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May 17, 2009: Double Shadow Transit on Jupiter


Double Shadow TransitIf you live in the Americas, you have an opportunity to see two of Jupiter's moons cast a double shadow on Jupiter's disk in the wee morning hours before dawn on Sunday, May 17. As seen from mid-northern latitudes, the moon and Jupiter rise in tandem over your southeastern horizon around 2 a.m. local daylight savings time. For the precise rising time of the moon and Jupiter in your sky, check out the links on this EarthSky almanac page. (Photo Credit: Wikipedia)
The moon will make it easy to find Jupiter in the morning sky on May 17, with Jupiter beaming nearby as a blazing point of light that's far brighter than any star. Jupiter is the 4th brightest heavenly body to light up the nighttime, after the sun, moon and Venus. With a backyard telescope or even binoculars, you can often see Jupiter's 4 largest moons - Io, Europa, Ganymede and Callisto. Now and again, you might miss seeing a moon or two, because these moons routinely swing in front of and behind Jupiter.
It is extremely difficult to see a moon of Jupiter transiting (crossing in front of) Jupiter's disk through a telescope. However, it is easier to see the moon's shadow transiting this world, appearing as a small, darkish spot on Jupiter's surface. Io and Callisto's shadows will simultaneously cast their shadows on the surface of Jupiter from 3:56 a.m. to 5:24 a.m. Eastern Daylight Time (EDT), on Sunday morning, May 17. Callisto's shadow transits from 12:36 a.m. to 5:24 a.m. EDT and Io's shadow transits from 3:56 a.m. to 6:11 a.m. EDT.

Moon Transits follow Shadow Transits

This may surprise you, but Io and Callsito won't be in front of Jupiter for most of their simultaneous shadow transit show. You'll see Io and Callisto to the side of Jupiter as as their shadows cross Jupiter's disk. The moons transit Jupiter after their shadows do. The table illustrates:

Transit Times in Eastern Daylight Time*

Jupiter MoonShadow TransitMoon Transit
Callisto12:35 a.m. to 5:24 a.m.1:34 p.m to 6:19 p.m.
Io3:55 a.m. to 6:11 a.m.5:15 a.m. to 7:32 a.m.

Jupiter at Western Quadrature

On May 17, Jupiter is only one day past western quadrature. At western quadrature, the Sun-Earth-Jupiter make a 90-degree angle in space, as shown on this diagram. At western quadrature, Jupiter reaches the meridian (due south as seen from the northern hemisphere) about 6 hours before the Sun reaches the meridian at solar noon. At western quadrature, the shadows of Jupiter's moons are slanted a maximum 11 degrees in front of their direction of travel in front of Jupiter. Therefore, the moon shadows cross Jupiter a maximum amount of time before the moons themselves do whenever Jupiter is at or near western quadrature.
Io is the innermost of Jupiter's 4 major moons, so the lag period between Io's shadow transit and the moon transit is less than that of the outer 3 moons. Callisto, on the other hand, is the outermost of these 4 major moons, so the lag time between Callisto's shadow transit and the moon transit is greater than that of the 3 inner moons.

Eastern Quadrature on November 10, 2009

On November 10, Jupiter will be at eastern quadrature. At this juncture, the shadows of Jupiter's moons will be pointing 11 degrees behind their direction of travel, so the moons will transit Jupiter a maximum amount of time before their shadows do. Jupiter was last at eastern quadrature on October 6, 2008.
* All times from page 238 of the Observer's Handbook 2009, though I converted UT to EDT

copyright 2009 by Bruce McClure

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