Rare Triple Transit! There’ll be 3 Moon Shadows on Jupiter on January 24th, 2015

Play the skywatching game long enough, and anything can happen.

Well, nearly anything. One of the more unique clockwork events in our solar system occurs this weekend, when shadows cast by three of Jupiter’s moons can be seen transiting its lofty cloud tops… simultaneously.

How rare is such an event? Well, Jean Meeus calculates 31 triple events involving moons or their shadows occurring over the 60 year span from 1981 to 2040.

But not all are as favorably placed as this weekend’s event. First, Jupiter heads towards opposition just next month. And of the aforementioned 31 events, only 9 are triple shadow transits. Miss this weekend’s event, and you’ll have to wait until March 20th, 2032 for the next triple shadow transit to occur.

Hubble spies a triple shadow transit  on March 28th, 2004 . Credit: NASA/JPL/Arizona.
Hubble spies a triple shadow transit on March 28th, 2004 . Credit: NASA/JPL/Arizona.

Of course, double shadow transits are much more common throughout the year, and we included some of the best for North America and Europe in 2015 in our 2015 roundup.

The key times when all three shadows can be seen crossing Jupiter’s 45” wide disk are on the morning of Saturday, January 24th starting at 6:26 Universal Time (UT) as Europa’s shadow ingresses into view, until 6:54 UT when Io’s shadow egresses out of sight. This converts to 1:26 AM EST to 1:54 AM EST. The span of ‘triplicate shadows’ only covers a period of slightly less than 30 minutes, but the action always unfolds fast in the Jovian system with the planet’s 10 hour rotation period.

The view at 6:41 UT/1:41 AM EST. Credit: Created using Starry Night Education software.
The view on January 24th at 6:41 UT/1:41 AM EST. Credit: Created using Starry Night Education software.

Unfortunately, the Great Red Spot is predicted to be just out of view when the triple transit occurs, as it crosses Jupiter’s central meridian over three hours later at 10:28 UT.

The moons involved in this weekend’s event are Io, Callisto and Europa. Now, I know what you’re thinking. Seeing three shadows at once is pretty neat, but can you ever see four?

The short answer is no, and the reason has to do with orbital resonance.

The orbital resonance of the three innermost Galilean moons. (Credit: Wikimedia Commons).
The orbital resonance of the three innermost Galilean moons. (Credit: Wikimedia Commons).

The three innermost Galilean moons of Jupiter (Io, Europa and Ganymede) are locked in a 4:2:1 resonance. Unfortunately, this resonance assures that you’ll always see two of the innermost three crossing the disk of Jupiter, but never all three at once. Either Europa or Ganymede is nearly always the “odd moon out.”

To complete a ‘triple play,’ outermost Callisto must enter the picture. Trouble is, Callisto is the only Galilean moon that can ‘miss’ Jupiter’s disk from our line of sight. We’re lucky to be in an ongoing season of Callisto transits in 2015, a period that ends in July 2016.

Perhaps, on some far off day, a space tourism agency will offer tours to that imaginary vantage point on the surface of one of Jupiter’s moons such as Callisto to watch a triple transit occur from close up. Sign me up!

Jupiter currently rises in late January around 5:30 PM local, and sets after sunrise. It is also well placed for northern hemisphere observers in Leo at a declination 16 degrees north . This weekend’s event favors Europe towards local sunrise and ‘Jupiter-set,’ and finds the gas giant world well-placed high in the sky for all of North America in the early morning hours of the 24th.

2AM local Credit: Stellarium.
Jupiter rides high to the south at 1:45 AM EST for the US East Coast. Credit: Stellarium.

Look closely. Do the shadows of the individual moons appear different to you at the eyepiece? It’s interesting to note during a multiple transit that not all Jovian moon shadows are ‘created equal’. Distant Callisto casts a shadow that’s broad, with a ragged gray and diffuse rim, while the shadow of innermost Io appears as an inky black punch-hole dot. If you didn’t know better, you’d think those alien monoliths were busy consuming Jupiter in a scene straight out of the movie 2010.  Try sketching multiple shadow transits and you’ll soon find that you can actually identify which moon is casting a shadow just from its appearance alone.

