Royal Observatory Greenwich blog
The Sky Tonight – August 2008

The highlight this month is Jupiter, rising in the South-East at sunset before commuting towards the South-West overnight. Take a look through binoculars and you will be able to see up to four moons around the planet. Take another look just a few hours later, and you may notice that the moons have moved slightly in their orbit around Jupiter.

Every August, the Earth ploughs through dust and debris left behind by comet Swift-Tuttle – and this produces the Perseid meteor shower. It’s a bit like driving a car through snow fall, where the car is planet Earth, and the snow flakes are the bits of cometry dust, that burn brightly and briefly when they hit the atmosphere.

It’s called the Perseid shower because, during August, the Earth is moving towards the constellation Perseus as it orbits around the Sun at 70,000mph. So lots of meteors appear to be coming from that direction!

At this time of year, the Earth is on the side of the Sun which allows us to see constellations such as Hercules, Cygnus and Lyra. We are also on the side of the Sun that is nearer to the centre of our galaxy – so we get a great view of the Milky-way arching over head this time of year. But you will have to go somewhere dark to see it at its best, well away from any inefficient lighting.

In Cygnus, the star that makes up the head of the swan is called Alberio. Although it looks like a lone star to the unaided eye, a small telescope or binoculars reveals that this is actually a double star. It is the stark difference in colours that makes this double star such a delight. Although there has been no sign of any orbital motion of the last hundred years of observation, calculations suggest that they orbit around each other once every around 70,000 years! The pair are visible with a small telescope, so take a look.

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