Ganymede transits coming up

[Stu]

Having seen a nice moon and shadow transit of Io last night, I was reminded that whilst Io and Europa appear bright against the Jovian surface, Ganymede and Callisto appear dark, so are much easier to follow past the darkened limb. I often find it all but impossible to track Io or Europa once they are in the brighter central portion of the planet.

Anyway, I recall once, many moons ago (🤪) I watched a triple shadow transit, and was surprised to see what I thought was a fourth shadow, but it turned out to be Callisto. On closer inspection, it appeared to be not as dark or as sharply defined as the shadows although conditions were pretty poor as I recall. The fact that it has a low albedo means that it is much easier to track across the disk.

Actually I just checked and the event was nearly nine years ago! 24th January 2015 in the early morning. Must have been mad!

So, what is my my point I hear you ask, do hurry up man!! Well, apparently there are no transits of Callisto until 2025 (please prove me wrong someone 🙏), but there are some of Ganymede coming up which will be worth looking out for.

25th November 2023 between midnight and 1.19am (actually starting in 24th at 11.58pm)

2nd December 2023 between 3.17am and 4.44am - LOW

6th January 2024 between 9.04pm and 10.56pm (the most convenient one by far, so bound to be cloudy!)

14th January 2024 between 12.54am and 2.49am - LOW

The November 25th and Jan 6th are best in terms of Altitude, both around 45 degrees at transit.

So, if you want to track a Galilean moon right across Jupiter, do it the easy way and choose a dark one to follow. Let us know if you have any luck with this 👍

[John]

Very interesting 🙂

I think it also makes a difference at what exact latitude on Jupiter that the transit occurs on and the levels of contrast of the surface features beneath the moon as it transits. 

For example, a few years ago the SEB more or less vanished leaving the GRS "hanging" against a pale surface. The disk of a moon transiting at that jovian latitude may have been harder or easier to see against that pale surface than it would have been against the darker SEB that would normally have been running across the disk (as it is currently).

I think we can forecast quite accurately the latitude that a moon disk will transit at but maybe not so much the extent / variation in surface features against which the moon will transit ?

 

Edited November 7, 2023 by John

[Stu]

12 minutes ago, John said:

Very interesting 🙂

I think it also makes a difference at what exact latitude on Jupiter that the transit occurs on and the levels of contrast of the surface features beneath the moon as it transits. 

For example, a few years ago the SEB more or less vanished leaving the GRS "hanging" against a pale surface. The disk of a moon transiting at that jovian latitude may have been harder or easier to see against that pale surface than it would have been against the darker SEB that would normally have been running across the disk (as it is currently).

 

Very true! Let’s see how these pan out. They are against the polar regions which are a little darker but should still stand out.

I’ve just found this fact sheet which actually puts Callisto much darker than Ganymede, which means these transits may be harder to spot though still worth trying for I think.Callisto seems to be 0.19, Ganymede 0.44, With Jupiter at 0.53 I think. Io is 0.62 and Europa 0.68

https://nssdc.gsfc.nasa.gov/planetary/factsheet/joviansatfact.html

https://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html

[Stu]

Some more good info here, confirming that it should be visible due to its large size and albedo.

https://www.cloudynights.com/topic/493940-why-is-ganymede-so-dark/
 

🤞hopefully this won’t turn out to be a damp squib 🤣🤣

[John]

Well it's good to know which nights are likely to be cloudy over the coming 3 months ! 😁

[josefk]

5 minutes ago, John said:

Well it's good to know which nights are likely to be cloudy over the coming 3 months ! 😁

it's the (roller coaster of) hope that kills you 😄😪😄😪

[johnturley]

1 hour ago, Stu said:

aving seen a nice moon and shadow transit of Io last night, I was reminded that whilst Io and Europa appear bright against the Jovian surface, Ganymede and Callisto appear dark, so are much easier to follow past the darkened limb. I often find it all but impossible to track Io or Europa once they are in the brighter central portion of the planet.

Anyway, I recall once, many moons ago (🤪) I watched a triple shadow transit, and was surprised to see what I thought was a fourth shadow, but it turned out to be Callisto. On closer inspection, it appeared to be not as dark or as sharply defined as the shadows although conditions were pretty poor as I recall. The fact that it has a low albedo means that it is much easier to track across the disk.

 

 

 

 

 

 

Agree, last night the shadow of Io was very clearly visible through both my 14in Newtonian, and Esprit 150, but I couldn't make out the disc of Io on the Jovian disc through either instrument. I didn't however start observing Jupiter until around 23.00, so missed the part when Io was close to the limb, and more easily visible.

However I observed a transit/shadow transit of Ganymede a couple of months ago, and in this case the disc of Ganymede was quite easy to spot.

John 

Edited November 7, 2023 by johnturley

[Stu]

1 hour ago, josefk said:

it's the (roller coaster of) hope that kills you 😄😪😄😪

Much easier to abandon all hope before the event, that’s what I normally do!