University Lowbrow Astronomers

Shadows on Jupiter.

by Mark Deprest
Printed in Reflections: October, 2001.

Jupiter with two of its moons, Io and Europa

October 27, 2001 at 0:48 am the shadows of two of Jupiter’s moons (Io and Europa) cast their shadows on the face of Jupiter and transit in tandem with the Great Red Spot!  This is just one of many MSE’s (Multiple Shadow Events) in this year’s Jovian apparition.  This article by Mark Deprest describes these events and some of the dynamics behind them.  Graphics produced by Guide 7.0 by Project Pluto.

Early in the 1600’s Simon Marius and Galileo Galilei pointed the newly invented telescope toward the heavens and specifically at the planet Jupiter.  What they saw was both amazing and sacrilegious, there seem to be small objects “orbiting” this giant planet, actually casting shadows on the face of Jupiter.

Late in 2000 Rex Graham, Senior Editor of “Astronomy” magazine, is glancing through his copy of “The Royal Astronomical Society of Canada’s:  Observer’s Handbook 2001” edited by Rajiv Gupta, when he notices that in October of 2001 there will be 10 Double Shadow Transits (DST) of the Galilean moons.  To him, this seemed like a rather large number of Double Shadow Transits to all happen in one month, time to check with the experts, the best of these is Reta Beebe of New Mexico State University and discipline leader of the International Jupiter Watch:  Atmosphere Discipline.  A quick little e-mail to her asking for a source to confirm this phenomenon, which got forwarded to Mark Vincent, Ph.D. one of Beebe’s colleagues at NSMU and a former President of the University Lowbrow Astronomers of Ann Arbor, Michigan.  Vincent then forwarded the e-mail to the, then current President of the Lowbrows, me.  The e-mail went like this:

Hi Mark,
I suspect you’d have a good answer to this request from Astronomy Magazine to Reta Beebe to me to you....
Gotta Go, Meeting in a Minute,
-Mark Vincent

Dr. Beebe,
Sorry about bugging you with such a mundane question, but do you know what source would list shadow transits on Jupiter?  I ask because in October 2001 there will be 10 double-shadow transits, according to the new “Observer’s Handbook 2001,” edited by Rajiv Gupta.  Ten sounds like a lot, but I would like to check into it.
Rex Graham
Senior Editor, Astronomy Magazine

Well, I did have a very good source for confirming this info, and this is the kind of project, that I like to do.  I went to my favorite astronomy software Guide 7.0 by Project Pluto, for the info.  Guide 7.0 would provide me with a text file of all the Galilean satellite events for any period of days needed.  This file included the moon involved, the type of event (occultations, eclipses, transits, and shadow events), the date, and the start and end times of each event.  This was more information than I needed, but for just the one month, it wasn’t too difficult weed through the excess data, to confirm the 10 DST’s.  But, was this some kind of unusual event or was this something that happens on a regular basis?  When I started leafing back through older copies of the “RASC’s Observer’s Handbook,” I noticed that there was a pattern to these MSE’s (Multiple Shadow Events), that approximately every 13 months or so, an increase in the frequency of the MSE’s occurs.

So, I went back to Guide 7.0 and asked for a complete Jovian year (12 Earth years) worth of events, this produced a rather large file containing some 27,890 event records.  I needed to weed-out a lot of data this time, so that all that I would have remaining were the shadow events.  I used Microsoft Excel to create a spreadsheet that I could export into Microsoft Access, which I used to eliminate everything except the shadow events.  This created a file of 8896 records that included both the start and end times of each shadow event.  It was just a matter of picking out those events when the start of one shadow event was followed by the start of another shadow event without the end of the first.  With that done, I now had a spreadsheet that showed 271 DST’s and 3 TST (Triple Shadow Transits) for the Jovian year starting January 1st 1994 and running through December 31st 2005.  I was able to confirm this 13 month pattern which is known as “Shadow Season” and it can last for about 35 to 48 days.  During a shadow season Multiple Shadow Events can average a frequency of one every 2.36 days.  DST’s are the most common, but occasionally TST’s are possible, the last TST that happened was on November 11th 1997, and involved Io, Ganymede, and Callisto.  The next TST will occur on March 28th 2004 and the same three moons will be involved.

