Starwatch: Mars at its closest

Mars as seen from Earth in 1999 – the last time the two planets were similarly placed. Photograph: NASA/STScI

Mars comes closer to the Earth on 14 April than at any time since early 2008. Even so, it appears only 15.2 arcsec across through our telescopes and we need good observing conditions, and some patience, to appreciate its surface detail. Just don't expect as sharp a view as that in our Hubble image which dates from 1999, the last time that Mars and the Earth were similarly placed.

Mars was at opposition on 8 April when, by definition, it stood opposite the Sun in the sky so that it rose in the E as the sun set and was highest in the S in the middle of the night, about 01:00 BST. Mars, though, follows a relatively eccentric orbit and is now inbound from its farthest point so that its closest approach has been postponed by a few days. However, at 92.4 million km, it is only 450,000 km closer on the 14th than it was on the 8th.

There is no mistaking the distinctive glow of the Red Planet as it stands 5° above the full Moon on the evening of the 14th and shines at magnitude –1.4 to rival Sirius. Mars lies 9° NW of Virgo's main star Spica and is tracking westwards to pass 1.4° S of the famous binary star Porrima, or Gamma Virginis, on 3 May.

Overnight on the 14/15th, the Moon approaches Spica and begins to enter the fringe of the Earth's shadow as it sets at dawn for Britain. The resulting lunar eclipse is total for the Americas between 03:07 and 04:25 EDT, with at least some of the Moon's disc lying within the southern part of Earth's dark umbral shadow between 01:58 and 05:33 EDT.

Since Mars takes 687 days to orbit the Sun, the Earth overtakes it, and it comes to opposition, every 2 years and 7 weeks on average. That eccentric orbit, though, means that some oppositions are better than others. It may be closer than it has been since 2008, but in 2016 it is closer still and in 2018 it comes within 58 million km of us and its small globe, only 6,792 km across, swells to 24 arcsec. On the other hand, both these upcoming oppositions occur with Mars further S and lower in our sky. Indeed, in 2018 it is 2° further S in our summer night sky than is the Sun at midwinter – far from ideal for telescopic study.

The day on Mars is 40 minutes longer than on the Earth, so our view changes only a little if we observe at the same time from night to night. As in the Hubble image, the north polar cap is obvious as a white button, while other surface features process slowly across the disc as the planet turns. One of the more prominent, Syrtis Major, is the dark wedge that lies near the meridian on the image. This low-level shield volcano has darker rocks which are wind-swept relatively clear of the lighter-hued and rusty iron-rich sand and dust that blanket large areas of the surface. If we observe near midnight BST on the 22/23 April, then our view of Mars and of Syrtis Major should be almost identical to that in our image.

The north polar cap, mainly of water ice and tipped 22° towards us, was larger during the Martian winter when it had an additional dusting of frozen carbon dioxide. However, the summer solstice in Mars' northern hemisphere occurred two months ago, and much of the carbon dioxide has sublimed back into the atmosphere.