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Mon, 23 May 2016

Mars comes close

Mars (centre), with Saturn to its left, photographed among the stars of Scorpius on 4 May. Antares was then immediately below Mars, though Mars moves westward during the month.

While Jupiter is still well visible in the south-western sky, and is the brightest object in the night sky apart from the Moon, Mars and Saturn are attracting our gaze in the opposite part of the sky. They are both low down in the south-east in the late evening. Mars is distinctly orangey, while Saturn, to its left, is slightly yellow.

Mars comes close to Earth every two years – what's called opposition, meaning that it is directly opposite the Sun in the sky. It reached that point on 22 May, and was at its brightest on that date, though its closest point to Earth will be on 30 May. The difference is due to the fact that Mars's orbit is not completely circular but is slightly elliptical. Mars orbits the Sun more slowly than Earth, so about every two years Earth catches up with the Red Planet and overtakes it on the inside, as it were.

What's more, this is the closest that Mars has been to Earth since 2005, because Mars is currently near the point of its orbit that is closest to the Sun. In 2018 it will be closer still at opposition, and will be almost at the largest that it can ever be seen from Earth. The graph below shows how the diameter of Mars (measured in seconds of arc) varies over the years. The peaks are oppositions, and at the opposition of 2016 it is larger than at any time since 2005.

By the way, if you hear a story that Mars will be so close that it's as big as the Moon, read this page which debunks the myth and explains why people have been misled ever since 2003!

Even at this opposition, small telescopes will show its disc, but seeing detail on the planet requires a magnification of about 75 and, more importantly, steady conditions in our own atmosphere.

With the planet being so low, you have to look through a considerable chunk of our atmosphere to see the planet, and any turbulence spoils the view. This is what astronomers refer to as seeing -- the steadiness or otherwise of the view. The same thing gives rise to twinkling of stars. In good seeing, even a fairly small telescope (say 100 to 130 mm aperture) will show some of the dark markings on Mars. Take a look at the photo below. But in bad seeing all you will see if a shimmering blob, looking as if it is immersed in a fast-flowing stream. So choose your moment!

The same goes for Saturn, which reaches opposition on 3 June. You should be able to spot its famous rings with almost any telescope, but seeing detail on the disc requires good seeing.

They are both currently in the constellation of Scorpius, though outshining its brightest star, Antares. The name Antares actually means 'Rival of Mars', a reference to its reddish colour, as it is a red giant star. You can compare Mars and Antares for yourself this year.

Above: Mars, photographed using a 130 mm Sky-Watcher reflecting telescope, on 4 May. Below, Saturn on the same occasion.


Added by: Robin Scagell