Westerly observers will face a tougher proposition, as Jupiter will be medium low in the daylight sky. Use of a GPS scope or noting Jupiters position a night or so prior at dusk in relation to a foreground object, such as a poll or house peak, will help with acquisition.

Here’s a break down of the timeline, in Eastern Daylight Savings Time;

4:40 PM- Callisto Shadow ingress.

8:12 PM- Io Shadow ingress (double transit begins).

8:38 PM- Callisto Shadow egress (double transit ends).

12:28 AM- Io Shadow egress.

This is an excellent photographic opportunity; it’s also worth comparing the differences in shadow darkness and sizes. Close in Io will appear large and diffuse, while distant Callisto will appear tiny and dark.

This weeks’ astro-word of the week is transit.  Generally speaking, an astronomical transit occurs when a smaller body moves across the face of a larger one from the viewers’ line of sight. Some typical transits are the Galilean moons passing in front of Jupiter, or an interior planet such as Mercury or Venus crossing the disk of the sun. The term also refers to passage of an object across the celestial meridian, the imaginary line that runs overhead from due north to south. Such instruments fixed to view this area are known as Transit Telescopes.


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