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Posted by Nick Hudd at 14:39 on 2010 Nov 10
Can anyone tell me please just how accurate predictions are for the Galilean satellites? I have both Skymap and Starry Night, and have also consulted various other handbooks and magazines. They do not agree to within some minutes I notice. Also, this evening (10th November) some sources give an occultation of Ganymede with later re-emergence, followed after that re-emergence by an eclipse and re-emergence. Other sources give only the occultation disappearance followed by the re-appearance from eclipse. The latter I suspect to be wrong, on a superficial examination of angles etc (I am no mathematician). There will obviously be variation in orbits etc but what is an accurate source, and is there a website which is updated frequently for these phenomena (as we get data for comets, asteroids etc)?Nick Hudd. Tenterden, Kent.
Posted by Steve Holmes2 at 23:35 on 2010 Nov 12
I suspect that the accuracy of "Jupiter’s moons" predictions is mainly down to the accuracy of the programming platform used to produce them rather than any basic differences in the theory used. All "non-commercial" predictor programs seem to use the same set of equations and methods originally devised by Jean Meeus and so, all other things being equal, should give the same answers. The fact that they don’t (to some small degree) may be down to the differences between the maths libraries for Java, JavaScript, C+, QBASIC etc. The orbital parameters of the moons themselves will probably not change greatly, not least because their periods are locked together by gravitational resonances.Additional factors influencing the relative accuracy of such things as the timing of transits would be the precision to which the answer is specified (a difference of just 1sec could change a "rounded" time by a whole minute); whether the the latitude and longitude of the observer are taken into account; the model used to represent the shape of the globe of Jupiter; and whether the timings were relative to the leading edge, centre or trailing edge of the moon: this alone can make a difference of +/-1 minute.As to which source is the most accurate, this can of course only be determined by comparison with "real life"! However, I would certainly expect the predictions to be accurate to +/- 1 minute. On the other hand, one is not going to begin observing precisely at the moment an event is predicted to start, and so the fact that different sources differ by a few minutes is surely not of any practical consequence. For truly accurate data one must consult sources deriving from internationally acknowledged organisations. For example, the BAA Handbook takes its timings from the French Institut de Mechanique Celeste et de Calcul des Ephemerides. These organisations use much higher precision calculations than "Meeus-type" programs but often supply their data only in tabular format, not usually online, and not interactively.There are many places on the Internet where predictions for the Galileans can be obtained (for example the ‘Sky & Telescope’ website) but the reason that sites giving ephemeris information for comets, asteroids etc. are perhaps easier to find is surely that the orbits for such bodies are not always well-known, being liable to change due to perturbations and as new observations come in. Reliable long-term models for them are thus not possible.I myself wanted to make some observations of shadow-transit events at the recent close opposition, so I also had to rely on timing predictions. My usual source is the (rather old!) program "The Planets" but I also use a number of other such programs as well as Starry Night. The predictions made by these programs were consistent to about +/- 1 minute but, as stated in the post, not all programs identified the occultation and eclipse events correctly – this is presumably down to the skill of the programmer though, rather than any output accuracy issues.Before composing this reply I investigated other programs on the Internet and found a couple that seemed quite good. "Galileo" and "Jupiter2" cover basically the same ground as each other, though each has its own "unique selling points". Their predictions are identical to each other (bearing in mind that Galileo does not use seconds whereas Jupiter2 does), and when I checked several transit events from the BAA Handbook for 2011 their predictions were exactly as listed there. They cannot, however, produce lists of satellite events (though Galileo will find the next one) so for that I return to "The Planets" – this will list not only all events for a given period but also only those actually visible from a given location.The Planets may be found at http://www.cpac.org.uk/download.aspGalileo may be found at http://astrosurf.com/durey/galileo.htmlJupiter2 may be found at http://www.astrosurf.com/rondi/programmes/index.htmHope this is helpful! Steve Holmes Laxfield, Suffolk UK
