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Jupiter at its best for 2015

Jupiter’s moons in transit. NASA, ESA, Hubble Heritage Team, CC BY-SA

This month is a great time to catch Jupiter shining brilliantly in the evening sky. And for the next few weeks it will be its best and brightest for the year.

The planet reaches opposition at 5:09am on Saturday morning, February 7 (AEDT). At that time, the sun is located on one side of the Earth and Jupiter is found directly opposite, on the other side of the Earth.

This positions Jupiter opposite the sun and on the the night-side of the Earth. Hence, the planet rises as the sun sets, is overhead around midnight (when the sun is directly below us) and sets as the sun rises. But not only do we get to see Jupiter all night long, we also see it at its brightest.

Being the largest planet, Jupiter is always easy to see. It is only rivalled by Venus, which is brighter than Jupiter because it is much closer and its thick atmosphere reflects sunlight really well.

But around the time of opposition Jupiter really dazzles. This is because, with both Earth and Jupiter located on the same side of the sun, the two planets are closest together for the year. This year they will be separated by 650-million km.

By August, Earth will have swung around the sun, but Jupiter won’t have moved that far. This will bring Jupiter into solar conjunction.

The two planets will be found on opposite sides of the solar system, taking them furthest apart for the year. At that time the distance between them will be almost 960-million km. And of course, we won’t see Jupiter because it will appear with the sun in the daytime sky.

Not all oppositions are created equal.

The best oppositions occur when Jupiter is near perihelion or closest to the sun. It makes sense, because if Jupiter is slightly closer to the sun, then opposition will also bring the planet a little closer to Earth.

Jupiter’s last perihelion passage was in March 2011 and the next will be in January 2023, in step with its 11.9 year orbit.

So for the next few years, oppositions aren’t as favourable as they could be. But don’t let this put you off. Look towards the north-east in the evening and it’s a beautiful sight to see Jupiter shining so brightly.

Jupiter is easily the brightest object in the north-eastern sky at sunset. Museum Victoria/Stellarium

It’s also really easy to notice that Jupiter’s light is much steadier than the light from any nearby stars. Jupiter’s increased angular size makes it less susceptible to turbulence in our atmosphere. Or in other words, stars twinkle easily, but planets don’t.

Moon shadows

We now know of 67 moons orbiting Jupiter, but the planet’s four largest moons – Io, Europa, Ganymede and Callisto – are worlds in their own right. They are easily seen in a good pair of binoculars or a small telescope, as tiny star-like objects, strung in a line that can cross either side of Jupiter.

On January 23, the Hubble Space Telescope (HST) captured a rare triple transit shadow - the shadows of Europa, Callisto and Io were seen moving across the surface of Jupiter.

Triple transit shadows can only occur if Callisto is one of the moons involved. This is because, Io, Europa and Ganymede - which all orbit closer to Jupiter than Callisto - are locked in a 1:2:4 orbital resonance. Hence, it is impossible to have all three moons suitably positioned together.

However, there is a problem with needing Callisto for a triple transit. Callisto’s orbit is slightly tilted so that a lot of the time its shadow misses the planet. Shadow transits of Callisto occur in seasons – the current one began in 2013 and will continue until 2016. A list of shadow transits can be found here, although currently they favour North America and are not visible across Australia.

Eclipses and Occultations

What’s more, this triple shadow transit occurred during a period when the Earth and sun are crossing Jupiter’s equatorial plane. This occurs, about every five to six years and during this time Jupiter’s moons can appear to overlap one another.

Sometimes an eclipse is produced when the shadow of one moon passes across another. At other times, the line-up causes an occultation when one moon passes in front of another and obscures it from sight.

Io lines up with Callisto’s shadow. NASA, ESA, Hubble Heritage Team

Just before the HST began observing the triple shadow transit, Io had been eclipsed by Callisto. The first image taken by the telescope shows Io just after it has moved out of Callisto’s shadow.

It’s always interesting to follow the changing positions of Jupiter’s moons through a telescope. I like to imagine what it must have been like for Galileo, as he tracked the moons for the very first time.

On early Saturday morning, around the time of opposition, Europa and Ganymede will be very close together. Europa is set to occult Ganymede (or move in front of it) around 6am (AEDT).

Jupiter’s largest moons appear as four tiny points of light through a telescope. On Saturday morning Europa will pass by Ganymede as shown in the close up on the right. Museum Victoria/Starry Night

The Institute of Celestial Mechanics and Calculation of Ephemerides, in Paris, runs an international campaign to collect data obtained by amateur astronomers during occultations and eclipses of Jupiter’s moons. The observations are used to track how the orbits of Jupiter’s moons are slowly changing over time, as the moons interact gravitationally with each other and Jupiter.