Feeds:
Posts

## Jupiter at opposition

A few days ago I was contacted by the Western Mail to give some information about the upcoming opposition of Jupiter, which will happen on March 8. The article to which I contributed can be found here, but I thought I would add some more detail here in my blog to the few brief sentences I was asked to contribute to the Western Mail article.

When we say that a planet is at opposition, what we mean is that it is in the opposite direction in the sky to where the Sun is, as shown in the figure below. In fact, the Sun-Earth-planet make a straight line when a planet is at opposition, with the Earth being the middle object in the straight line. The other straight line configuration formed by the Sun-Earth-planet is when the planet lies beyond the Sun with the Sun in the middle. We call this configuration conjunction.

Only planets further from the Sun can be at opposition; Mercury and Venus can only ever be at conjunction, either on our side of the Sun (inferior conjunction) or on the other side (superior conjunction).

Only planets outside of the Earth’s orbit (superior planets) can be at opposition, and this is usually the best time to see them

The average distance from the Earth to the Sun, just a little under 150 million kilometres, is called the Astronomical Unit (AU). It is a convenient unit to use when discussing the Solar System. On this scale Jupiter, which is roughly five times further away from the Sun that the Earth, is 5 AUs.

It takes Jupiter roughly 12 years to orbit the Sun. This means that it appears to move through one zodiacal constellation each year, as there are 12 zodiacal constellations.  This year it is in Leo, last year (2015) it was in Cancer, and next year (2017) it will be in Virgo. This also means that, whilst it is currently visible in our winter and spring skies, in 6 years’ time it will become more of a summer object and disappear from our winter skies.

Although Leo is a fairly easy to find constellation (much easier than Cancer), Jupiter outshines all the stars in Leo. In fact, even when at its furthest from Earth, Jupiter outshines all of the stars in the sky, even Sirius. The only point-like object in the sky which can appear brighter than Jupiter is Venus, which is currently in the evening sky towards the west and setting within a few hours of sunset. You will never seen Venus in the middle of the night, whereas Jupiter is currently visible nearly all night.

How Jupiter should appear through a small telescope. The Galilean moons (Io, Europa, Ganymede and Callisto) should be clearly visible, and if you are lucky you may also see the bands and the great red spot

## The varying brightness of Jupiter

Because oppositions happen when Earth is passing Jupiter, and in a year Jupiter has moved about one twelfth of its path around the Sun, each opposition of Jupiter is spaced by about 13 months. The opposition in 2015 was on February 6 and in 2017 it will be on April 7. When Jupiter is at opposition, it is about 4 AUs from Earth, whereas when it is at conjunction it is about 6 AUs from Earth.

Although this is 50% further away, it means that Jupiter’s brightness in the sky does not vary as much as Mars, which varies a huge amount between when it is at opposition and when it is near conjunction (I say ‘near’ as you cannot see a planet when it is at conjunction). When at opposition, Jupiter has a magnitude of about -2.5 (it doesn’t vary much as both Earth and Jupiter have quite circular orbits); when it is near conjunction its magnitude is about -1.7. This is a luminosity difference of $10^{0.4(2.5-1.7)} = 10^{0.32} = 2.1$, so just over twice as bright when at opposition compared to when it is near conjunction.

Compare this to Mars, which brightens considerably when at opposition. This is because Mars is much closer to Earth, with an orbit which is roughly 1.5 AUs. If it were a circular orbit (which it is not, Mars has a more elliptical orbit than either the Earth or Jupiter), this would mean that at opposition it would be about 0.5 AUs from Earth, but when near conjunction it is about 2.5 AUs, so about 5 times further away! If you move something 5 times further away its brightness goes down by a factor of $5^{2}=25$! This is why the brightness of Mars varies so much in different parts of its orbit.

## When can I see Jupiter in March?

When a planet is at opposition not only is it at its brightest, but it is also in the night-time sky essentially all night. It transits the local meridian around midnight, and when an object transits the meridian it is at its highest point in the sky. But, even a month or two before opposition or a month or two after opposition, Jupiter is in the sky for most of the night. It is easy to see in the late evening sky at the moment, and as we move past March and into April it will rise earlier and earlier and so be easily visible soon after sunset. In May it will already be up quite high by the time the Sun sets.

Jupiter is well worth seeing, even without a telescope or binoculars. Remember, if you are not sure whether you are looking at Jupiter just ask yourself is it the brightest point-like object in the sky. If it is, and it is not over towards the west just after sunset, then you are almost certainly looking at Jupiter. Compare its light to the stars near it; you will notice that Jupiter’s light is steady whereas the stars near it usually twinkle. Stars twinkle, planets don’t.

If you have access to either a small telescope or binoculars you should be able to see the Galilean moons. These were first seen by Galileo in January 1610, and provided the first piece of evidence that not everything orbited the Earth. Io, the closest of the four to Jupiter, takes less than two days to orbit its parent planet, and so even in a few hours you can see a shift in its position. This never ceases to amaze and excite me, even though I have seen it dozens of times. If you are really lucky, you may make out the bands of Jupiter and even its great red spot. I have seen the great red spot a few times. It is a truly memorable sight to see it with your own eyes rather than a photograph in a book or on the web.