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    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Jupiter’s moons pull a vanishing act – and other neat dance moves!

(For an observing report on this event, go here.)

Jupiter’s four bright moons always put on a great show – ask Galileo – changing positions slowly in the course of the night. But for viewers in North America the night of September 2-3 offered an unusual opportunity because the four bright moons all played hide-and-seek at once – something that happens  about 20 times a century and I suspect much less for any given geographic area. That said, the following video and text I believe remainr elevant andhelpful for anyone interested in watching Jupiter’s moons. The  video is an animated simulation depicting events of one night. It was created in Starry Nights Pro astronomy software and time has been speeded up so  that what you see in about four minutes is what actually takes place in about seven and a half hours.

(For a complete explanation of what’s going on in this animated simulation – and what you should see September 2-3, 2009 – you can take this shortcut  to  the sequence of events below.)

Here’s why I love watching  Jupiter’s four brightest moons:

  • You can see them with any small telescope – even binoculars if you can hold them real steady.
  • They do something! Most astronomical objects don’t change much over our lifetimes. Jupiter’s moons, as you can see from the video simulation, go through significant changes in a single night.
  • These four bright moons played a major role in changing our view of the universe.
  • They even helped us determine the speed of light a couple hundred years ago, something next to impossible to determine on Earth without modern, sophisticated instruments.
  • For the telescope user they:
  1. duck in and out of Jupiter’s shadow (eclipse)
  2. hide behind the planet and suddenly pop out (occultation)
  3. cross in front of the planet providing a challenge for telescope users to spot them  (transit)
  4. and from time to time they cast their shadows on the giant planet – shadows visible in a backyard telescope as pefect round circles
  • Hubble and modern spacecraft have shown us that Jupiter’s moons are full of surprises.  No two are alike and all four are different than what scientists imagined before the spacecraft got out there and gave us an up close and personal view.
  • All of which is incredibly awesome when you understand that the little lights you see moving with grace, precision, and predictability are complete worlds in themselves the size of our moon or larger. (Ganymede is about 1.5x the diameter of our moon.) Newton is playing the tune, and the moons do the dance – music of the spheres indeed!


You can enjoy Jupiter’s bright moons any night the planet is visible. What’s special about the night of September 2 and 3 of 2009 is they’re going through their complete routine in one night and for a couple hours all four of them are out of sight for all practical purposes, but still providing an interesting challenge for the telescope user. What’s more, Jupiter is very close to the moon on this particular night  and very bright in the southeastern sky,  so it is easy to find. And any small telescope will reveal the moons. Seeing them with binoculars is possible, but it takes sharp eyes and a steady hand. I’ve never been able to see them with binoculars unless I can steady the binoculars on a tripod, or against the corner of a building or some other support.

The moons were discovered by Galileo 400 years ago and he didn’t waste any time writing about his findings in his “Starry Messenger.” What he had to say shook up the religious/philosophical/scientific establishment of the day. Although Copernicus had argued otherwise more than 50 years before, the common belief remained that the Earth was the center of the universe and everything revolved around the Earth. But a few nights of observing Jupiter’s moons and that whole business of us being at the center of everything went out the window. Obviously Jupiter was at the center of its own little system and these moons were revolving around it, not us.

The  13-day-old Moon and Jupiter dominate the eastern sky near the horizon for North American observers on September 2, 2009. This screen shot from Starry Nights software captures the positions, but don't get the idea that Jupiter looks thatbig - the size represents its brightness - and its is fa rbrighter than any of the stars, though to the naked eye it will look like a star.

The 13-day-old Moon and Jupiter dominate the eastern sky near the horizon for North American observers on September 2, 2009. This screen shot from Starry Nights software captures the positions, but don't get the idea that Jupiter looks that big - the size represents its brightness - and its is far brighter than any of the stars, though to the naked eye it will look like a star.

Now, with any small telescope, you can travel in the footsteps of Galileo, observe Jupiter’s moons, and make your own drawings. This month (September, 2009) is a good time to start because Jupiter appears in our eastern sky as the Sun is setting in our western sky. By about half an hour after sunset Jupiter will put in an appearance. The only thing brighter than it in the eastern evening sky this month is the moon. And on the evening of September 2 folks in North America will see a nearly full moon pretty close to Jupiter – exactly how close depends on where you are and when you look. On the East Coast at 8 pm EDT the moon will be less than 3 degrees from Jupiter in the southeast and pretty close to the horizon. About 7 hours later it will be in the southwest and the moon will still be with it, but the separation will have increased to about five degrees. (Typical binoculars have a field of view of about 7 degrees, so you should easily see both the moon and Jupiter in the same binocular field.  The question – which I honestly can’t answer – is how easy will it be to see Jupiter’s moons with our own moon shining so close to it? Will the moon drown them out? I’m pretty  confident this won’t be a problem for telescope users. It may make it  difficult for binoculars users since the moons of Jupiter are roughly as bright as the faintest stars we can see with our naked eye.)

What should you look for an when?

First, if you want to know which of Jupiter’s moons is where on any given night, use this neat little online utility provided by Sky and Telescope magazine.

Now the basics. Jupiter has 63 moons, but only four of them are easily seen in small telescopes. Here are their names – in order moving outward from the planet – and links to more details about each.

Sequence of events September 2-3

Here is the schedule of events in  EDT, with  24-hour format Universal Time in parenthesis. (Data is from a listing in the September Sky and Telescope.)  If clouds, sleep, or work cheat you out of a live view, you still might refer to the following list as you watch the animation.)

September 2

7:19 pm (23:19)Callisto is occulted – goes behind the planet. (This will be in daylight for US observers, but when you start observing Jupiter later,  know that Callisto is already behind it.)

