Observing Alert: Rare Triple Transit Of Jupiter’s Moons Happens Friday Night (Oct. 11-12)

Talk about a great fall lineup. Three of Jupiter’s four brightest moons plan a rare show for telescopic observers on Friday night – Saturday morning Oct. 11-12. For a span of just over an hour, Io, Europa and Callisto will simultaneously cast shadows on the planet’s cloud tops, an event that hasn’t happened since March 28, 2004.

Who doesn’t remember their first time looking at Jupiter and his entourage of dancing moons in a telescope? Because each moves at a different rate depending on its distance from the planet, they’re constantly on the move like kids in a game of musical chairs. Every night offers a different arrangement.

Jupiter and its four brightest moons seen in a small telescope. Credit: Bob King
Jupiter and its four brightest moons seen in a small telescope. Credit: Bob King

Some nights all four of the brightest are strung out on one side of the planet, other nights only two or three are visible, the others hidden behind Jupiter’s “plus-sized” globe. Occasionally you’ll be lucky enough to catch the shadow of one of moons as it transits or crosses in front of the planet. We call the event a shadow transit, but to someone watching from Jupiter, the moon glides in front of the sun to create a total solar eclipse.

Since the sun is only 1/5 as large from Jupiter as seen from Earth, all four moons are large enough to completely cover the sun and cast inky shadows. To the eye they look like tiny black dots of varying sizes. Europa, the smallest, mimics a pinprick. The shadows of Io and Callisto are more substantial. Ganymede, the solar system’s largest moon at 3,269 miles (5,262 km), looks positively plump compared to the others. Even a small telescope magnifying around 50x will show it.

Jupiter on Sept. 24 with its moon Europa (at left) casting a pinhead black shadow on Jupiter's clouds. Credit: John Chumack
Jupiter on Sept. 24 with its moon Europa (at left) casting a pinhead black shadow on Jupiter’s clouds. Credit: John Chumack

The three inner satellites – Io, Europa and Ganymede – have shadow transits every orbit. Distant Callisto only transits when Jupiter’s tilt relative to Earth is very small, i.e. the plane of the planet’s moons is nearly edge-on from our perspective. Callisto transits occur in alternating “seasons” lasting about 3 years apiece. Three years of shadow play are followed by three years of shadowless misses. Single transits are fairly common; you can find tables of them online like this one from Project Pluto or plug in time and date into a free program like Meridian for a picture and list of times.

Because Io, Europa and Ganymede orbit in a 4:2:1 resonance (Io revolves four times around Jupiter in the time it takes Ganymede to orbit once; Europa completes two orbits for Ganymede's one) a "quadruple transit" is impossible. Credit: Matma Rex / Wikipedia
Because Io, Europa and Ganymede orbit in a 4:2:1 resonance (Io revolves four times around Jupiter in the time it takes Ganymede to orbit once; Europa completes two orbits for Ganymede’s one) it’s impossible for all three to line up – along with Callsto – for a “quadruple transit”. Credit: Matma Rex / Wikipedia

Seeing two shadows inch across Jupiter’s face is very uncommon, and three are as rare as a good hair day for Donald Trump. Averaged out, triple transits occur once or twice a decade. Friday night Oct. 11 each moon enters like actors in a play. Callisto appears first at 11:12 p.m. EDT followed by Europa and then Io. By 12:32 a.m. all three are in place.

Catch them while you can. Groups like these don’t last long. A little more than an hour later Callisto departs, leaving just two shadows.  You’ll find the details below. All times are Eastern Daylight or EDT. Subtract one hour for Central time and add four hours for BST (British Summer Time):

* Callisto’s shadow enters the disk – 11:12 p.m. Oct. 11
* Europa – 11:24 p.m.
* Io – 12:32 a.m.
** TRIPLE TRANSIT from 12:32 – 1:37 a.m.
* Callisto departs – 1:37 a.m.
* Europa departs – 2:01 a.m.
* Io departs – 2:44 a.m.

Looking at Jupiter from high above the plane of the solar system, we can picture better how shadow transits and eclipses happen. Credit: Garrett Serviss from "Pleasures of the Telescope" (annotations: Bob King)
Looking at Jupiter from high above the plane of the solar system in this diagram from more than a century ago, we can better picture how shadow transits and eclipses happen. The tiny disk of Io and the shadow of Ganymede are seen in transit; Callisto is about to be eclipsed by Jupiter’s shadow.  Credit: Garrett Serviss from “Pleasures of the Telescope” (annotations: Bob King)

The triple transit will be seen across the eastern half of the U.S., Europe and western Africa. Those living on the East Coast have the best view in the U.S. with Jupiter some 20-25 degrees high in the northeastern sky around 1 a.m. local time. Things get dicier in the Midwest where Jupiter climbs to only 5-10 degrees. From the mountain states the planet won’t  rise until Callisto’s shadow has left the disk, leaving a two-shadow consolation prize. If you live in the Pacific time zone and points farther west, you’ll unfortunately miss the event altogether.

