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Here’s another astounding solar system image. The big guy in the picture is obviously Jupiter. But see that misshapen thing to its upper right? That’s Ganymede, its biggest Moon — in fact, Ganymede is slightly bigger than Mercury.
Ganymede, like any other body that big, is a sphere. So why does it look weird? Because, in that image, it was slipping into Jupiter’s shadow! If you had been standing on the moon’s surface at that time, you would see Jupiter in silhouette, blocking the Sun in your sky. It’s a solar eclipse, writ large. Very large: from Ganymede’s viewpoint Jupiter would be nearly 8 degrees across– 15 times larger than the Moon appears in our own sky! If I outstretched my arm, my palm would just cover Jupiter.
Wow.
That image above is just one of a series of images of that eclipse taken by gifted amateur astronomer Fabio Carvalho.
Looking at the whole series, I am reminded again that the planets in our sky are worlds like the Earth, places that exist, and not just points of light at night. They have moons, eclipses, weather, some even have seasons … and most amazing of all to me is that little disk of Ganymede. I grew up looking at that moon — and its three siblings Io, Europa, and Callisto– in my telescope, and they were never more than dots. It wasn’t until the Pioneer and Voyager probes that they became worlds in their own right. And now technology has advanced so much that even someone on Earth can spot their faces. You can even see some detail on Ganymede! Fantastic!
I’ll remind readers again that pictures like this, and many more, can be found on the Bad Astronomy and Universe Today bulletin board, too.’
July 12th, 2006 at 12:56 am
Hey man… very cool article… I added your feed to my google reader! I like the site a lot!
July 12th, 2006 at 2:58 am
Off-topic: Just FYI, with this post Feedburner began inserting advertisements into the RSS feed. The BA wants me to have a free Motorola Razr! Joy.
The meta tags emitted by the blog software itself, which e.g. Firefox uses to offer to automatically add the feed to its bookmarks, are still pointing at the no-longer-functional badastronomy.com/bablog/feed/atom/.
July 12th, 2006 at 4:40 am
I use:
feeds.feedburner.com/BadAstronomyBlog?format=usm
as the feed link with firefox. There should be an http:// in front of that.
July 12th, 2006 at 5:35 am
Oh!
What a site What a sight!
July 12th, 2006 at 6:24 am
There are also some gorgeous things over at the NASA/JPL Huygens-Cassini page. Some of the pictures of the moons and the rings are astounding.
http://saturn.jpl.nasa.gov/home/index.cfm
July 12th, 2006 at 6:31 am
How big would Jupiter appear if you were standing on Callisto? Show your work.
July 12th, 2006 at 11:28 am
When we had clear skies here (in Florida), I’d often watch Jupiter and spot what the moons were doing. The times for occulations and eclipses always seem to take place when it’s already daylight here.
I haven’t been able to get a view of Jupiter (or any other object) due to our rainy season. Darn.
BTW, isn’t there a site for seeing what a view of another planet from one of it’s moons would be? I’d wonder how Jupiter would look from the major moons.
I also wonder if on the night side, one could spot Jupiter’s ring.
July 12th, 2006 at 2:04 pm
Those are some very nice photos.
Kevin, the angle subtended by Jupiter at an observer on Callisto can be derived approximately from some simple trig. A line from the observer to Jupiter forms the adjacent side of a right-angled triangle whose opposite side is Jupiter’s radius. Therefore, the angle made by half of Jupiter is the tangent of d/r where r is Jupiter’s radius and d is the distance. From http://www.nineplanets.org I get Jupiter’s radius as 71,492 km and the distance between Jupiter and Callisto as 1,883,000 km (don’t know if this is to the centre of mass or to the cloud tops, but it is only about a 4% difference). Thus, the angle subtended by half of Jupiter at an observer on Callisto is tan(1883000/71492) = 0.5 deg, which suggests that Jupiter will be about 1 degree wide in the sky of Callisto.
Erm, can someone check my maths? I’m surprised it is so small (compared with the BA’s figure for the appearance of Jupiter from Ganymede).
July 12th, 2006 at 5:44 pm
Thanks for the info, Nigel. So based on that, I’m guessing Jupiter from Callisto would appear about twice as big as our Moon from Earth. I tried to determine Jupiter’s relative size by using Celestia, but I couldn’t tell if I did it right.
July 12th, 2006 at 6:58 pm
“If I outstretched my arm, my palm would just cover Jupiter. Wow.”
For most people, the ‘wow’ would be that Jupiter would appear so small, even up close. In astronomical paintings, you see planets or moons looming over a quarter of the sky. I once figured the angular size of Jupiter from the inner moons, and it was approximately the size of a basketball at arm’s reach. Disappointing.
July 13th, 2006 at 10:49 am
Correct me if I’m wrong, but wouldn’t a lunar eclipse be fairly common between Ganymede and Jupiter? I look at Jupiter a lot, and it seems like the four moons orbital plains are fairly close to the ecliptic (Unlike our erratic moon. Yikes!) So given Jupiter’s immense apparent diameter, and Ganymede’s short “month†(about 7 days by my observations) wouldn’t Jupiter pass between Ganymede and the sun about once every 7 days?
The reason I’m curious is that I have never seen one of Jupiter’s moons mysteriously “wink out†like in the picture. Does this mean eclipses are not common events in the Jovian system?
Anybody?
July 13th, 2006 at 11:16 am
By the way,
Nigel, I checked your math, you need to take the inverse tangent of 71398/1883000, not the tangent.
Start with right triangle CJS. Where C is Calisto, J is Jupiter’s center, S is Jupiter’s surface, and angle A is the angle formed between J and S as seen from Calisto.
Tan(A) = (71398 / 1883000)
Solve
Tan(A) = .0379
Inverse Tan of .0379
A = 2.17 Degrees
Then double that to find Jupiter’s apparent diameter. About 4.34 degrees.
So Jupiter is about 50% as wide as seen from Calisto, as compared to Ganymede. This makes sense since Calisto is not quite twice as far away from Jupiter.
Everyone makes mistakes in Trigonometry ☺
July 13th, 2006 at 12:28 pm
Great post. Jupiter is amazing.
July 13th, 2006 at 4:53 pm
Those are impressive images, at first I thought they were from Hubble. Unfortunatly on Ganymede there isn’t enough air to play Pink Floyd’s “Dark Side of the Moon”.
As far as a web site to see what it looks like to be standing on another world, I don’t know of one but Redshift 2, 3, 4 etc. allows you to put yourself on any other body in the solar system. Jupiter is quite conspicuous on its satellites.
July 14th, 2006 at 3:12 pm
Max,
OK, I think I can see where I went wrong. I think I actually made two mistakes (got adjacent over opposite instead of opposite over adjacent, so didn’t notice that I had done a tan on that ratio instead of the inverse tan). Thanks.
So, Kevin, Jupiter is around 4.5 degrees wide as seen from Callisto.
July 14th, 2006 at 5:07 pm
No problem, Nigel.
4.5 degrees, so about 8.5 times the size of the full moon.