The Dutch Observatory

January 19, 2015 on 7:57 pm | In Jupiter | Comments Off

The Tale of the Triple Shadow Transit
……… and a whole lot more……

Some of us are hoping for clear skies to see the  Triple Shadow Transit on Jupiter in the early hours of 1/24/2015.
However, that is only part of the story, and here is the rest.

The fun starts around midnight when the shadows of Io and Calisto are visible on Jupiter and the moon Io is already in front of the Jovian disk near the Eastern limb.  As time goes on you will notice that the distance between the shadows declines as they transit across.  This is because the shadow from Calisto, which is much farther away from Jupiter than Io, moves relative slower on the planetary disk than the shadow from Io. Around 0:40 AM, Io’s shadow will have caught up to the shadow of Calisto and they will merge to one shadow.  However at the same time, Calisto’s shadow ”beam” will also partially eclipse the moon Io, so Io will become partially dark, which shows up from our vantage point as a small shadow against Jupiter behind it.  This shadow will pop up out of nowhere, and is only visible for a few minutes.  In the meantime around 1:15 am, Io has moved ahead and has caught up with Calisto’s shadow, and now it will start to cover, and block our visibility of the shadow of Calisto on Jupiter.  Io will not totally cover the shadow, but the change of the ”dark” shadow should be very well noticeable.  Finally, at 1:30 I would expect Europa’s shadow as well as the moon Calisto have moved onto the planetary disk as seen from Earth and you have arrived at your “Triple Shadow Transit”.

So you thought you’d see three shadows only?  Now you know what to look for in the 90 minutes leading up to the Triple Shadow Transit.  And once you have seen this, think again what you just witnessed.  This is why I am so excited this year about the chance to see what those many “Mutual Events” of the Jovian Moons create for us to observe from Earth.
Complicated?  I’ve added some screen shots of what will happen to make it better understandable.
Clear Skies,
Theo Ramakers
Images generated with WinJUPOS.