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EI2GYB > ASTRO 05.10.25 14:05l 124 Lines 8120 Bytes #57 (0) @ WW
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Subj: Catching the October Action With Jupiter's Moons
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Catching the October Action With Jupiter's Moons
Watch Jupiter and its major moons undergo a complex series of events in October.
Jupiter and its moons are busy in October. If skies are clear, be sure to set your alarm and follow the largest planet in our solar system this month. While massive Jupiter always warrants a view through even a small telescope, its four major Galilean moons warrant special interest, as we're in the midst of a season of rare double shadow transits.
These occur thanks to the 1-2-4 orbital resonances involving the three inner moons: Io, Europa and Ganymede. Outermost Callisto is the odd man out, being the only Galilean moon whose orbital tilt makes its shadow miss Jove on occasion. Callisto just resumed shadow transits this past summer, leading up to mutual transit and eclipse season for the moons next year. These are bidecadal affairs, as the plane of the orbit of Jupiter's moons appear edge on as seen from the Earth.
Jupiter currently rises in the constellation Gemini the Twins at around midnight local time, and sits high in the sky due south at sunrise. Jupiter actually passes quadrature 90 degrees west of the Sun this month on October 17th. This enhances the scene, as the shadow of the massive gas giant and its moons are cast far off to one side, as far an angle as can be seen from the Earth.
Expect this angle to shrink to nearly directly behind the planet at opposition on January 10th, 2026. 2025 is a rare 'skip' year for oppositions of Jupiter. This last happened in 2013, and won't happen again until 2037.
Fun fact: In 1676, Danish astronomer Ole R›mer was confounded by errors in the predictions for phenomena involving the Galilean moons, and correctly deduced that the discrepancies were because of the difference in distance when Jupiter was at quadrature versus opposition, and light itself was taking time to transit and reach the Earth.
Observing these events at the eyepiece - even a small telescope will reveal the moons as Galileo first saw them with his crude instrument in 1610; but you'll need good seeing and a fair amount of magnification (100x or better) to tease out the tiny ink-dot shadows. Robert Hooke may have made the first documented observation of a shadow of one of the major moons Jupiter (Callisto) in 1666. The freeware planetarium program Stellarium is a good resource to see the current configuration of the moons.
If you watch the shadow play of the moons dancing across the face of Jupiter long enough, you notice a subtle difference in the shadows of the moons themselves. Innermost Io casts a pin-prick inky dot, while the shadow of outermost Callisto has fuzzy, indistinct edges. Can you tell which moon is which, just by looking at the shadow it casts?
It's amazing to think: floating above the Jovian cloud-tops, you'd see a total solar eclipse as these shadows drift on by. Eclipse season gets even more complex during mutual transit season, as one moon passes in front of the other. It's often said that total solar eclipses of Earth are rare, but the Jovian moons see a similar situation as well, with a moon barely covering the Sun as seen from their respective surfaces.
Special events of interest in October mainly involve Io pairing up with Europa every three days, and Io with Ganymede every seven days. Conversely, the inner three moons also hide in front of or behind Jove several times in October, giving the system a strange appearance of having only one moon: Callisto.
Watch for another strange sight in October, as the +7th magnitude star HIP 37422 photo-bombs the scene from October 12th to 14th, appearing as a psuedo-5th 'moon'. HIP 37422 actually passes behind Jupiter on October 14th.
Probably the highlight of the month goes down on October 18th, with Io and Europa both casting shadows, followed by a quick hand off to Europa and Callisto. This juuuuust misses qualifying as a triple play by only five minutes! Still, this will make for a great event to capture by video.
Finally, here are the events to watch for in October 2025.
In mid-October, Jupiter rises at around 11:30 local, which translates to 3:30 (Universal Time) UT for Boston, 22:30 UT for London, and 17:30 UT for Bangkok. If an event listed happens a few hours after those times and you're near those respective longitudes, you should be able to see it.
Jovian Moon Events for October 2025
Oct 4th-Io and Europa both cast shadows from 6:47 to 8:18 UT.
Oct 6th-Io and Ganymede both cast shadows from 1:14 to 1:35 UT.
Oct 6th-Jupiter appears to only have one moon (Callisto) from 3:37 to 4:45 UT.
Oct 7th-Io and Europa both cast shadows from 19:43 to 21:37UT.
Oct 8th-Io, Europa, Ganymede and Callisto are in 1-2-3-4 order from 8:05 to 16:58 UT.
Oct 9th-Jupiter appears to have one moon (Callisto) from 17:27 to 17:41 UT.
Oct 11th-Io and Europa both cast shadows from 8:39 to 10:58 UT.
Oct 12th-The +7th magnitude star HIP 37422 appears on the scene as the 5th 'moon' of Jupiter from 13:34 to Oct 14 1:13 UT.
Oct 13th-Io and Ganymede both cast shadows from 3:07 to 5:27 UT.
Oct 13th-Jupiter occults HIP 37422 from 11:59 to 14:48 UT.
Oct 14th-Io and Europa both cast shadows from 21:36 to 23:54 UT.
Oct 16th-Jupiter appears to only have one moon (Callisto) 18:38 to 19:17 UT.
Oct 17th-The Galilean moons are in 1-2-3-4 order from 17:38 to 22:42 UT.
Oct 18th-Io and Europa both cast shadows from 10:37 to 12:51 UT.
Oct 18th-Europa and Callisto both cast shadows from 12:56 to 13:32 UT.
Oct 20th-Io and Ganymede both cast shadows from 6:14 to 7:19 UT.
Oct 21st-Io and Europa both cast shadows from 23:56 to 1:47 UT.
Oct 22nd-The Galilean moons are in 1-2-3-4 order from 13:18 to 20:47 UT.
Oct 23rd-Jupiter appears to have one moon (Callisto) from 20:14 UT to 21:30 UT.
Oct 25th-Europa and Io both cast shadows from 13:13 to 14:44 UT.
Oct 29th-Io and Europa both cast shadows from 2:32 to 3:41 UT.
Nov 1st-The Galilean moons are in 1-2-3-4 order from 00:56 to 15:37 UT.
Nov 1st-Io and Europa both cast shadows from 15:50 to 16:37 UT.
Nov 4th-Callisto casts its shadow across Jupiter from 6:56 to 10:40 UT.
Nov 5th-Io and Europa both cast shadows across Jupiter from 5:08 to 5:34 UT.
Nov 8th-Io and Europa both cast shadows across Jupiter from 18:26 to 18:30 UT.
From there, the moons fall slowly out of sync. The next major multiple shadow transit season isn't until the end of 2026.
Also, Saturn's moon Titan is wrapping up its own game of shadow play, with a final shadow event on October 6th.
Be sure to catch the shadow drama at Jupiter this month, coming to a sky near you.
More info & videos at:
https://www.universetoday.com/articles/catching-the-october-action-with-jupiters-moons
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