Mars is one of the brighter planets and you will have no problem finding it in the heavens when it is in a dark sky. It does, however, vary considerably in brightness because its distance from us changes hugely over time.

Mars' path in 2010 (click to enlarge)

When far apart, Mars appears like a fairly average bright star. At its closest, it shines as one of the brightest objects in the sky and its reddish colour will be quite apparent.

Crowds queue to view Mars at Wroughton, in the UK, in 2005

The close encounter with Mars was particularly close in 2003 when its apparent diameter in the sky grew from just 4 arcseconds to a mighty 25 arcseconds. When Mars was at its closest in December 2007, it was a fraction under 16 arcseconds. Mars came to a distance of 88 million km (55 million miles) this time compared to 56 million km (35 million miles) in 2003.

Astronomers talk about brightness as a number called magnitude. The higher the number the fainter the object. Each whole number is about 2.5 times brighter or fainter than the next. The brightest stars in the sky are around magnitude 1. The faintest you can see without a telescope on a dark night are around magnitude 6. The Sun shines at a minus magnitude, -27. Against this scale, Mars’ brightness varies from about 2 when it is at its furthest from us to a brilliant -3 when at its very closest.

Our chart shows the path of Mars as it slowly faded brightens on its next approach to Earth. Opposition will occur on 29 January 2010 when the planet will reach a magnitude (apparent brightness) of -1.2.

Around opposition, Mars wil be seen to perform a curious loop in the sky. Astronomers call this retrograde motion. There is no trickery on Mars’ part – this happens when the faster-moving Earth overtakes the Red Planet on the inside track. It is a bit like passing another car on the highway – you are both heading in the same direction but the other vehicle appears to slip backwards against the background.