On the line – August 2009

Claire Bretherton: Hello, I'm Doctor Claire Bretherton and welcome to the latest edition of Big Questions Answered. Today we're going to be answering the question that crops up every year around this time, so over to you John.

John: Hello, my name is John. I've received an email saying that the planet Mars will be as large as our Moon to the naked eye during the month of August 2009. It will be the closest it has been to the earth in almost 5000 years. I'd like to know if this is true or not?

Claire: Thank you John, and as I said, that is a question that crops up around this time every year, but unfortunately, the email you received is completely wrong. Now, it is true that Mars did get particularly close to the earth on 27 August 2003. It reached a distance of just under 56 million km, the closest it had been in nearly 60,000 years.

Now, Mars takes around two Earth years to go around the Sun. So every year and a half or so, the Earth and Mars actually line up in their orbits. But the orbits of Earth and Mars are not actually circular; they're actually slightly elliptical, so sometimes when they align, their orbits are closer together than at others. And on this particular date in 2003, their orbits were very close together indeed.

Now the last time we think Mars got this close was on 12 September 57,617 BC - a very long time ago. And it's likely to get as close again on 28 August 2287.

Now the other part of the email suggested that Mars would look as large as a full Moon in the sky. This would look spectacular, but unfortunately, it's just not possible; the Moon is much, much closer and even though Mars is larger than the moon, about twice the radius, it's much, much further away.

The Moon is only around 380,000 km away, so even at this close approach where Mars reached 60 million km away, it can never look anywhere near as large as the Moon in the sky and it will always be much fainter; so even at its brightest, the full Moon will be over 7000 times brighter than Mars in the sky.

Now, if you do want something impressive to look out for in August, then try looking out for the Perseids meteor shower. Now meteors are known as shooting stars, but they're not really stars all. Actually, they're bits of dust and grit coming into the Earth's atmosphere and burning up as they go, creating a flash of light across the sky.

Now, the Perseids meteor shower is caused when the Earth travels through the trail of debris left behind by the comet Swift Tattle; and it's very prolific, peaking on the 12 August.

If you go outside and give your eyes some time to adapt, then you may see one or two meteors every minute.

Well, do keep your questions coming, the number is 0208 123 9911 and hopefully we can catch up again soon.