The orientation of Earth's nighttime shadow at mid-triple transit. Credit: Created using Orbitron.
The orientation of Earth’s nighttime shadow at mid-triple transit. Credit: Created using Orbitron.

Other mysteries of the Galilean moons persist as well. Why did late 19th century observers describe them as egg-shaped? Can visual observers tease out such elusive phenomena as eruptions on Io by measuring its anomalous brightening? I still think it’s amazing that webcam imagers can now actually pry out surface detail from the Galilean moons!

Photo by author.
The 2004 triple shadow transit. Photo by author.

Observing and imaging a shadow transit is easy using a homemade planetary webcam. We’d love to see someone produce a high quality animation of the upcoming triple shadow transit. I know that such high tech processing abilities — to include field de-rotation and convolution mapping of the Jovian sphere — are indeed out there… its breathtaking to imagine just how quickly the fledgling field of ad hoc planetary webcam imaging has changed in just 10 years.

The moons and Jupiter itself also cast shadows off to one side of the planet or the other depending on our current vantage point. We call the point when Jupiter sits 90 degrees east or west of the Sun quadrature, and the point when it rises and sets opposite to the Sun is known as opposition.  Opposition for Jupiter is coming right up for 2015 on February 6th. During opposition, Jupiter and its moons cast their respective shadows nearly straight back.

Did you know: the speed of light was first deduced by Danish astronomer Ole Rømer in 1671 using the discrepancy he noted while predicting phenomena of the Galilean moons at quadrature versus opposition. There were also early ideas to use the positions of the Galilean moons to tell time at sea, but it turned out to be hard enough to see the moons and their shadows with a small telescope based on land, let alone from the pitching deck of a ship in the middle of the ocean.

And speaking of mutual events, we’re still in the midst of a season where it’s possible to see the moons of Jupiter eclipse and occult one another. Check out the USNO’s table for a complete list of events, coming to a sky near you.

And let’s not forget that NASA’s Juno spacecraft is headed towards Jupiter as well., Juno is set to enter a wide swooping orbit around the largest planet in the solar system in July 2016.

Now is a great time to get out and explore Jove… don’t miss this weekend’s triple shadow transit!

Read Dave Dickinson’s sci-fi tale of astronomical eclipse tourism through time and space titled Exeligmos.

Seeing in Triplicate: Catching a Rare Triple Shadow Transit of Jupiter’s Moons

The planet Jupiter is always fascinating to watch. Not only do surface features pop in and out of existence on its swirling cloud tops, but its super fast rotation — once every 9.9 hours — assures its face changes rapidly. And the motion of its four large Galilean moons is captivating to observe as well. Next week offers a special treat for well-placed observers: a triple shadow transit of the moons Callisto, Europa and Ganymede on the evening of June 3rd.

The view at 19:00 UT/3:00 PM EDT on June 3rd. Credit: Starry Night Education Software.
The view at 19:00 UT/3:00 PM EDT on June 3rd. Credit: Starry Night Education Software.

Now for the bad news: only a small slice of the planet will witness this rare treat in dusk skies. This is because Jupiter starts the month of June 40 degrees east of the Sun and currently sets around 11 PM local, just 3 hours after local sunset. Never fear, though, it may just be possible to spy a portion of this triple transit from North American longitudes with a little careful planning.

The action begins on June 3rd at 15:20 Universal Time as Callisto’s shadow slides on to the disk of Jupiter, to be followed by Europa and Ganymede’s shadow in quick succession hours later. All three shadows are cast back onto the disk of Jupiter from 18:05 to 19:53 UT, favoring European and African longitudes at sunset.  The final shadow, that of Ganymede, moves off the disk of Jupiter at 21:31 UT.

The hemisphere of the Earth facing towards Jupiter from the beginning of the triple shadow transit to the end. the red line marks the day/night terminator. Credit: Stellarium.
The hemisphere of the Earth facing towards Jupiter from the beginning of the triple shadow transit to the end. the red line marks the day/night terminator. Credit: Stellarium.

