A Shadow on Jupiter


Image exposure:
25.4 ms per frame @ zero gain
Best 10% of 1500 video frames
Image date:
2021-09-01 (10.28 pm AEST)

Jupiter has nearly eighty natural satellites, the largest of which are the four Galilean Moons: Io, Europa, Ganymede and Callisto – all easily visible in binoculars or a small telescope. Io is rocky, inhospitable and highly active geologically with hundreds of active volcanoes, caused by the tidal forces induced by its proximity to Jupiter itself.

Because it orbits very close to Jupiter, Io’s orbit of the planet is quick and lively – a mere 1.77 days – and so it regularly transits across the face of Jupiter as seen from Earth. When this occurs, Io itself is not seen – but its shadow on Jupiter is very prominent to astronomers.

It just so happened that one of these transits was in progress while I was taking the video which I used to create the feature image above. The shadow is clearly visible on Jupiter and in the unlikely event that any creature existed on Jupiter, they would experience an eclipse of the Sun as the shadow passed across them.

I would like to say that this observation was planned but to be quite honest I’ve taken a number of shots of Jupiter over the years and never recorded a transit, so my success this time was due more to the law of averages than any foresight on my part.

I took a total of six videos over the course of about fifteen minutes and converted each to a single still image using Autostakkert software. I then combined the resulting six still images to a single gif image as seen below:

Image shows (a) Rotation of Jupiter; and (b) the moving shadow of Jupiter’s satellite Io. The Great Red Spot is also visible near the top of the planet.
This animation consists of images obtained from stacking six videos captured between 10.21 pm and 10.35 pm on 2021-09-01
Image: Roger Powell

Here is an animated orbital diagram borrowed from Wikimedia Commons, which show how the four Galilean Moon orbits are arranged:

The four Galilean moons of Jupiter:
Io (red); Europa (blue); Ganymede (Yellow); and Callisto (Cyan).

Orbital diagram sourced from Wikimedia Commons.

Here is a single still image which I took during Io’s transit of Jupiter. The transit is obscured because Jupiter was deliberately over-exposed to show the other three Galilean Moons:

Jupiter (over-exposed to enable capture of satellites).
Europa (lower), Callisto (near Jupiter) and Ganymede (top).

Io was in transit across Jupiter and is not visible in this image.
Image: Roger Powell
2021-09-01 at 10.40 pm (AEST)

Transits not only occur across Jupiter, they also occur across the face of the Sun, so here is an image I took of the Transit of Venus in 2012 to prove it.

Thanks for reading 🙃

Cosmic Focus Observatory

Above us only sky….

Telescope:SkyWatcher Esprit 120 mm apochromatic 3-element refractor; 840 mm f/l @ f/7.
Optics:Field flattener; Astronomik light pollution filter, 2x Powermate.
Mount & Guiding:SkyWatcher EQ6-R Pro mount.
Imaging camera:ZWO ASI 071 MC cooled.

Images © Roger Powell
I’m one of the founder members of Macarthur Astronomical Society and current webmaster.
Cosmic Focus Observatory, 34° S

Acknowledgements to the cartoonist.


    1. I know from some of your posts that you use a similar processing technique to me. I used this opportunity as a test to determine the optimal percentage of frames to stack in Autostakkert. I used 10%, 15%, 20% and 25% and when I compared the images, I struggled to find any difference between them!
      I ended up choosing 10%, for no particular reason other than my observation of the curve gradient on the frame quality graph.

      Liked by 1 person

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