April 17, 2017: It’s Jupiter season!

Although for some it’s tax season, for we astronomers, Jupiter season is finally here!  Hooray!

First of all, I’d like to express my appreciation to everyone who visited my blog over the past few days to read my post about the amazing Dragonfly Telephoto Array.  That post was the most viewed post I’ve ever written so far – well over 400 views, and over a dozen people thought so much of it that they shared it on their own Facebook pages.  I am humbled that you all enjoyed my article so much.  Thank you!  Unfortunately, this blog post won’t be anywhere near as deep or interesting.

Once a year, every year, like the swallows returning to Capistrano, Jupiter returns to dominate our skies.  For the past few weeks, Jupiter has been rising at an early enough time (before about 7 – 7:30 pm) so that by another early enough time (about 9-10pm), it has risen high enough in the sky to get above the murk and turbulence of the lower part of the atmosphere, above at least 30 degrees in elevation, to allow glorious detail to be seen for the rest of the night, for as long as you care to observe.

And the details have been glorious of late.  Even observing early last week, in godawful seeing that would scarcely let me get to 123x, I saw my first moon shadow transit of the season – barely.

Here in Denver, the seeing is greatly affected by the nearby Rocky Mountains, less than 40 miles west of town.   As weather fronts move through, there are high winds quite often, over 50 mph, and those winds whip through the gaps in the mountain range and mess up the seeing but good.  The atmosphere becomes completely unsteady, obscuring all but the largest details.  And early last week, the wind was sure whipping around, all right.

Just a few days ago, I was toodling around on one of my astronomy group’s pages on Facebook as is my wont to do, when someone mentioned that Jupiter was up, the seeing was terrific, the moons were doing something funky, and that we should all get out there and have a look.  Of course, this was pretty much meaningless to me – and everyone, for that matter – as this person was nowhere near Denver, and had no idea what my seeing was like – especially after the gross, disgusting seeing I had just experienced a few days earlier.

But he was so excited about it that I decided to throw caution to the nonexistent wind and have a look.  He was right!  And there it was!  The seeing was magnificent.  A sight to behold, to be sure.  I was able to use my 8mm Televue Plossl to get up to 193x with no loss of sharpness or clarity.  And I saw this:

jupiter arrow christopher sullivan brighter callisto
Jupiter’s arrow.  Photo credit:  Christopher Sullivan, 4/14/17, around 10pm.

Take a good look – three of Jupiter’s Galilean moons (Ganymede, Europa, and Callisto, from top to bottom) are in a triangular formation on the right, while the fourth (Io) is aligned on the left, making an almost perfect arrow formation.  Cool!  I’ve never seen the moons like that before.  It’s interesting how much dimmer Callisto is than the other three Galilean moons in the photo.  It does not look this much dimmer to the eye through a telescope.  This is because one place where our eyes beat a camera is in dynamic range – we can see a much broader range of brightnesses than a camera can.

Now, I can’t see quite as much detail visually in my Mak as is shown in that photo.  For example, I can’t see that there’s a sort of split in the South Equatorial Band, almost like a Jovian Cassini Division.  I can see that there’s something going on there, but not quite a split.  I could easily see four bands, with mottling and detail especially present in the main two Equatorial ones.  And the Great Red Spot was prominent and easy to see.  No barges, though.  And, of course, no color.

The next day, I was out on the lawn in front of Chamberlin Observatory for an “in-reach” session with the DAS.  As opposed to an outreach session for the general public, this was for the members – older, experienced hands helping out the newbies, teaching them about the sky and how to use their telescopes.  I was there to help out a beginner with some problems he was having with his mount.  And once again, all eyes were on Jupiter.  It really is something.

The night after, off of my balcony, it was Jupiter again, as the great seeing continued to hold up.  The air was absolutely, completely still.  One of the nice things about stalking Jupiter this way is that because it’s so bright, I don’t need to dark adapt at all.  Just pop out to the balcony every few minutes to see what’s up.

First I saw Europa’s shadow transit.  As the transit finished up, Europa was just a little dot popping off the left side like some kind of giant space pimple on the face of Jupiter.  The GRS was on full and lovely display.  Then, just a couple of hours later, Io followed suit with its own shadow transit.  It still amazes me that I can see a 2000 mile wide shadow from a distance of about 400 million miles.

As you would expect, these moon shadow transits can occur in pairs or even triplets, where two or three shadows can be seen on the moon at the same time.  However, the orbital resonances between the moons preclude there from ever being a quadruple shadow transit.  The last triple shadow transit was on the night of January 23-24, 2015.  Unfortunately, I was clouded out for that one, and had to watch it on video from a remote telescope.  The next triple won’t be until March 30, 2032 – a pretty long wait.

But, of course, double shadow transits occur much, much more frequently.  I apologize to my readers outside of North America, but I could only find a list of double transits viewable from here.  This year, there will be double shadow transits visible in North America on:  May 18th starting at 11:55pm EDT, with both shadows lasting 49 minutes; on May 26th, at 1:47am EDT, lasting 72 minutes; on June 2, at 10:21pm EDT, lasting 55 minutes; and, on June 19, at 10:04pm EDT, lasting 35 minutes.  These last two will be two hours earlier here in Denver, just after sunset, and will be starting when it’s most likely still too bright out to get the contrast needed to see the shadows; especially that fourth one.  But the first two look very solid for viewing.  Mark your calendars!


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