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Tue, 11 Apr 2017

What's that bright star?

Jupiter is the bright star below the Moon in this view from Kendal, Cumbria, on 8 April by Stuart Atkinson

Even people who aren't habitual stargazers are likely to spot a bright star in the evening sky at the moment, hanging in the southern sky late in the evening. This star is actually the planet Jupiter, which is dominating the sky through the night at the moment, and is particularly bright and attention-grabbing because it is at its closest to us for some time, and is at "opposition". That's the astronomical term for describing how it lies exactly opposite the Sun in the sky and so rises as the Sun sets. What it means for sky-watchers is that Jupiter is impressively big and bright in the sky, brighter than any other star or planet on view after dark.

The actual date of opposition was 7 April, when the planet was due south at midnight GMT, which is actually 1 am BST. But it will be around throughout the summer, though getting slightly more distant each night. So this is the best time to view it and see it at its most magnificent.

Though you really need a telescope, even a small one, to see any detail on Jupiter, just binoculars will show you its brightest moons, strung out on either side of the planet. There are four in all, Io, Euroopa, Ganymede and Callisto, all orbiting the planet. This means that they change position from one night to the next, so if you can view the planet two nights running you will see the moons in a different orientation. The diagram below shows the movement of the moons over just one day. Though the two views look quite different, you can see that only Io, the closest moon to Jupiter, has changed sides, while the others have simply moved closer or farther away. 

movement of Jupiter's satellites

Jupiter on 6 April

Jupiter on 6 April, with moons Callisto (upper left) and Io (right). The Great Red spot is visible near the centre of the disc. Photo: Robin Scagell

Spot the Spot

If you study Jupiter through a telescope, look out for the dark belts running across the middle of the planet. These are cloud belts – Jupiter is a gas planet, with no solid surface. Notice that Jupiter's rapid rotation speed of about 10 hours has flattened the planet somewhat. You might even see the Great Red Spot (GRS), a storm that has been raging on Jupiter since 1878, and which is visible on the photo shown above. This has been diminishing in size in recent years, though gaining in redness. 

How small a telescope do you need to see the GRS? It was seen through a 90 mm refractor recently, and probably a 75 mm telescope will show it. If you do spot the Spot through a small telescope, please let our Planetary Section Director, Alan Clitherow, know about it. Please give the date and time and instrument details. 


Added by: Robin Scagell