Countdown to Conjunction: Venus and Jupiter

If you’ve been outside in the evening over the past few weeks you’ll have noticed that there are two very bright “stars” close together, following the Sun as they set one after the other in the west. Those two bright dots are not stars at all; they’re planets. The brighter of the two is Venus, which at the moment is below and to the right of the other dot, which is Jupiter.

Tonight they are around ten degrees apart in the sky, but over the next week they’ll get closer and closer, as Venus whizzes and Jupiter crawls round the Sun, until on 15 March they’ll be in conjunction, only 3 degrees apart.

Jupiter and Venus in conjunction 15 March 2012, around 1930

On the days either side of 15 March (say between 08 and 19 March) they’ll be very close too. In fact it’s worth watching this celestial merry-go-round in action every clear evening over the next few weeks as the planets move towards and then away from each other in the sky. Towards the end of March though it’ll become harder to see them both as they disappear into the glare of sunset. If you’ve got clear skies and a good western horizon it’s worth looking out for the thin crescent Moon which will appear between the two planets on the night of 25 March.

Venus

Venus, the second planet out from the Sun, is about the same size as the Earth, just a little smaller. It’s the hottest planet in the solar system, with a thick atmosphere of carbon dioxide gas (94.6% is CO2, the rest is mainly nitrogen) which traps most of the light from the Sun that shines on it, super-heating the atmosphere to around 460°C (733K). At ground level this thick, hot atmosphere creates a pressure over 90 times greater than sea-level pressure on Earth. High in Venus’ atmosphere float clouds of sulphuric acid, which is all we see when we look at Venus from the Earth.

Seen from here on Earth, the size and shape of Venus in our sky changes as we both orbit the Sun. At its closest to Earth Venus is “only” 38 million km away, and its disk is 66 arc seconds across, while at its furthest from us it’s 260 million km away, and it shrinks to around 10 arc seconds. On top of this, its phase changes from full (when it’s directly opposite the Sun as seen from Earth) to new (when it’s directly between us and the Sun) and back again. Of course when it’s in either of these positions we won’t see it, as it will be in the sky right next to the Sun. We see Venus best when it’s far to the west of the Sun (when it’s seen in the evening) or far to the east (when it’s seen in the morning). The furthest west and east points as seen from Earth are called maximum elongation, and at these points Venus presents a half phase to us.

Due to the reflectivity of its clouds, and its proximity to us, Venus is the brightest planet as seen from Earth. Venus appears brightest in our sky, at around -4.5 magnitudes, when it’s 68 million miles from us and presents a crescent phase.

During the 15 March conjunction Venus will have a brightness of -4.2 magnitudes.

Jupiter

Jupiter, the largest of all the planets in the solar system, is a gas giant, into which the Earth would fit well over 1000 times. It has a family of 66 moons (at the last count), the largest four of which, Io, Europa, Ganymede and Callisto, are big enough that they would be considered planets if they didn’t themselves orbit a planet. Jupiter orbits the Sun at a distance of around 780 million km, and so it’s surface is very cold, around -150°C (123K). Its atmosphere (and the whole planet is atmosphere, with no solid surface) is made up of around 90% hydrogen and 10% helium.

Seen from Earth, Jupiter presents a disk of between 30 and 50 arc seconds, making it (usually) bigger than Venus as seen from Earth. Unlike Venus, Jupiter always presents a full phase towards us, upon which we can see (through even a small telescope) cloud features including the famous Great Red Spot. The gases in Jupiter’s atmosphere are not as reflective as Venus’ clouds, and Jupiter is a considerable distance further away, and so it appears fainter in our sky, despite presenting a large disk to us. The brightest it gets is around -3 magnitudes, about four times fainter than Venus at its brightest.

During the 15 March conjunction Jupiter will have a brightness of -2 magnitudes, compared to Venus at -4.2 magnitudes, and so Venus will appear around 7 times brighter.

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