From Spaceweather.com April 22 edition. ******************************Jupiter is about to be edge-on to the sun, and that means unusual things are happening. Here's an example recorded by Australian astronomer Anthony Wesley on April 19th. "It's an eclipse of Ganymede by Callisto," he says. Callisto is off-screen, stage left, but its circular shadow can be seen moving across the disk of Ganymede. Actually, look again. Just before Callisto's shadow appears, the shadow of Io partially eclipses Ganymede as well. Wesley captured two of Jupiter's moons eclipsing a third in only 10 minutes. Unusual, indeed. This is happening because Jupiter is nearing its equinox on May 2nd; the sun is crossing Jupiter's equatorial plane. Around this time, the orbits of Jupiter's moons line up with the sun, allowing their shadows to fall across one another. Astronomers call it "mutual event season." During the season, which lasts until August 2021, astronomers can see not only eclipses, but also occultations. That's when the physical disk of one moon blocks another. The last mutual event season occurred in 2015; the next won't come until 2026. According to France's Institute for Celestial Mechanics and Computation of Ephemerides (IMCCE), there are 85 more mutual events between now and the end of the 2021 season. Some of the best may be found in this table from the Cambridge University Press. Only experienced astrophotographers will be able to make movies as detailed as Wesley's. However, even casual stargazers with ordinary backyard telescopes can see moons winking in and out as the shadow play unfolds. Look for Jupiter low in the southeast before dawn. ****************************** As an aside, Jupiter has 53 named moons and 26 unnamed moons for a total of 79, making it the moon leader in our solar system...until October 2019, when the discovery of 20 new outer moons put Saturn in the lead, at 82.
Of course, this takes us down the path of defining what a "moon" is, kinda like deciding whether Pluto is or is not Planet #9.
Didn't they decide a few years ago that Pluto is just a big ball of ice? And don't they say that Io may be capable of supporting human life? Or did I dream that?
I believe I read that Io is high in volcanic activity. I'm not sure what that might mean for supporting life (as we understand it to be.) From the web: Regarding Pluto: NASA's website says it is more than just a ball of ice, but it's size pretty much excludes it as being called a planet. The comments on that 2010 pronouncement are pretty funny. Substitute "COVID" for "Pluto" and they're pretty relevant today. "Scientists are the experts. How could you idiots argue?"
It looks like a bunch of you are spacateers, knowing all this stuff about the planets and moons, and what's this thing called lo?
It's pronounced "Eye Oh." Io. It's one of Jupiter's moons. I just read it's the 3rd largest of the 4 original moons of Jupiter discovered by Galileo (Io, Europa, Ganymede, and Callisto), and is the 4th largest moon in the solar system. Ganymede is the largest moon in the solar system, being larger than the planet Mercury. Of course, discovering that things revolved around the Sun and not around the Earth would get you in trouble at the time. For me, much of this is refreshing the stuff I knew as a kid, or it is instantly acquired internet knowledge. I get emails from Spaceweather.com on this kind of stuff, as well as solar events (flares, magnetic storms, etc.) Hal is the astronomer around here. I have a telescope I keep threatening to upgrade, but nothing of the scale that Hal has.