This list also produced an unusual event that I like to call an ODST or Overlapping Double Shadow Transit, which started on September 21, 1997 at 18:38 UT and lasting until 02:26 UT on September 22, 1997.

  1. Europa’s shadow first appeared on the face of Jupiter at 18:38 UT and Callisto’s shadow began its transit at 19:22 UT.
  2. Then Europa’s shadow left the face of Jupiter at 21:29 UT, only to be replaced at 22:46 UT by Ganymede’s shadow.
  3. The shadows of Ganymede and Callisto made their way across the face of Jupiter until 00:14 UT when Ganymede’s became the lone survivor of this “Shadow Dance.”  At 02:26 UT the shadow of Ganymede left the “dance floor.”
  4. While all this was going on the GRS (Great Red Spot) was making its cruise across the Jovian face, not to forget that the moons themselves transited also.  Now that would have been a real treat to observe.  But unfortunately from my point of view most of it happened while Jupiter was still below my horizon.

I’m sure by now that somebody is asking, is it possible for all four Galilean moons to cast their respective shadows on the face of Jupiter at the same time?  Answer:  No.  In Fred Price’s book, “The Planet Observer’s Handbook, he shows a wonderful little formula that pertains to the correlation between the mean daily orbits of Io, Europa and Ganymede.  That expression is:

Li + 2Liii - 3Lii = 180 degrees

Which basically means that if Io and Ganymede are transiting then Europa is behind Jupiter.  Price goes on to state that these three Galilean moons’ motions are commensurate and that, “they are precise and permanent and the above relation always holds and follows from mutual gravitational perturbations between all three satellites.”  Callisto, however, does not have any effect on this mo tion relationship, so the maximum number of Galilean Moon Shadows that can appear on the face of Jupiter at the same time is three.

Now the question is:  Why the 13-month gap between “Shadow Seasons?”  To answer this question you need to find out if all 4 of the Galilean moons cast a shadow on the face of Jupiter every time they orbit.  We also need to know how often and for what length of time each moon casts its shadow on the face of Jupiter.  It should follow that the longer and more often a moon casts its shadow of Jupiter’s face the better the chance of a MSE.

The first question was answered in Fred Price’s book, where I read that twice, for 3 years, in a Jovian year (12 Earth years), Callisto castes no shadow, nor does it transit or exhibit eclipses or occultations.  This is because the distance of Callisto from Jupiter and the tilt of Jupiter’s equatorial plane (which the Galilean Moons orbit very close to) can be great enough for not only its shadow, but also the entire moon to appear above or below the planet.  Also, its shadow can at times only just graze the northern or southern limbs of Jupiter, so its time on the face of Jupiter varies from approximately 25 minutes to 4 hours & 50 minutes when it transits the equator.  All of this coupled with a period of almost 18 days between transits means that there is a very limited amount of time for Callisto to be part of a MSE.

Obviously the other Galilean moons are also subject to variances in the length of time their respective shadows fall on the face of Jupiter, but to a lesser degree.  Io, Europa and Ganymede create a shadow transit on every orbit.  Ganymede’s transit time can vary from 2.25 hours to almost 3.5 hours and its orbital period is about 7 days.  Europa’s orbital period is approximately 3.5 days and its shadow transits last between 2.5 hours and 3 hours.  Io because of its close orbit to Jupiter shows very little variances in the length of time its shadow transits, about 2 hours and has an orbital period of about 42.5 hours.  All of these factors play into the 13 months between Shadow Seasons and contribute to a number of sporadic DST’s throughout the time between seasons.

When I am asked, what is my favorite thing to observe?  The answer is always “the ever changing Jupiter, with its 4 Galilean moons.”  As you can see, (in just about any size scope), Jupiter and it moons are source of continuously changing views and for a few months every 5.93 years when Earth and Jupiter are co-planar even the moons can cast their shadows on each other.  This will next happen in 2002 and it’s a wonderful opportunity to visually compare the differences in the sizes and albedos of the Galilean moons.

For more information on Jupiter and its moons, I suggest “The Planet Observer’s Handbook” by Fred W. Price and published by Cambridge University Press, “Jupiter:  The Giant Planet” by Reta Beebe and published by Smithsonian Institution Press.  You can also find a wealth of information from the Internet, where there are as many different websites about Jupiter as Saturn has moons.


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