Wed, 02 Nov 2016
Mars chases Earth
Mars (top) on 20 October. Photo by Robin Scagell
Mars has been in the news, with the ESA Exomars mission. You can see the Red Planet for yourself by looking to the south-south-west during evening twilight, as it's currently easily visible to the naked eye after sunset.
Right now, Mars is chasing Earth in its orbit, but it's a race that it is doomed to lose because Earth has the advantage of being closer to the Sun than Mars. The laws of planetary motion decree that the closer a body is to the Sun, the faster it moves in its orbit. We overtook Mars in its orbit back in May, and we've been pulling away ever since. Mars is gamely following us in its outer orbit, but eventually it will lag behind too much to be visible, and will disappear into the evening twilight sometime next May.
Until then, you can track it as a noticeably red object moving slowly against the starry background as the autumn constellations give way to those of winter and then spring. But don't expect too much if you look at it through a telescope. It is a real disappointment, because of its distance and low altitude in UK skies. During November it is about 7 seconds of arc across – that's about one seventh of the diameter of Jupiter as seen in the sky – and is about 200 million km away. By the end of the year it will have shrunk to 5 arc seconds in diameter and about 245 million km away. Even those with large telescopes will struggle to see more than a tiny but bright pale orange blob, shimmering as Earth's atmosphere trembles, though the farther south you are the higher in the sky and the better view you will get.
So be content just to see Mars, if you haven't done so previously, and keep an eye on it over the next few months. Next January and February it'll be close to Venus, which will be very much brighter in the evening sky, which will act as a guide to finding Mars. And look forward to the summer of 2018, when Mars will come closer to Earth than since 2003, and will be a great sight in telescopes.
Added by: Robin Scagell