From the Eastern Time Zone Jupiter will be well-placed in the eastern sky around the time of mid-transit. Created with Stellarium
From the Eastern Time Zone Jupiter will be well-placed in the eastern sky during the transit. Created with Stellarium

Key to seeing all three shadows clearly, especially if Jupiter is low in the sky, is steady air or what skywatchers call “good seeing”. The sky can be so clear you’d swear there’s a million stars up there, but a look through the telescope will sometimes show dancing, blurry images due to invisible air turbulence. That’s “bad seeing”. Unfortunately, bad seeing is more common near the horizon where we peer through a greater thickness of atmosphere. But don’t let that keep you inside Friday night. With a spell of steady air, all you need is a 4-inch or larger telescope magnifying around 100x to spot all three.

The March 28, 2004 triple transit. Shadows from left: Ganymede, Io and Callisto. You can also see the disks of Io (white dot) and Ganymede (blue dot) in this photo taken in infrared light by the Hubble Space Telescope. Credit: NASA/ESA
The March 28, 2004 triple transit. Shadows from left: Ganymede, Io and Callisto. You can also see the disks of Io (white dot) and Ganymede (blue dot) in this photo taken in infrared light by the Hubble Space Telescope. Credit: NASA/ESA

If bad weather blocks the view, there are two more triple transits coming up soon – a 95-minute-long event on June 3, 2014 starring Europa, Ganymede and Callisto (not visible in the Americas) and a 25-minute show on Jan. 24, 2015 featuring Io, Europa and Callisto and visible across Western Europe and the Americas. That’s it until dual triple transits in 2032.

 

Asteroid Might be Visible to Naked Eye on Feb. 17

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An asteroid could be visible with binoculars, or even the naked eye on Wednesday, February 17, 2010. No, it’s not coming close to Earth, although this second most massive object in the asteroid belt will be at its closest point to Earth in its orbit, about 211,980,000 kilometers (131,700,000 miles) away. Asteroid Vesta – one of the asteroids that the Dawn spacecraft will visit – will be at opposition on Wednesday, meaning it is opposite the sun as seen from Earth, and is closest to us. Vesta is expected to shine at magnitude 6.1, and that brightness should make it visible for those with clear skies and a telescope, but perhaps even those blessed with excellent vision and little or no light pollution. Vesta will be visible in the eastern sky in the constellation Leo, and will continue to be visible — although less so — in the coming months.

What makes this space rock so prominent these days? Along with its relative proximity at this point, a full half of the asteroid is being bathed by sunlight when seen from Earth, making it appear brighter. Another attribute working in the observer’s favor is that Vesta has a unique surface material that is not as dark as most main belt asteroids – allowing more of the sun’s rays to reflect off its surface.

For more info about observing Vesta, check out this article from Sky & Telescope.

If you get lucky enough to see Vesta, and want to learn more about it, check out this info on the Dawn mission website. Dawn is currently motoring its way through the asteroid belt, will begin its exploration of Vesta in the summer of 2011.

Source: JPL

Nova Sagittarius 2008 Is Brightening!

In case you didn’t catch the New Nova In Sagittarius alert the other day… You might want to pay a little closer attention because it is brightening by leaps and bounds! Captured 4 days ago by our friends at Macedon Ranges Observatory and shared exclusively with UT Readers, the up-to-the-minute reports show it is now clearly a binocular object and may have even reached unaided eye visibility.

AAVSO Special Notice #106
April 25, 8:09 am EST

According to reports, Nova Sgr 2008 continues to brighten,
with the last measurements from Alexandre Amorim
indicating about V=6.5 on 20080423.0993.

Luckily, many AAVSO program stars are in Saggitarius,
and if you use a B-scale or larger field with VSP, you
will find many sequence stars from which to choose.
Keep watching this nova as it brightens; few professional
telescopes can observe this bright, while with a pair
of binoculars it is an easy target. We will be uploading
a BVRI calibration in a few days, in plenty of time
to follow the decline.

This nova’s coordinates are RA 18:05:58.90 Dec -27:13:56.3 . For those who would like to try their hand with binoculars? Aim just a couple of fingerwidths north of the tip of the “teapot” spout. It will by far be the brightest in the field. Use the included map – the circled area is the rough location and the magnitudes are set so that anything that appears brighter than what you see in the circle will be the nova.

Be sure to drop our friends at Macedon Ranges Observatory a few lines and let them know how much we all appreciate seeing this well ahead of the rest of the world’s news!