Disapearing Galilean moons

[ian61]

This is probably going to seem odd BUT… As a science teacher I am trying to follow through classic experiments – one I remember from my youth was that it should be possible to plot the positions of the Galilean moons of Jupiter nightly and thus deduce their orbital periods and approximate orbital radius by plotting on graph paper.  The last bit is fine, I can handle circular/simple harmonic motion and graph in a spread sheet but I have hit two problems…

A Estimating distance – I have been trying to use planetary diameters as this is a constant that is independent of magnification.  This is fine as long as the distance is quite small but I find at about 4/5 I start to struggle are there any tips out there?

B I think I am losing the moons more than I should be. I started looking on 13^th May and up to the 15^th saw all four easily at low magnification on the 16^th things were a bit hazy, I think I have 4 but 17^th  definitely only 3. On 18^th all 4 clearly visible, ditto 20^th, but 23^rd only 2 and 25^th only 2. Last night 30^th in a gap just before the clouds formed a grey dome above us I again saw all 4 quite easily.  Now there are two obvious explanations, not mutually exclusive but of course I will not see them in transit or if they get too far away – According to my Norton’s Star Atlas Callisto averages about 26 planetary radii (13 diameters away) but I should be able to see that easily as the plane of the orbits is quite close to being in line with the visible banding. The other moons are all much closer.  I can’t believe they are hiding behind the planet quite so much or am I missing something obvious here?

Ian

[Davey-T]

Stellarium shows the moons .

Dave

[Stu]

The moons either transit or are occulted by Jupiter once each orbit so it is quite common for less than 4 moons to be visible.

Io has an orbital period of 1.77 days, Europa 3.57 days, Ganymede 7.16 and Callisto 16.69 days according to SkySafari. So the inner moons are the ones which are most often not easily visible.

Transits can be seen, particularly near the limb of Jupiter which is darker. Callisto is also very dark so appears to be very similar to a shadow transit when in front of the planet.

Hope that helps a little.

Stu

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[jabeoo1]

S&T always print this excellent diagram.  Here they are all 'hiding'.

[D4N]

To get the distance I would image it.

Get a cheap web cam and remove the lens, either get an adaptor or glue some 1.25" inch pipe to the front so it will slot into the focuser tube.

Just make sure you get the exposure right to see the moons without turning the disc into a blob. Or take a low exposure clip to get accurate disc diameter and a higher one to show the moons. Then combine the two images.

Remember that the apparent size of the planet will change over time so you can't directly compare pictures taken too far apart unless you adjust the scale but you can still make individual measurements.

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[ian61]

I suppose I just havn't yet got my head around how fast that these moons are actually travelling.

Just loaded Stellarium - looks great once I figured out how to stop it moving in real-time, but are those positions true - I suppose these days its a simple thing to do to get such things correct with access to the data.

Thanks,

Ian

[D4N]

Stellarium is very accurate so long as you tell it the right time and position.

On point B remember that not only will the moons pass in front of and behind Jupiter but they will also pass through the planets shadow at which point they will be far to dim to see.

The orbits of the Galilean moons are known very accurately. In fact observing their transits used to be a good way of determining longitude before we got accurate chronometers.

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[Owmuchonomy]

The orbital periods of the inner 3 moons are perfectly synchronised in the ratio of 1:2:4 as indicated above. SkySafari and an App called Jupiters Moons for IOS devices produce remarkably accurate simulations validated by visual observation. They can even flip the view to suit your scope type.

[ian61]

Many thanks for all the replies I am getting - I am certainly building up my software collection! I used Stellarium to look back in time and it definitely is correct as its pictures married up with my sketches wonderfully.  You can't see it for real but it is amazing how fast they are going if you go back or forward even an hour or so you can see the difference. Brilliant!

Ian

[Davey-T]

On 3rd June starting around 16.20 is a rare triple shadow event, Callisto, Ganymede and Europa's shadow on the disc, worth trying to image.

Dave

Not in SGL calender, bit of an oversight 

[Stu]

On 3rd June starting around 16.20 is a rare triple shadow event, Callisto, Ganymede and Europa's shadow on the disc, worth trying to image.

Dave

Not in SGL calender, bit of an oversight

Should be sorted now, thanks for pointing it out. Hope it's clear!!

[Avocette]

+1 for S&T's Jupiter's Moons App on iPhone and iPad. They also publish a Saturn's Moons App.