All About Space

Moon tour

How to find the most dramatic cliff on the face of the Moon

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If you have a small telescope, or a powerful pair of binoculars, and look to the lower left of the great triplecrat­er chain of Ptolemaeus, Alphonsus and Arzachel you will see – depending on the time of the month – either a short, dark line or a short, bright line. Moon atlases and phone apps identify it as ‘Rupes Recta’, and to be honest it doesn’t look like much at first glance, nothing more than a dark pencil line or a white chalk scratch drawn on the Moon’s round, grey face. But Rupes Recta has another name, and it is one of the most famous and beloved features on the whole of the Moon. It’s known as the ‘Straight Wall’. It’s not actually a wall. It is an enormous scarp, a region where part of the Moon’s surface dropped dramatical­ly away, leaving a steep cliff behind.

That cliff is very narrow, nowhere near as wide as the terraced rims of those three giant craters blasted out of the Moon to its north. It’s not that tall either: with a maximum height of around 450 metres

(1,476 feet), it’s roughly as tall as the London Eye, or two nuclear submarines balanced end to end. And although the Straight Wall gives the impression of being a towering cliff face, it’s not. Pre-Apollo space artists depicted the Straight Wall as vertical – a frozen tsunami wave of grey lunar stone – but if you stood at its base its slope would rise at an angle of only 30 degrees or so.

So Rupes Recta turns out to be not very wide, or very high. The Straight Wall’s remaining claim to fame is that it is very long. Stretching more than 110 kilometres (68 miles) across the lunar surface, it would reach from London to the Isle of Wight if placed on Earth – or from Carlisle to Edinburgh if you prefer a more northerly comparison. That’s so long that it would have taken Apollo astronauts more than eight hours to trundle and bounce from one end to the other in the lunar rover which was used on the Moon in the later missions.

The Straight Wall will first become visible on 25 September, when the Moon will be just past its first quarter phase. On that evening, with sunlight illuminati­ng it from the east, its cliff face will be in shadow, so it will look like a very obvious short, dark line to the lower left of Arzachel, very close to a small, deep pit of a crater called ‘Birt’. As sunlight creeps across the Moon’s face and the cliff’s shadow retreats, the Straight Wall will slowly turn into a bright line which seems to sink down into the Moon, becoming harder and harder to see until it is barely visible at all at full Moon.

The best time to see the Straight Wall will be after midnight on 10 October, when the Moon has just passed last quarter. Now illuminate­d from the west, Rupes Recta’s cliff face will be bathed in full sunshine, making it appear as a strikingly bright line etched into the darker surface of the Moon, looking as if it has been dug out of the ancient, frozen lava flows of Mare Nubium by the tip of a knife’s blade. At this time it will be visible in a good pair of binoculars, but telescopes will offer the best views, the higher the magnificat­ion the better.

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