Jupiter double transits - 30th December and 6th January

[Pierre_C]

Hi all.

I am relatively new to planetary observing.

I have noticed there is a double transit of Jupiter’s moons (Io and Ganymede) on the evening Monday 30 December, but it happens before civil twilight begins in Eastern Australia. There are also double shadows before and during civil twilight, but the double act ends before nautical twilight begins.

Would it be worth looking at from an urban location, or would the amount of light in the sky make it too difficult to see?

I will be using a standard 8-inch Dobsonian telescope.

The AEDT times are:

7:08 Io transit ingress
7:43pm Io shadow ingress
9:20pm Io transit egress
9:55pm Io shadow egress



6:24pm Ganymede transit ingress
8:28pm Ganymede transit egress
8:37pm Ganymede shadow ingress
10:52pm Ganymede shadow egress



9:15pm civil twilight begins in Melbourne
9:54pm nautical twilight begins in Melbourne
10:38pm astronomical twilight begins in Melbourne


Thanks for your thoughts.

[TimHarbour]

Hi mate.
I live in Bortle 2-3 skies in Wauchope NSW.
Last week I was observing Jupiter through our 10” dob and there was no problem seeing Jupiter’s moons as they’re quite bright.
When I added the Barlow lens to my 9mm plossl I noticed quite clearly the shadow of one of its moons on the surface. The moons/shadows transition across very quickly under observation.
You may find the more limiting factor to be the atmospheric seeing conditions that will limit the magnification you can use, which can change through the evening too.
At least you’re armed with the info of it happening? I only found it by chance.
Where did you get the info for Monday?
Regards, Tim.

[Pierre_C]

Thanks, Tim.

I found the local times in the Jupiter Moon Events tables in the back of ‘Astronomy 2024 Australia’ and augmented them with adjusted UTC times from the Royal Astronomical Society of Canada’s ‘Observer’s Handbook 2024’. I recommend both books if you are interested in this type of detail.

I obtained the local twilight times from the ‘Sky Tonight’ app.

I have since received a copy of ‘Astronomy 2025 Australia’ for Christmas and have found another double act on Monday 6 January that is later at night and may get around some of these twilight issues – a double transit and single shadow at around 10pm.

The AEDT times on Monday 6 January are:


8:54pm Io transit ingress
9:38pm Io shadow ingress
11:06pm Io transit egress
11:50pm Io shadow egress



9:45pm Ganymede transit ingress
11:50pm Ganymede transit egress



9:15pm civil twilight begins in Melbourne
9:54pm nautical twilight begins in Melbourne
10:37pm astronomical twilight begins in Melbourne


Quasar Publishing, Astronomy Australia 2025:
https://quasarastronomy.com.au/product/astronomy-2025/