Moonwatch - The Straight Range

TYPE: Mountain range
SIZE: 90km length, 1800m max altitude
AGE: 3.2 to 3.85 billion years old
LOCATION: Latitude 48°N, longitude 20°W
RECOMMENDED EQUIPMENT: 4-inch reflector

My lunar offering this month is an extraordinary little mountain range, unlike anything else on the Moon. The famous Victorian selenographer WR Birt named it the Straight Range because of its shape, and this name we use today, though official lists use the Latin form: Montes Recti.

Whenever illuminated it is easy to find on the northern edge of the Mare Imbrium about halfway between the crater Plato and Cape Laplace, and is therefore in view after first quarter for a considerable part of each lunation (see Glossary, page 105). It consists of a line of peaks arranged in a straight line – and it really is straight, so that it stands out. It is some distance inside the Mare, and there are no other mountains anywhere near it.

Though the range is about 90km long, it is nowhere more than about 20km wide. There are between 20 and 30 obvious individual peaks, the highest of which rise to no more than 1,800m. The brightest peak is at the western end.

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“One can almost appreciate why some of the early observers suggested that it might be an artificial structure!”

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A very small telescope will show this unusual feature well, and one can almost appreciate why some of the early observers suggested that it might be an artificial structure! The Mare area all round it is relatively smooth, with only one craterlet of any size between it and the coast; the conspicuous 20km impact crater Le Verrier lies well to the south.

When I come across anything at all linear on the Moon’s surface, I tend to search for associated ghost rings. Thus the famous Straight Wall in the Mare Nubium (which is not a wall, it is a 110km fault in the mare) is obviously linked with an ancient ring that has been covered with lava. I once spent a long time looking for something significant with the Straight Range. I failed to find it. Eventually I came to the conclusion that it must be part of a very old ring whose walls – or the remnants of them – might be concentric with a larger structure defined by the rims of the Apennines and Caucasians.

Subsequently I found that the same suggestion had been made, much earlier, by Bill Hartmann. I think it is probably correct; the history of this part of the Moon is not too easy to interpret, but at least we can be sure that the major factor has been the catastrophic impact that produced the Mare Imbrium.

Photography of this area is always worthwhile. The smoothness of the terrain north of the Straight Range contrasts sharply with the roughness of the region between Imbrium and the northern Mare Frigoris; you can also capture Plato, the clumpy Teneriffe Mountains, and Cape Laplace at the edge of the Sinus Iridum. At the next lunation prepare your camera, wait until just after half-Moon, and hope for a clear sky.