Four hundred years ago when Galileo discovered the moons of Jupiter, the satellites appeared in his primitive telescope as tiny, almost infinitesimal specks of light pirouetting around the giant planet. Their discovery transformed 17th century cosmology and made Galileo famous, but he never saw them as anything more than star-like pinpricks. The “Galilean satellites” were second-class citizens in the heirarchy of known worlds. What would Galileo say now?
On August 16th, Philippine astrophotographer Christopher Go used a modern 11-inch Celestron telescope to photograph Io casting its shadow on Ganymede.
“I captured this rare event through a hole in the clouds,” says Go. “It was a lucky clearing!”
In the movie, Io and Ganymede reveal themselves as fully-formed worlds with surface markings and a spherical shape. Io’s circular shadow cuts a dark swath across Ganymede, transforming that giant moon (it is larger than Mercury) into a succession of crescents rarely seen by observers. Indeed, as far as we know, no telescope on Earth or space has ever photographed one of Jupiter’s moons casting its circular shadow so clearly across another.
“While imaging the shadow transit, I took the time to photograph Jupiter itself,” says Go. “The Great Red Spot, an anticyclone twice the size of Earth, was very prominent.”
At this point, one imagines Galileo would jump up and exclaim – “bring me a telescope!” If only we could. August 2009 is a superb time to watch the giant planet. Jupiter is at its closest to Earth and outshines every star in the night sky. Backyard optics reveal giant storms, clouds, moons, moon shadows and occasionally an explosive surprise. Jupiter is in the South at midnight, in mid-to-late August 2009:
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I had a 12-inch reflector for a couple of years. It took up a fair amount of space in my kitchen, as it was about 4' long and 6 ' high but was fabulous for looking at the moon and you could see the rings of saturn and moons of Jupiter. The number of stars you could see with the 'scope compared with the naked eye was amazing.
1AM, January 22ndBright moon outside, 40 degrees, and lots of fluffy white clouds racing across the sky. It’s been raining all day and pretty much quit a few hours ago, but every now and then a brief shower comes out of those clouds. It’s rained here so much the past two and half months that it doesn’t seem to know how – or when – to quit.So – the question is, do I chance it or not? It wouldn’t be the first time I got rained on while trying to get a quick look – but it’s not something I recommend for a telescope!After getting a scope ready in the house and debating with myself for another half hour, I decide to risk it.So out I go onto a very wet deck with a 72mm AstroTech refractor mounted on an old Polaris mount and a stout wooden tripod. I have four eyepieces in bolt cases stowed in my pockets: 16.8mm, 10.5mm, and 7mm Farworski Orthos, and a Take 5mm LE.I start a quick tour of the sky, guided by wherever the holes in the clouds are, and after about thirty minutes of that, I’m beginning to think about giving up since the clouds aren’t. But as I turn around towards the north, what do I see twinkling back at me but Mizar!Aha! A huge hole in the clouds, too!So I spin the scope around to the north quickly, get lined up, look into the 16.8mm eyepiece (26x), and there it is – Mizar A and B just barely split, with the moonlit sky peaking out between them, and Alcor over in the other corner of the eyepiece.I shift gears to the 10.5mm (41x) and take another look. Still a pleasing sight, but I prefer a bit more distance between the Mizar pair, and I swap the 7mm (61x) into the diagonal.Wow! I drop into my observing chair and just stare – Mizar has me mesmerized now. Actually, to be fair, it’s the scope that had done it.I haven’t spent much time with this dimunitive little devil because the weather didn’t cooperate for about five weeks after I got it, and then I used it mainly as an over-sized finder on top of another scope. So I’m just beginning to get familiar with this small white tube.What amazes me is how it’s both a great wide field scope and a great double star scope. Usually those two things aren’t done well by a single scope. But this one does both – extremely well. A 25mm Plossl gives me almost a three degree field of view and 17x. Star fields at that magnification are just ravishing. The dimmer ones are like the heads of silver pins gleaming from the top of a pin cushion – just very fine, brilliant points of light.And doubles at high magnification in it leave me searching for words to describe the experience – as is the case now when I place the 5mm Tak (86x) in the diagonal. The bright sky background, caused by the moon and also by the moisture in the air which is scattering its light, makes both the Mizar pair and Alcor look white – and the contrast between those startlingly white globes of light and the bright sky has me completely captivated. I sink into my chair and don’t move from it for about fifteen minutes.