11:43 pm (3:43 Sept. 3) Io is occulted – goes behind the planet. (This is fun to watch – how long does it take Io to vanish? )

11:58 pm  (3:58 Sept. 3)Europa transit begins on the opposite side of the planet from where you saw Io vanish a few minutes before. Seeing the moon against the bright disc of the planet is possible in small telescopes, but varies in difficulty depending on exactly what part of the planet is behind the moon, some parts being darker in hue than others. But keep in mind, these moons are so far away that even in a large, backyard telescope they barely show a disc under ideal conditions. So what you are look for is a point of light not much different than a star.

September 3

12:43 am (4:43)Ganymede follows Europa onto the planet’s disc. Is it easier to see than Europa? It’s significantly larger, so might be a tad easier, but again, it takes a large telescope to show the moons as even a small disc.

12:56 am (4:56)Europa’s shadow follows it onto the face of Jupiter – but it may be easier to see as it gets nearer the center. It also may help you pick up the disc of Ganymede – the shadow of Europa during the first half of its journey is very close to the disc of Ganymede – at least as shown by the Starry Nights Pro simulation.

2:29 am (6:29)Io pops back into view – but look where it is! It’s not close to the planet when it does this because after it was hidden by the disc (an occulation) it went into the planet’s shadow – technically, an eclipse. So what you see is it emerging from the shadow, already some distance out from the planet.

2:42 am (6:42) Ganymede’s shadow enters the disc.  Notice how far behind Ganymede it is? Europa’s shadow was much closer to it. Why? Because Europa is much closer to the planet then Ganymede and here’s proof!

2:49 am (6:49) Europa emerges from the disc – but if you have not been able to follow it when it was on the disc it may be difficult to pickup for few minutes because it will still be close to Jupiter and lost in its glare – at least to the smallest telescopes and binoculars. However, Europa’s shadow will still be visible on the disc for almost another hour.

3:47 am (7:47) Europa’s shadow goes off the disc.

At this point East Coast observers are seeing a Jupiter that is very close to the southwestern horizon and the moons will be difficult to observe. For my location – at 71 degrees longitude (Westport, MA). the giant planet sets about 4:30 am.

4:20 am  (8:20)Ganymede’s transit ends, but it’s shadow is still on the planet.

4:35 am (8:35) – At last! Here comes Callisto! For me it has been out of sight all night. It goes behind the planet before it is dark enough to see it and doesn’t come out until after Jupiter has set. However, folks farther west should certainly see this exit event.

6:21 am (10:21)Ganymede’s shadow exits the planet’s disc. Hey – that’s all folks – for tonight. But there are many other nights when there are interesting events.

Be sure to check Sky and Telescope Javascript utility.  Any night Jupiter is well placed for observing I always check this little utility to see if there are any neat events coming up at a convenient time. Nights like September 2-3, 2009 are rare. But with four moons there are frequent times when one or the other is doing something interesting.

How to time a light beam!

Oh – and about determining the speed of light. Think about it.  I believe Galileo once took a stab at this by stationing observers facing one another from different mountain peaks. They then uncovered a lantern at a predetermined time.  No luck. Light is much too fast for this kind of experiment. Hey – light could go completely around the Earth more than seven times in a second! But here’s how Jupiter’s moon helped determine the speed of light more than 300 years ago! These kinds of discoveries always leave me in awe at how brilliant the discoverer’s were and how precisely they were able to make observations with tools that were not nearly as good as the inexpensive telescopes available to anyone today.  The  account which follows can be read in full here. It is from a posting by Michael Fowler of the University of Virginia Physics Department. One more thing to appreciate as you watch Jupiter’s moons.

The first real measurement of the speed of light came about half a century later, in 1676, by a Danish astronomer, Ole Römer, working at the Paris Observatory. He had made a systematic study of Io, one of the moons of Jupiter, which was eclipsed by Jupiter at regular intervals, as Io went around Jupiter in a circular orbit at a steady rate. Actually, Römer found, for several months the eclipses lagged more and more behind the expected time, but then they began to pick up again. In September 1676, he correctly predicted that an eclipse on November 9 would be 10 minutes behind schedule. This was indeed the case, to the surprise of his skeptical colleagues at the Royal Observatory in Paris. Two weeks later, he told them what was happening: as the Earth and Jupiter moved in their orbits, the distance between them varied. The light from Io (actually reflected sunlight, of course) took time to reach the earth, and took the longest time when the earth was furthest away. When the Earth was furthest from Jupiter, there was an extra distance for light to travel equal to the diameter of the Earth’s orbit compared with the point of closest approach. The observed eclipses were furthest behind the predicted times when the earth was furthest from Jupiter.

From his observations, Römer concluded that light took about twenty-two minutes to cross the earth’s orbit. This was something of an overestimate, and a few years later Newton wrote in the Principia (Book I, section XIV): “For it is now certain from the phenomena of Jupiter’s satellites, confirmed by the observations of different astronomers, that light is propagated in succession (note: I think this means at finite speed) and requires about seven or eight minutes to travel from the sun to the earth.” This is essentially the correct value.

Less successful was an idea they had much later that they could solve the problem of finding one’s longitude – while at sea – by observing the moons of Jupiter. This was very seriously pursued  because knowing longitude is critical to navigation and the accepted method involved using a very precise clock that kept correct time throughout a long sea voyage. It was hard enough to make a very precise clock – but one that retained its precision when subjected to the knocking about and unavoidable moisture that was part of any long voyage by sail? Nearly impossible. (See the wonderful book, “Longitude,” by Dava Sobel, for the story of how they did solve this.)  But that said, as you watch the moons of Jupiter with a small telescope, try to imagine doing this on the heaving deck of a sailing ship with a telescope that is significantly cruder than the one you buy today! Then imagine that your life may depend on the result! Makes you appreciate GPS if nothing else 😉

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