Jupiter's Red Spot? A disappointment and not red!
Jupiter rides high in the sky all night throughout April. A month past opposition, now is the easiest time for evening observers to get a look at its intricate, ever-changing cloud bands, its moons, and their shadows.
Since some readers may still be shaking out a new Christmas scope, and others may have been too busy looking at the deep sky to pay much attention to Jupiter, this month I'll talk about how to see detail on Jupiter, what to look for, and some terminology so you can decipher what other Jovian observers write about.
("Jovian", if you're curious, comes from Jovis or Iovis, the Latin genitive form of the word Jupiter. The genitive, or possessive, form is the same form we use when we refer to a star like Theta Orionis or Alpha Lyrae.)
Getting started with Jupiter observing is easy. First, there's no trouble finding it - it's that extremely bright object to the east or nearly overhead in early evening. Point your telescope at it. What's the first thing you notice? Probably the Galilean moons - four of them, named Io, Europa, Ganymede, and Callisto. (Jupiter has many more moons, but these four are all we'll see in most amateur telescopes.) Of course, you may not see all four on any given night: some of them might be hiding behind the planet, or even passing in front of it (I'll talk about that shortly). The moons move fast: make a note of where they are, then watch, as you continue to observe Jupiter, how much change you can see in the moons even over periods of less than an hour.
You might notice that one moon is brighter than the others. That's Ganymede, the biggest of all the moons in the solar system. You can probably resolve it as a disk in your telescope, which is quite a feat for a moon that's more than four hundred million miles away from us.
Besides the moons, what do you notice about the planet? No matter how small your scope, you'll probably notice two reddish bands ringing the planet. Those are Jupiter's Equatorial Bands, one just north and one just south of the Jovian equator, and designated the NEB and SEB (north equatorial band and ... you know).
The NEB and SEB are surprisingly different. Look carefully at them, and see if you can see differences in intensity, width, and color. They vary from year to year, which is part of the fun of watching Jupiter, but they're always somewhat different from each other.
You've probably heard about Jupiter's "great red spot", and you'll want to see that, of course. You'll probably be disappointed the first time: for one thing, it's not very red. The GRS sits in the SEB, and most of the time, it's not really any redder than the SEB itself. If you haven't seen the GRS before, find out when it's supposed to be facing us - use software if you have it (check shallowsky.com for an online applet) or use the chart in one of the astronomy magazines - pay close attention to the SEB and look for a place where the band splits. It splits because the GRS, a huge, rotating storm like a cyclone three times the size of the Earth, sits right in the middle of the band, so the high-level winds that make up the SEB (think of it like our jet stream) skirt around the outside edges of the GRS, then rejoin on the other side. On a really steady night, with excellent optics, sometimes you can see turbulent swirls in the wake of the GRS where the two parts of the SEB rejoin.
Now that you know where the equator is, look near the poles. You'll probably be able to see darker areas near both poles - the polar regions, designated NPR and SPR. They're quite extensive, not little wimpy things like Mars' polar caps.
Finally, look over the planet's disk. When Jupiter's Galilean moons pass between the sun and the planet, the shadows they cast are visible on the planet as small, very sharp black spots. If you see a moon shadow, watch it as it crosses the disk. Did I mention the moons move fast? They take a few hours to cross Jupiter's disk, so you can watch their progress over the course of an observing session.
If you see a moon shadow, try to find the moon that's casting it. If you don't see a moon just off the edge of Jupiter's disk, then the moon is probably transiting the disk itself. It can be challenging to see Jupiter's moons in transit - sometimes they will stand out as a darker spot against a light area, or as a lighter spot against one of the equatorial bands. They're almost always easier to see when near the limb (the edge of the planet), and watching a moon enter or exit a transit is always a pretty sight.
Sometimes you can even see two moons and shadows transit at once. There are a couple of choice double shadow transits this month, of Io, Europa, and both of their shadows: the 5th starting at 9:54, and the morning of the 13 at a half hour after midnight. The one on the fifth also features the GRS, leading the moons by a couple of hours.
Once you're finished looking at Jupiter, there's plenty more to look at in the shallow sky this month. Saturn continues an excellent pass, reaching its most northern declination - 22 degrees 49 minutes - on April 2.
Venus continues its brilliant evening apparition, shining at magnitude -4.5 high in the evening sky. On April 3, it passes only half a degree south of the Pleiades, close enough to see both in the same low-power telescope field - should be a beautiful sight! Much fainter Mars stands near it, setting at about 11pm.
Mercury is visible very low in the evening twilight during the first week of April, then disappears into the sun's glare as it speeds to inferior conjunction on the 16th.
Uranus, Neptune, and Pluto are observable in the morning sky.
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