Photos: Jewels from Jupiter

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  1. Jupiter loses a stripe

    The weather on Jupiter is changeable, as these before-and-after pictures show. The photograph on the left shows Jupiter as seen in June 2009. The photo on the right, taken on May 9, 2010, reveals that one of the planet's prominent dark cloud belts has faded away. The lightening of the South Equatorial Belt is due to atmospheric changes. Both pictures were taken by Anthony Wesley, an amateur astronomer in Australia. (Anthony Wesley via The Planetary Society) Back to slideshow navigation
  2. Family portrait

    Launched in 1989, the Galileo spacecraft has photographed Jupiter as well as several of the giant planet's satellites. Here's a montage that shows Jupiter's Great Red Spot and the four largest moons. From top, they are Io, Europa, Ganymede and Callisto. (NASA) Back to slideshow navigation
  3. Cratered Callisto

    Callisto is considered the most cratered celestial body in the solar system. The false-color overlay at right exaggerates the moon's surface features, including the Valhalla impact structure near the center of the disk. (NASA) Back to slideshow navigation
  4. Dark face

    Colors are enhanced in this view of Ganymede's trailing hemisphere, highlighting the moon's polar caps. The violet color indicates where small particles of frost may be scattering light on the blue end of the spectrum. (NASA) Back to slideshow navigation
  5. Cloudy weather

    The mosaic at left shows the true colors of the cloud patterns in Jupiter's northern hemisphere. The rendition at right uses false colors to represent the height and thickness of the cloud cover. (NASA) Back to slideshow navigation
  6. This is the Spot

    A true-color picture captures the subtle shadings of Jupiter's Great Red Spot, a massive, long-lived storm system in the planet's thick atmosphere. (NASA) Back to slideshow navigation
  7. A big splash on Europa

    A computer-generated perspective view shows the Pwyll impact crater on Europa, an ice-covered moon of Jupiter. The heights are exaggerated, but the central peak indicates that the crater may have been modified shortly after its formation by the flow of underlying warm ice. (NASA) Back to slideshow navigation
  8. A blast at Io

    This image of Io, thought to be the solar system's most volcanically active world, shows the plumes of two eruptions. One plume can be seen at the very edge of the disk, the other is puffing up from the dark volcanic ring near the center of the disk. (NASA) Back to slideshow navigation
  9. Lava light

    An active volcanic eruption on Jupiter's moon Io flares in an image taken in February 2000 by the Galileo spacecraft. The dark L-shaped lava flow to the left of center marks the site of energetic eruptions in November 1999 at Tvashtar Catena, which is a chain of giant volcanic calderas. The two small bright spots at left side of image are sites where molten rock is exposed to the surface at the toes of lava flows. (NASA) Back to slideshow navigation
  10. Crazy quilt

    The thin crust of Europa's Conamara region is criss-crossed by craters, cracks and lines - indicating that the surface ice was repeatedly disrupted. The colors, which are enhanced in this view, show where light ice crystals and dark contaminants have settled onto the surface. (NASA) Back to slideshow navigation
  11. A moving moon

    In a picture taken in April 2001 by NASA's Cassini spacecraft, the moon Io looks like a marble set against the background of Jupiter. Io is the giant planet's third-largest satellite. (NASA) Back to slideshow navigation
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updated 8/25/2010 1:52:00 PM ET 2010-08-25T17:52:00

For many weeks, the planet that has dominated our evening sky has been brilliant Venus, visible low in the west-southwest sky for about 90 minutes after sunset. But after Venus sets, it is Jupiter that takes over for the rest of the night, outshining everything in the night sky but the moon. 

This week, Jupiter — the solar system's largest planet — rises around 8:45 p.m. local daylight time. On Thursday evening, if youre facing east soon after 9 p.m., you'll see the nearly full moon standing about 6 degrees above Jupiter. Your clenched fist held at arm's length measures about 10 degrees in width, so just over half of a fist will separate Jupiter from the moon. 

