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MERCURY passed in front of the sun two weeks ago in a rare transit event. This week, we get a chance to see the planet at night. But you will have to set your alarm.
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Mercury and Venus are known as inferior planets, because they orbit closer to the sun than Earth does. On 28 November, Mercury reached its greatest western elongation. This is the point in its 88-day orbit of the sun when the distance between the sun and Mercury as seen from Earth is the biggest it gets. This apparent distance is also known as the angular separation. In the following days, the planet dips a little lower in the sky, but you should still get a good view.
Seeing the planets depends on them being as far from the sun as possible, because when they are right next to it, the light they reflect is outshone by the sun itself. This matters even more for Mercury than it does for Venus, because it is much smaller and fainter than its neighbour, and so close to the sun. The rocky planet, with a cratered surface rather like our moon鈥檚, is less than 5000 kilometres in diameter. That makes it smaller than Jupiter鈥檚 moon Ganymede, and Titan, which orbits Saturn.
Mercury鈥檚 maximum angular separation varies between 18 and 28 degrees according to how close it is to the sun in its highly elliptical orbit (the absolute maximum separation is shown in the illustration). For Venus, it is between 45 and 47 degrees. Mercury鈥檚 angular separation is smaller because its average orbit is about 58 million km from the sun, while Venus鈥檚 is 110 million km and Earth鈥檚 is about 150 million km.
Mercury鈥檚 proximity to the sun makes it hard to see most of the time, with the best viewing opportunities being just before sunrise or just after sunset.
While the greatest elongation is on 28 November, Mercury will be visible for two weeks after this, rising 2 hours before sunrise and reaching 10 degrees above the horizon 45 minutes before sunrise. People in both hemispheres should get a look, but those in the north will have the best view. To spot Mercury, try to find the constellation Libra and the planet should be there.
Binoculars will give you a better view, but you can see the planet with the naked eye. At the time it rises, Mercury will have a magnitude of 鈭�0.44, so should be visible to most people with clear skies regardless of light pollution. Because it is close to the horizon, it will shine with a pink hue, like the setting sun. That is because Earth鈥檚 atmosphere scatters light coming through it, reducing the amount of blue light reaching your eyes.
The nights are getting longer in the northern hemisphere, so next week is a guide to the best way to watch the aurorae there.
What you need
Binoculars
A good view of the eastern horizon
For next week
A dark night at high latitude
Next in the series:
2 How to watch the Leonid meteor shower
3 Venus and Jupiter in conjunction
4 Mercury at its greatest elongation
The best time to see the planet directly
5 How to see the Northern Lights
6 Find the Andromeda galaxy
7 How to see Santa (the ISS) on Christmas Eve
Stargazing at home online
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