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

CC BY-SA 4.0
2021-12-31T18:54:05.693Z
"...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. https://en.wikipedia.org/wiki/Galilean_moons#Determination_of_longitude
uhoh
CC BY-SA 4.0
2021-12-31T18:59:12.877Z
@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.
giardia
CC BY-SA 4.0
2021-12-31T19:00:58.633Z
I'm suggesting that this is a necessary part of an answer to the OP's question as asked.
uhoh
CC BY-SA 4.0
2021-12-31T19:04:41.197Z
Yes the thermal properties of the surfaces of the moons can be studied that way, similar to what's done with Mercury. You might add this technique as an additional answer to [Can James Webb take pictures of our solar system? In what ways will they differ from/compliment those from Hubble?](https://space.stackexchange.com/q/57492/12102) cf [Why the thermal imaging of Mercury's surface requires a telescope on a jet flying through an eclipse?](https://space.stackexchange.com/q/22411/12102) and [Friendly temperatures at day/night junction in Mercury?](https://space.stackexchange.com/q/33397/12102)
uhoh
CC BY-SA 4.0
2021-12-31T19:15:47.160Z
thanks for these pointers !! this is a fun and informative stie
giardia
CC BY-SA 4.0
2021-12-31T19:41:19.773Z
...and your posts make it *even more fun and informative!*
uhoh
CC BY-SA 4.0
2022-01-01T17:49:43.323Z
This is a really great answer @giardia, thank you!  I'm surprised there's enough of an "offset" for an eclipse and occultation to be distinct events.  Very interesting.  I suppose the only time when they're NOT distinct is when Jupiter and Earth are at opposition.
prideout
CC BY-SA 4.0
2021-12-31T18:46:31.403Z
2021-12-31T19:08:31.650Z
2021-12-31T19:08:31.650Z
giardia
<p>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?</p>
<p>For example, maybe there's some sort of visual phenomenon that I could observe during the entry (or exit) of the event.</p>

CC BY-SA 4.0
2021-12-31T04:01:28.060Z
Include spec of the equipment for better answer
Kavin Ishwaran
CC BY-SA 4.0
2021-12-31T05:08:38.793Z
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.
Pierre Paquette
CC BY-SA 4.0
2021-12-31T13:36:20.827Z
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.
JohnHoltz
CC BY-SA 4.0
2022-01-01T23:40:15.260Z
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.
uhoh
CC BY-SA 4.0
2022-01-01T23:57:39.130Z
[Do we have enough tags to "span the space" of astronomical eclipses and transits? Is something missing or needs changing?](https://astronomy.meta.stackexchange.com/q/828/7982)
uhoh
<jupiter><eclipse-non-earth-related><io><transits-other-than-sun>
Can you distinguish a transit from an eclipse when observing Galilean moons?
prideout
# Can you distinguish a transit from an eclipse when observing Galilean moons?

(Asked by: username_0 on 2021-12-31)

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.

**Tags:** jupiter, eclipse-non-earth-related, io, transits-other-than-sun

> **Comments:**
> **username_1**: Include spec of the equipment for better answer  
> **username_2**: 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.  
> **username_3**: 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.  
> **username_4**: 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.  
> **username_4**: [Do we have enough tags to "span the space" of astronomical eclipses and transits? Is something missing or needs changing?](https://astronomy.meta.stackexchange.com/q/828/7982)  

---

## 1 answers

### Answer by username_5 on 2021-12-31. Score: 2 | Accepted answer

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.

> **Comments:**
> **username_4**: "...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. https://en.wikipedia.org/wiki/Galilean_moons#Determination_of_longitude  
> **username_5**: @username_4 - 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.  
> **username_4**: I'm suggesting that this is a necessary part of an answer to the OP's question as asked.  
> **username_4**: Yes the thermal properties of the surfaces of the moons can be studied that way, similar to what's done with Mercury. You might add this technique as an additional answer to [Can James Webb take pictures of our solar system? In what ways will they differ from/compliment those from Hubble?](https://space.stackexchange.com/q/57492/12102) cf [Why the thermal imaging of Mercury's surface requires a telescope on a jet flying through an eclipse?](https://space.stackexchange.com/q/22411/12102) and [Friendly temperatures at day/night junction in Mercury?](https://space.stackexchange.com/q/33397/12102)  
> **username_5**: thanks for these pointers !! this is a fun and informative stie  
> **username_4**: ...and your posts make it *even more fun and informative!*  
> **username_0**: This is a really great answer @username_5, thank you!  I'm surprised there's enough of an "offset" for an eclipse and occultation to be distinct events.  Very interesting.  I suppose the only time when they're NOT distinct is when Jupiter and Earth are at opposition.  

---

