On 12th October this year (2013), a rare triple transit of Jupiter’s moons were visible as they glided across the globe of Jupiter’s upper atmosphere.
Jupiter completes a full rotation in 9 hours 55 minutes and this causes the gas giant to bulge due to the centrifugal forces of this rapid spin – an effect you can also see through a small telescope.
When any one of the larger of Jupiter’s 67 known moons – particularly one of the Galilean Moons, Io, Europa, Ganymede & Calisto – pass in front of the planet they can be seen together in alignment, known as a transit. These single moon transits happen very regularly and can be seen with a large scope most nights.
However, the moons also cast a shadow on the planet, depending on where the sun is in relation to the moon and the gas giant. This contrasts so well that the shadow can be seen as a small black dot on the sphere of Jupiter. And this makes it easier to see in smaller scopes.
I took this image of Jupiter and Ganymede on 3rd December 2013, the time of opposition – when the Earth was directly between the sun and Jupiter. And the image on the left is a reconstruction of a transit when the planet is far from opposition (I had clouded skies for the actual triple conjunction). Note how aligned the moon appears to the planet in the first image in order to cast a shadow, compared to the second image when it’s much further to the side in its orbit. Only the spherical nature of the planet prevents the moon and its shadow from being completely level at this point at opposition.
Fortunately, due to the internet having no regard for the time honoured tradition of clouds preventing astronomers from enjoying rare events, Kristoffer Åberg was able to take video footage of the triple conjunction, combine the individual frames to create a super-resolution image and send it to us online.
These images were taken in Kumla, Sweden just before dawn on 12th October 2012.
Kristoffer said of these captures:
I got up at 5am in the hope that conditions would be good for taking pictures of the event. By the time I had assembled all the equipment it was 6am and I made three different recordings with different exposure times. This resulted in a total of about 6,000 images. When I had finished, the sun began to rise and it became too light to continue.
After processing each clip separately in Registax 6, I was left with three separate images of the planet. These three were edited and joined in Photoshop and you can see the result in the picture.
The top is 300x magnification in RGB, but the moons appear relatively poorly here.
The middle image is digitally enlarged 2x with colour balance / contrast adjusted to accentuate the shadows and moons better.
The bottom image is a black and white version with annotation. I made an attempt to calculate the angular diameter to see how the resolution of the magnified image was. ~1400km/pixel are the enlarged versions. Furthermore, I even took the planet and moons magnitudes at the moment of imaging.
These triple transits are quite rare, occurring only once or twice a decade, so definitely an opportunity not to be missed if the conditions permit.
Double transits are more frequent and single transits happen on a nightly basis – depending on where you are in the world. But regardless of their frequency, whether a single or a triple transit, they’re great fun to observe.
If you want to image a Jovian transit for yourself, all you need is a scope (any scope over 3” will suffice), a goto mount (an alt az goto is fine as you will only take 1-2 minute videos so field rotation won’t matter), a webcam or planetary imaging camera and a Barlow lens to magnify the image. Have a look at our tutorials section for details on capturing and processing planetary images.