The following video simulation begins at around 15:00 UT just prior to the ingress of Callisto’s shadow and runs through 22:00 UT:

Triple shadow transits of Jupiter’s moons are fairly rare: the last such event occurred last year on October 12th, 2013 favoring North America and the next won’t occur until January 24th, 2015. Jean Meeus calculated that only 31 such events involving 3 different Jovian moons either transiting Jupiter and/or casting shadows onto its disk occur as seen from Earth between 1981 and 2040. The June 3rd event is also the longest in the same 60 year period studied.

The 1:2:4 orbital resonance of the Jovian moons Io, Europa and Ganymede. Credit: Wikimedia Commons.
The 1:2:4 orbital resonance of the Jovian moons Io, Europa and Ganymede. Credit: Wikimedia Commons.

Can four shadow transits occur at once? Unfortunately, the answer is no. The inner three moons are in a 1:2:4 resonance, meaning that one will always be left out of the picture when two are in front. This also means that Callisto must be included for any triple shadow transit to occur. Next week’s event sees Callisto, Europa and Ganymede crossing in front of Jupiter and casting shadows onto its disk while Io is hidden behind Jupiter in its enormous shadow. Callisto is also the only one of the four large Jovian moons that can “miss” the disk of Jupiter on certain years, owing to the slight inclination of its orbit to the ecliptic. Callisto thus doesn’t always cast a shadow onto the disk of Jupiter, and we’re currently in the middle of a cycle of Callisto shadow transits that started in July of 2013 and runs through July 2016. These “Callisto transit seasons” occur twice during Jupiter’s 11.8 year orbit, and triple shadow transits must also occur within these periods.

So, what’s a North American observer to do? Well, it is possible to spot and track Jupiter with a telescope in the broad daylight. Jupiter rises at around 9:20 AM local in early June, and the waxing crescent Moon passes 5.4 degrees south of it on June 1st. The Moon stands 30 degrees from the planet on June 3rd, and it may be juuusst possible to use it as a guide to the daytime event. A “GoTo” telescope with precise pointing will make this task even easier, allowing you to track Jupiter and the triple shadow transit across the daytime sky from North American longitudes. But be sure to physically block the blazing June Sun behind a building or structure to avoid accidentally catching its blinding glare in the eyepiece!

The orientation of Jupiter the Moon and the Sun at 4PM EDT on June 3rd. Credit: Stellarium.
The orientation of Jupiter, the Moon and the Sun at 4PM EDT on June 3rd. Credit: Stellarium.

Do the shadows of the moons look slightly different to you? A triple shadow transit is a great time to compare them to one another, from the inky hard black dot of the inner moons Europa and Io, to the diffuse large shadow of Callisto. With practice, you can actually identify which moon is casting a shadow during any transit just by its size and appearance!

A study of three multi-shadow transits: last year's (upper left) a double shadow transit from early 2014 (upper right) and 2004 (bottom. Photos by author.
A study of three multi-shadow transits: last year’s (upper left) a double shadow transit from early 2014 (upper right) and 2004 (bottom). Photos by author.

Shadow transits of Jupiter’s moons also played an interesting role in the history of astronomy as well. Danish astronomer Ole Rømer noted that shadow transits were being observed at slightly different times than predicted depending on the distance of Jupiter and the Earth, and made the first rough calculation of the speed of light in 1676 based on this remarkable insight. Celestial navigators were also intrigued for centuries with the idea of using the phenomena of Jupiter’s moons as a natural clock to gauge longitude. It’s a sound idea in theory, though in practice, it proved tough to make accurate observations from the pitching deck of a ship at sea.

Jupiter captured near the daytime Moon. Photo by author.
Jupiter captured near the daytime Moon. Photo by author.

Miss the June 3rd event? There’s still two fine opportunities to see Jupiter do its impression of the Earth-Moon system and appear to have only one satellite – Callisto – on the evenings of May 30th and June 7th.

From there, Jupiter slides lower into the dusk as June progresses and heads towards solar conjunction on July 24th.

Let us know if you manage to catch sight of this rare event!

-Send those shadow transit pics in to Universe Today at our Flickr forum.