By Jove: Jupiter Reaches Opposition on February 6th

Jupiter +Great Red Spot as seen on January 22nd 2015. Credit:

Did you see the brilliant Full Snow Moon rising last night? Then you might’ve also noticed a bright nearby ‘star’. Alas, that was no star, but the largest planet in our solar system, Jupiter. And it was no coincidence that the king of the gas giants is near the Full Moon this February, as Jupiter reaches opposition this Friday on February 6th at 18:00 Universal Time or 1:00 PM EST.

As the term implies, opposition simply means that an outer planet sits opposite to the Sun. Mercury and Venus can never reach opposition. Orbiting the Sun once every 11.9 years, oppositions for Jupiter occur once every 399 days, or roughly every 13 months. This means that only one opposition for Jupiter can happen per year max, and these events precess forward on the Gregorian calendar by about a month and move one zodiacal constellation eastward per year.

Starry Night.
The apparent path of Jupiter through Spring 2015. Created using Starry Night Education Software.

Through a telescope, Jupiter exhibits an ochre disk 40” in diameter striped with two main cloud belts. The northern equatorial belt seems permanent, while the southern equatorial belt is prone to pulling a ‘disappearing act’ every decade of so, as last occurred in 2010. The Great Red Spot is another prominent feature gracing the Jovian cloud tops, though its appeared salmon to brick-colored in recent years and seems to be shrinking.

Jupiter rotates once every 9.9 hours, fast enough that you can watch one full rotation in a single night.

Photo by author
Jupiter near opposition in 2014. Photo by author.

It’s also fascinating to watch the nightly dance of Jupiter’s four large moons Io, Europa, Ganymede and Callisto as they alternatively cast shadows on the Jovian cloud tops and disappear into its shadow. Near opposition, this shadow casting activity is nearly straight back as seen from our perspective.  Here is the tiny ‘mini-solar system’ that fascinated Galileo and further convinced him that the Earth isn’t the center of the cosmos. Jupiter has 67 moons in all, though only 4 are within range of modest sized telescopes… Even 5th place runner up Himalia is a challenge near the dazzling disk of Jove at +14th magnitude.

Also watch for a phenomenon known as the Seeliger or Opposition Effect, a sudden surge in brightness like a highway retro-reflector in the night.

Opposition 2015 finds Jupiter just across the Leo-Cancer border in the realm of the Crab. Jupiter crossed from Leo into Cancer on February 4th, and will head back into the constellation of the Lion on June 10th. Jupiter then spends the rest of 2015 in Leo and heads for another opposition next year on March 8th.

Jupiter will also make a dramatic pass just 24’ — less than the diameter of the Full Moon — from Regulus on August 11th, though both are only 11.5 degrees east of the Sun in the dusk sky. Jupiter also forms a 1 degree circle with Regulus, Mercury and Jupiter 14.5 degrees east of the Sun on August 7th.

Jupiter reaches a maximum declination north for 2015 on April 7th at 18 degrees above the celestial equator. We’re still in a favorable cycle of oppositions for Jupiter for northern hemisphere viewers, as the gas giant doesn’t plunge south of the equator until September 2016.

Looking farther ahead, Jupiter reaches east quadrature on May 4th, and sits 90 degrees elongation from the Sun as the planet and its moons cast their shadows far off to the side from our Earthly perspective. We’re still also in the midst of a plane crossing: February 5th is actually equinox season on Jupiter! This also means that there’s still a cycle of mutual eclipses and occultations of the Jovian moons in progress. One such complex ballet includes (moons) on the night of February 26th.

February 26th. Starry Night
The close grouping of Io, Callisto and Ganymede on the night of February 26th. Created using Starry Night Education software.

And yes, it is possible to see the Earth transit the disk of the Sun from Jove’s vantage point. This last occurred in 2014, and will next occur in 2020.

But wait, there’s more. Jupiter also makes a thrilling pass near Venus on July 1st, when the two sit just 0.4 degrees apart. We fully expect a spike in “what are those two bright stars?” queries right around that date, though hopefully, the conjunction won’t spark any regional conflicts.

Jupiter, Regulus and the rising waning gibbous Moon on the evening of February 4th. Credit: Stellarium.

Solar conjunction for Jupiter then occurs on August 26th, with the planet visible in the Solar Heliospheric Observatory’s (SOHO) LASCO C3 camera from August 16th to September 6th.

Emerging into the dawn sky, Jupiter then passes 0.4 degrees from Mars on October 17th and has another 1.1 degree tryst with Venus on October 26th.

Looking for Jupiter in the daytime near the waxing gibbous Moon. Credit: Stellarium.
Looking for Jupiter in the daytime near the waxing gibbous Moon. Credit: Stellarium.

Let the Jovian observing season begin!

-Wonder what a gang of rogue space clowns is doing at Jupiter? Read Dave Dickinson’s original tale Helium Party and find out!

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.