Comments on: Two moons, circling

By: Mark Hansen

Mark Hansen — Thu, 27 Aug 2009 22:59:22 +0000

@kuhnigget (48),
Sorry to be posting late but;
The coalsack nebula was one thing I did enjoy looking out for when I drove freight trains from Sydney to Goulburn (NSW, Aust.). It wouldn’t be visible all the way out to Picton but as soon as you got to Tahmoor, there it was, a black patch that was blacker than the rest of the sky.
The Southern Cross is definitely a constellation (we don’t put any mere asterisms^* on our flag, thank you ) and readily visible even in capital cities here. Epsilon Crucis is usually not visible, however. Where I live (near Campbelltown, NSW) the skies are usually good enough to not only see E Crucis but right on the edge of visibility is NGC 4755 (the Jewelbox). The Great Nebula is also clearly a fuzzy blue cloud.

I’m surprised at you not recommending the Pleiades to Asimov Fan. There is an open cluster that lets you test how light polluted your skies are (and how good your eyes).

*_{For the pedants: I do realise that a constellation is also an asterism. What I mean is that a constellation is recognised as such, whereas an asterism is any image or pattern you care to trace out using whichever stars take your fancy. If you are still not happy with this, please take a number and wait for me to care.}

By: Plutonium being from Pluto

Plutonium being from Pluto — Tue, 25 Aug 2009 06:02:10 +0000

@ 53. bela okmyx :

Thanks for that!

By: Joe Meils

Joe Meils — Mon, 24 Aug 2009 16:08:30 +0000

Bless you, Phil… I was looking for a new astronomical subject to paint…

By: Two moons, circling « Xenophilia (True Strange Stuff)

Two moons, circling « Xenophilia (True Strange Stuff) — Mon, 24 Aug 2009 15:45:19 +0000

[...] via Two moons, circling | Bad Astronomy | Discover Magazine. [...]

By: Alan Kellogg

Alan Kellogg — Mon, 24 Aug 2009 05:31:28 +0000

Does anybody have the vaguest idea how far away Io passed Ganymede, damn it?

By: John Paradox

John Paradox — Sat, 22 Aug 2009 18:49:17 +0000

50. bkallee Says:

Someone remind me to close my mouth later… Jawdropping

Remember to close your mouth

(just feeling helpful today)

J/P=?

By: toasterhead

toasterhead — Sat, 22 Aug 2009 12:51:36 +0000

Why does Ganymede appear to darken as Io passes between us and it? Is it just an optical illusion, or is there a secondary shadow (Io’s penumbra, perhaps?) after the main shadow passes?

By: Vishnu

Vishnu — Sat, 22 Aug 2009 09:02:50 +0000

If Jupiter spins faster than Io, Ganymede or Europa can rotate, then Jupiter must be dragging them increasing their speeds. Then, why don’t their orbit gradually increase in size, as it happens with Earth’s Moon – as Moon moves away from Earth at a rate of a few centimeters every year. Or wait, does it happen them too? Or in general does it happen to any satellite whose time period around its parent planet is less than the time period of rotation of the planet itself.

By: Vishnu

Vishnu — Sat, 22 Aug 2009 08:57:20 +0000

I think this crossing would be repetitive as the Orbital periods of Io and Ganymede are in 1:4 resonance. They should have been crossing at the same point with respect to a reference frame fixed on Jupiter’s center. But yes, the transiting would be indeed a very rare event, as Io, Ganymede lining up with Earth is a different ball game altogether!

By: Bill Nettles

Bill Nettles — Sat, 22 Aug 2009 04:09:43 +0000

Since Ganymede has a larger orbital radius than Io, this pic had to be taken when Ganymede was on the back “half” of its orbit. Does anyone know whether Io was on the front side or the backside of its orbit? If it was frontside, that makes the sequence even more impressive because the two moons would have been passing in “opposite” directions rather than Io playing catchup (at 4X angular speed).