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If I look at Jupiter through a small telescope (or binoculars) while Io is overlapping Jupiter, is it possible for me to visually distinguish a transit from an occultation?

For example, maybe there's some sort of visual phenomenon that I could observe during the entry (or exit) of the event.

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  • $\begingroup$ Include spec of the equipment for better answer $\endgroup$
    – Kavin Ishwaran
    Commented Dec 31, 2021 at 4:01
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    $\begingroup$ Welcome to AstronomySE! Let’s get some definitions straight, here… A transit is when the satellites passes in front of the planet, whereas an occultation is when the satellite is in Jupiter’s shadow. So, yes, the two events are rather easily distinguishable from one another. In an occultation, you’ll see the satellite until it disappears in the shadow, basically “into thin air.” In a transit, you might be able to see the satellite in front of the planet’s disk; however, you’ll see it until it’s right in front of Jupiter—and after it clears the disk as well. $\endgroup$
    – Pierre Paquette
    Commented Dec 31, 2021 at 5:08
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    $\begingroup$ An eclipse is when it goes into the shadow. An occultation is when it dissapears or reappears from behind Jupiter. The beginning or end of the occultation does not need to be in shadow. So the question is accurate. When Io is on the limb, is there a way to tell if it is the beginning or end of a transit or occultation. $\endgroup$
    – JohnHoltz
    Commented Dec 31, 2021 at 13:36
  • $\begingroup$ There are two votes for closing this question and blocking anyone from posting answers for "Needs details or clarity: This question should include more details and clarify the problem. (2)" I think this kind of voting is extremely counterproductive. voting to leave open! Leave helpful comments and answers but don't shut this question down completely. It's certainly clear enough and has an excellent, informative, thorough answer that the OP has accepted. Everything is okay here. $\endgroup$
    – uhoh
    Commented Jan 1, 2022 at 23:40
  • $\begingroup$ Do we have enough tags to "span the space" of astronomical eclipses and transits? Is something missing or needs changing? $\endgroup$
    – uhoh
    Commented Jan 1, 2022 at 23:57

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With a small telescope/binoculars, it will be difficult or impossible to resolve details on the scale of Io's disk (about 1 arcsecond). So you wouldn't be able to use shapes in the image to distinguish the two scenarios.

If you can attach a color filter, you might be able to distinguish between transit and occultation. Both Io and Jupiter are yellowish, so a color filter might not help much. But the other moons have more of a blue/grey color, especially compared to Jupiter. So a blue/green filter might make one of the other satellites appear to have a higher surface brightness than Jupiter, making it easier to see whether a transit is taking place.

Jupiter also has a big shadow. When it's far from opposition, a satellite may be in eclipse (invisible in the shadow) either before or after occultation (invisible behind the planet). Whether eclipse happens before or after occultation depends on the observing geometry. What it would look like is a satellite fading out early, before it reaches Jupiter's limb, or a satellite fading in at a position not quite touching the planet's limb.

Of course, eclipses never happen just before/after transits, so an eclipse is something that could potentially distinguish the two events.

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    $\begingroup$ "...maybe there's some sort of visual phenomenon that I could observe during the entry (or exit) of the event." Jupiter has a big shadow and as far as I remember, when it's far from opposition as seen from Earth the satellites will wink on or off well-resolved before touching Jupiter's limb on one side. en.wikipedia.org/wiki/Galilean_moons#Determination_of_longitude $\endgroup$
    – uhoh
    Commented Dec 31, 2021 at 18:54
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    $\begingroup$ @uhoh - Yes, far from opposition I think these eclipses can be close to an hour long before/after the satellite is actually occulted by the planet, depending on satellite. This is actually great news for JWST, which can ONLY observe solar system targets far from opposition, because some interesting science can be done when moons are in shadow. $\endgroup$
    – giardia
    Commented Dec 31, 2021 at 18:59
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    $\begingroup$ I'm suggesting that this is a necessary part of an answer to the OP's question as asked. $\endgroup$
    – uhoh
    Commented Dec 31, 2021 at 19:00
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    $\begingroup$ thanks for these pointers !! this is a fun and informative stie $\endgroup$
    – giardia
    Commented Dec 31, 2021 at 19:15
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    $\begingroup$ ...and your posts make it even more fun and informative! $\endgroup$
    – uhoh
    Commented Dec 31, 2021 at 19:41

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