Posted by Nick Hudd at 08:17 on 2010 Nov 14
Thanks very much Steve. That is very useful and informative. I have been investigating this myself, and had discovered the Sky and Telescope site which has a great deal of handy data for observing. I have known about the site for ages but never explored it properly.I am not an experienced observer but my eyes tell me that eclipses and emergences take a noticeable time. My asking for the "best" prediction is largely down to having an enthusiastic "apprentice" (9), and making sure that (without saying what is going to happen) she gets the telescope for the relevant minute or two!! The eclipses and emergences particularly are an excellent demonstration, for a bright kid, of the relation of Jupiter, Sun, and Earth, and, indeed, of the relation of the satellites to each otherMany thanksNick
Posted by Steve Holmes2 at 17:06 on 2010 Nov 14
The "transitions" do indeed take a finite time – a couple of minutes. I recently took some images of a double re-emergence of Io and Europa and they both gradually brightened before becoming full visible. A good illustration of how big the Galileans are.Nice to see you have a 9yr-old interested in these things!Best regards, Steve
Posted by Nick Hudd at 10:55 on 2010 Nov 15
It has at least got her Mum a bit more academic street cred. To the "apprentice" Mum was just a maths teacher, but Mum actually has a PhD in physics, which daughter only realised through conversations with me about astronomy!Nick
Posted by Steve Holmes2 at 12:21 on 2010 Nov 15
I’m moving in the opposite direction – my 80+ yr-old mother is absolutely fascinated by all things celestial and so takes great delight in even my modest efforts. I think it’s what they call "outreach"!Steve
Posted by Nick Hudd at 17:48 on 2010 Nov 15
The one I’ve got involved outreach when the child found out that grandma could point out Ursa major, Cassiopeia and the area round Polaris for her as well. It began as an exercise in getting her Brownie badge in stargazing, but she never bothered with that and has got gripped by the astronomy itself. Nick
Posted by Steve Holmes2 at 14:57 on 2010 Nov 16
I’m not sure how late 9yr-olds are allowed to stay up these days but if you get a clear night on Wednesday (17th) and can avoid hypothermia (!), your "apprentice" might be interested in the disposition of Jupiter’s Galileans that evening. From 21:15 until 23:06, there will only be one moon visible! (Callisto – a long way to the east). Io is either occulted or in eclipse, Europa is in transit (as is its shadow) and Ganymede is occulted. The start is defined by Ganymede disappearing behind Jupiter, the end by Europa appearing again (both at the western limb). Io comes out of eclipse at 23:37 and Ganymede re-appears at the eastern limb at 00:15. [All times UTC]While such "only one moon visible" events are not particularly rare they are definitely unusual so should be worth observing. It could also be the start of a discussion about how often such an event should occur. With randomly-chosen orbit periods such a line-up would be very rare but of course the periods of the inner three Galileans are locked together by gravitational resonance and so line-ups actually happen much more frequently.Clear skies!Steve
Posted by Nick Hudd at 09:16 on 2010 Nov 17
I have been trying to catch some such events for her – exactly the reason for my original query, but as you say, the time of evening is a limitation for a child. Last week, there was a transit of Europa followed by shadow (easier to see than Europa) and occultations of Io and Ganymede, but later on Io came out of eclipse while Ganymede emerged from occ then went into eclipse then reappeared. As Ganymede was eclipsed about where Io emerged (in line of sight that is) it provided a striking example of the shadow extending diagonally away from where we are. Regrettably, she couldn’t come that evening and it would have meant probably an earlier and a very late session for her.Nick
Posted by Steve Holmes2 at 16:42 on 2010 Nov 17
I had also noticed the sequence of events you mention but circumstances prevented me from observing. This was also the previous "only one visible moon" [night-time] event, as you say. Looks as though the weather isn’t going to be kind this evening so we’ll have to wait another week. The next night-time event starts at 1:00am on 25th Nov, until 1:32am – not really a "daughter friendly time" though! That’s it for the current "season" however, as after 7 more days the situation is close but not quite there.Fascinating stuff!Steve
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