The two objects will remain visible through the rest of the night, peaking toward the south at around 3 a.m., at an altitude that measures more than halfway from the horizon to the point directly overhead (the zenith).

This sky map shows where to look to spot Jupiter and the moon Thursday night.

Image: Sky map
Starry Night Software
This sky map shows the relative positions of the moon, Jupiter, Uranus and Neptune in eastern evening skies, as seen on Aug. 26 from mid-northern latitudes. Under optimal conditions, Uranus and Neptune may be visible through binoculars — if you know exactly where to look.

Jupiter rules
In the coming weeks we will see Jupiter loom as large and as bright as it ever can get from our earthly vantage point, because it's nearing perihelion — that point in its 12-year orbit that places it nearest to the sun.

Jupiter now appears 11 percent larger and more than one and a half times brighter than it did back in 2005, when it was near aphelion (that point in its orbit farthest from the sun). [Photos of Jupiter and its moons.]

Even steadily held 7-power binoculars will show Jupiter as a tiny disk. A small telescope will do much better, while in larger instruments, Jupiter resolves into a series of red, yellow, tan and brown shadings, as well as a wealth of other telescopic detail. Amateur astronomers have been imaging this big planet all summer long as it has been approaching the Earth.

In less than a month, on Sept. 21, the planet will be in opposition. That's when Jupiter is nearest to Earth and shining in the sky all night long, from sunset to sunrise.

And don't forget Jupiter's four major moons, discovered 400 years ago by Galileo. They can be seen in any telescope and even binoculars. They orbit Jupiter so quickly (with orbital periods ranging from 1.68 days for Io to 16.7 days for Callisto) that they change their appearance from night to night.

Size (and distance) matters
When you look at the moon and Jupiter on Thursday night, you might ponder the difference in both their sizes and distances. 

The moon, of course, far outshines Jupiter — by more than 9 magnitudes, or a brightness ratio of 4,370 to 1. But the moon is also much smaller than Jupiter. The moon's diameter is 2,158 miles (3,473 kilometers), while Jupiter's is 88,846 miles (142,984 km).

What makes the moon loom so much larger and brighter is its distance.

On Thursday night, the moon will be 251,200 miles (404,270 kilometers) from Earth. But Jupiter will be 1,496 times more distant: 375.9 million miles (605 million kilometers) away.

Gyrating Jupiter?
I recently received an e-mail from Linda Francese of Brookfield, Conn., involving an interesting observation made by her son, which likely involved a sighting of Jupiter. She wrote:

"My son, Johnny woke me up at 2 a.m. to look at a 'star' that was moving. His friend had called him and told him about it. Johnny walked over to his house (he lives a house away) and they called another friend to come over. The three of them were watching this 'star' go back and forth. Do you know anything about this?"

I'm pretty certain that the "star" that Johnny and his friends saw was Jupiter. So why did it appear to move? Likely they experienced what is called the "autokinetic effect."

This is a phenomenon of human visual perception in which a stationary, small point of light in an otherwise dark or featureless environment appears to move.

Many sightings of UFOs have also been attributed to the autokinetic effect's action on stars or planets. Psychologists attribute the perception of movement where there is none to "small, involuntary movements of the eyeball." The autokinetic effect can also be enhanced by the power of suggestion: If one person reports that a light is moving, others will be more likely to report the same thing.

Currently, Jupiter is shining in the constellation Pisces, a star pattern that consists chiefly of faint stars.  Under a clear, dark sky with no moon nearby, Jupiter will appear to shine with little or no competition from other nearby stars. 

If a person stares at Jupiter over a span of perhaps 15 to 30 seconds, it's quite possible for the autokinetic effect to kick in and cause Jupiter to gyrate or perhaps move in a small circle. 

Next week, when the moon has moved out of Jupiter's vicinity and the surrounding sky is dark, try staring at Jupiter and see if it'll move for you like it did for Johnny and his friends.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

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