Wednesday, October 19, 2011

Jovian-Dual Shadow Transit for October 24, 2011

After having such a prolific season of simultaneous Jovian dual-transits last May-June, it will be hard to accept the fact that we have only two more this 2011 season: October 24, 2011 and October 31, 2011. Each transits are an exact week apart (although times differ) and both are dual-shadow transits. Io and Ganymede both partake in each event, as their orbits must be aligned perfectly just for this week. According to Simultaneous Transits of Galilean satellites and their shadows across Jupiter during 2011, a paper that lists each of dual-transit phenomena in full (can be found here), the October 24 dual-transit is the longest of the two (the other being October 31, respectively) with both moons transiting over 1 hour 18 minutes, with both shadows on the planet over the span of 58 minutes. A comprehensive photo shoot (courtesy of The Transits Page) is shown at the end of this article.

Image Credit: Transits Page; Text entered by AstronomicalEventsCalendar

Looking at the above image, we see the Galilean moons on October 24, 2011 at 00:00 UT (times forth are Universal Time, or UT). The transit begins 14:22 UT and ends comprehensively at 15:20 UT - a rather long dual-transit at 1 hour and 28 minutes, although the longest was on January 10, 2011 (2 hrs & 15 min). The next image shows the Jovian satellites at 14:00 UT, on the same date. Note how close Io and Ganymede are, and the shifting of Callisto and Europa.

Image Credit: The Transits Page; Text entered by AstronomicalEventsCalendar

In the above image, you can see Io and Ganymede about to enter transit. Callisto has shifted further away from Jupiter as it completes its orbit, and Europa is shifting down, as its orbit is completed. During transits with Ganymede, because of Ganymede's orbital eccentricy, it orbits near the south pole of Jupiter, making transits involving it shorter than transits with Europa. Europa and Io orbit very closely around the belt line of Jupiter, making dual-transits more facile; Callisto only dual-transited once this year, on January 10 (which was the longest, partially becasue Callisto is the farthest moon, and orbits slower than Europa, who shared the transit. Other than that, only Io transits the rest of 2011, with either Ganymede or Europa, see for yourself here.)

Before the transit begins, at 04:19 UT, Io and Ganymede will conjunct, as the picture below (first) shows us. Celestial Coordinates: [145.1" E; 20.6" S]. On the timing of the transit, Ganymede's shadow will ingress first at 14:06 UT (note before the actual dual-transit begins), and then Io's will at 14:22, signifying the start of the dual-transit. From there, Io will start its journey across Jupiter's luminous disk at 14:29 UT, and Ganymede at 14:34, starting the dual-transit of the moons.

Here, we can clarify the terms of dual-shadow transit, and dual-transit, becasue both can happen at the same time - the October 24 and 31 transits are great examples of these (among others). A dual-shadow transit occurs when two shadows of moons travel across the surface of Jupiter together (and yes this can also happen on any other planet with two or more moons, etc.), and then, when the two moons travel together, it is called a dual-transit. (The difference is just either the shadow or moon), but both have to cross Jupiter (or any other planet, etc.) at the same time.

You can see most of the events of this transit below, computer simulated, becasue 14:22 UT is 10:22 EDT; we here in America will not be able to view this transit (neither the 31). You can convert times here. {This is a rather sad fact for us here in America, but for those in East Asia, the Pacific, and some of the Middle East will enjoy seeing this treat. Jupiter here in America is high during night - and at Opposition, and it's very sad that we won't be able to see it!}


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