On Monday morning, 18 August 2014, in the eastern sky before sunrise you’ll see a very close conjunction of the two brightest planets, Venus and Jupiter.
They’ve been shining brightly in the pre-dawn sky for a while now, but as they trace out their separate orbits around the Sun they appear to move relative to one another, Venus the faster of the two. And they’re getting closer every day, until on Monday 18 August they’ll be at their closest, only 12 arcminutes apart, about one third of the diameter of the Moon.
This is closest conjunction in 15 years, and will be a very striking sight in the morning sky, but you’ll need to be up and about early to see it, about an hour before sunrise, around 0450 BST (sunrise is around 0550BST for most of the UK – Orkney gets an earlier sunrise at 0535, while the southwest of England have to wait till around 0605).
If you’ve got a pair of binoculars and a tripod, or even better a telescope, it’s really worth looking at these two planets. Venus is the brighter of the two, shining about twice as brightly as Jupiter through the morning twilight, but if you can magnify them (and you’ll catch them in the same field of view in a pair of binoculars), then Jupiter will be around three times the diameter of Venus (30 arcseconds compared to 10), and you’ll see Jupiter’s four largest moons as tiny points of light near the giant planet.
Don’t worry if you’re clouded out, or if you sleep in, on Monday morning; they’ll be close together in the pre-dawn sky for a few days afterwards too.
This evening, and for the next few evenings, just as the sky begins to darken after sunset, you’ve got a chance to see three of the five naked-eye planets side by side.
The two brightest naked eye planets (Venus and Jupiter) are close together, separated by only a few degrees, closing to 1° on 28 May (in what we call a conjunction). This should make them very easy to spot, low in the NW from around 30 minutes after sunset. In fact they’re close enough together that you could fit them both in one binocular field of view.
Mercury, however, might be trickier to spot. As the faintest naked-eye planet it will lurk in the twilight sky unseen for many people, just above the two brighter planets.
Remember, if you’re observing with binoculars or a telescope make sure you wait until the Sun has fully set
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.
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, 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.
Tonight, just after sunset, Venus, Jupiter and a thin crescent Moon will line up in the evening sky. If you’ve got clear views towards the west it’s really worth a look.
Thanks to the excellent Starwalk app for the above image.