Jupiter's June

With the exception of Mercury at only 0.03°, all the planets have some tilt to their axis. As any fifth grader should be able to explain to you, Earth has an inclination of 23½° and that is what gives us our seasons as we orbit the Sun. As amateur astronomers we can sometimes notice the tilt when observing some of our planetary neighbors. Mars' axial tilt is about a degree larger than Earth's and can present one hemisphere more favorably than another. For example, our best views of Mars are when it comes to opposition right around the time of its perihelion. As it turns out Mars is always close to its Winter Solstice at that point in its orbit, so we see  southern hemisphere features like Hellas and Syrtis Major better because they are tilted towards us while northern albedo markings such as Mare Acidalium are tough to discern (map). 

Hubble captures Titan's Transit - Feb 24, 2009

Saturn, with an inclination of 26¾° is the easiest example of noticing the affect of axial tilt. At its spring and fall equinoxes the rings all but disappear as we view the planet's equator straight on. At its solstices we are treated to the full grandeur of the ring system during maximal display. Another aspect is that only when Saturn approaches its equinoxes will the orbital planes of its moons begin to intersect the globe of the planet from our vantage point. It is only at those points in its ~29-year orbit that we get to see Titan transit and cast its large shadow upon the clouds below. While not as rare as a Venus transit, seeing our Solar System's largest moon cut in front of its home planet is an infrequent event (and one that is on my bucket list for the upcoming equinox!)

And where does Jupiter lie on the axial tilt spectrum? It comes in at a mere 3°, barely tipping towards or away from our view. We never get a nice look at its polar regions as with Saturn, it is consistently featuring its full-on view. Despite Jove's stingy axial tilt, the observant amateur astronomer can still discern evidence of the inclination, even with a modest telescope, by studying the Galilean moons. 

Yesterday (January 20, 2024) on Jupiter the Druids assembled at their Stonehenge to celebrate their Northern Summer Solstice, the maximal tilt of the planet's north pole towards the Sun. About a week earlier I was out imaging Jupiter (under very poor seeing) with a serendipitous alignment three out of the four Galilean moons. Io was about to slip behind the planet, while Europa had just start its trek across the planet's face. Ganymede stood nearby just off the limb awaiting its turn to begin transiting the planet.

Jupiter on Jan 13, 2024 7:39 p.m. EST

Like most planetary moons, the Galilean quartet have their orbital planes roughly aligned to Jupiter's equatorial plane (i.e., if we could see those planes it'd be similar to seeing Saturn's rings). But in my capture we note that despite having orbits lying in that equatorial plane, none of them appear near the planet's equator as they approach Jupiter. We see that the two that are about to cut in front of the globe will do so across Southern hemisphere cloud tops, while the one that is about to duck behind the planet appears at a northern Jovian latitude. This let's us know that Jupiter's northern hemisphere, and by extension the orbital planes of these moons, is currently tilted towards us.

There's another piece of information to be gleaned from the image taken on the 13th. If we use WinJUPOS to apply a grid overlay on Jupiter, we can more easily see that the distance the moons lie from the equator varies. Io is closest to the equator at roughly 20°, Europa is about double at ~38°, while Ganymede is doing a more polar crossing at about 55°. If we consider the schematic below that approximates how a set of orbits lying in the planet's equatorial plane might appear with a north-leaning planet tilt, we can see the significance of this. Io must be orbiting closest to the planet since it is nearest the equator. Europa must lie (very roughly) twice as distant. and Ganymede is orbiting at a distance perhaps some 2½ times that of Io. When we check our hypothesis we see that our analysis was an acceptable swag: 

• Io: 422 km
• Europa: 671 km
• Ganymede: 1,070 km

But what about Callisto, the farthest out of the 4 Galilean moons at 1,883 km? If Callisto had been in the frame we would have seen it floating above or below Jupiter given the combination of the moon's more distant orbit and the planet's current maximum northerly tilt (much like the green orbit in our schematic above). 

Of course now that Jupiter has passed the Northern Summer Solstice in its orbit it will be moving towards an Autumnal Equinox roughly 3 years from now. As we head there you'll see the moons gradually fall back towards transiting the planet along its equator, and Callisto will once again join her siblings in crossing the Jovian cloud tops from as seen from our home planet. 

So often we set up the telescope and take a just a quick peek at our target, not tarrying to inspect the view in the eyepiece nor record what was seen. So here's a challenge for you to do something more. Observe Jupiter when a Galilean moon event is set to occur (S&T has a great online tool to predict when these occur, with three opportunities this coming week on the 22nd, 24th, and 29th). In a notebook sketch what you see (and you do not need a large scope to see these events). Continue to do this over the next three years and you'll have a cool record that shows the shifting tilt of the planet as evidenced by the changing appearance of the 4 brightest moons when near or in front of the planet. While you won't get an award for your effort, I bet you'll feel a reward for being able to demonstrate some of the mechanics of our Solar System through a patient recording of what you've seen first-hand.