Moon tour
We head over to Mare Crisium for our lunar observations this month
If a prize was ever awarded for 'easiest feature to find on the Moon” it would surely be given to Mare Crisium, the Sea of Crises. It’s so big, so obvious, so well-placed that it actually takes a lot of effort to miss it. Even if you know absolutely nothing about the Moon and its surface features you’ll have seen Mare Crisium already, many times.
Named in 1651 by astronomer and lunar cartographer Giovanni Riccioli, after being known previously as 'The Caspian Sea' among other things. Mare Crisium doesn’t just have a special place in lunar science; it has featured heavily in science fiction, too. It was the setting for sci-fi master Robert Heinlein’s classic story The Moon Is A Harsh Mistress. It was also the setting of Arthur C Clarke’s story The Sentinel which inspired the film 2001: A Space Odyssey. Unfortunately, in real life, the lava plain has had no boots walk upon it so far. However, Mare Crisium has had several robotic visitors. In 1969, the Russian Luna 15 probe crashed there. In 1976, the Luna 24 probe, also Russian, landed successfully on the sea, collected precious samples of the rock and brought them safely back to Earth for analysis.
So, where can you find this landmark? If you have ever looked at a full Moon, or even a first quarter Moon, you’ll have noticed a small, dark oval shape at the top right of its disc, up at about the one o’clock position.
It’s really obvious through a small telescope, or just the smallest pair of binoculars, but even your naked eye is powerful enough to pick it out. This is Mare Crisium, a vast plain of ancient, dark lava which covers the floor of the Crisium Basin. In fact, calling it 'vast' doesn’t do it justice; Mare Crisium is more than 550 kilometres (342 miles) wide, and covers an area of roughly
176 thousand square kilometres (68 thousand square miles). It was formed between 3.9 and 4.5 billion years ago, when an asteroid struck the Moon so hard it didn’t just leave an enormous hole, but flooded it with a vast pool of melted rock and debris, which spread across its floor, cooled and set, leaving behind the dark 'sea' we see today.
Mare Crisium will start to become visible on the evening of 20 November, when sunlight starts to pour across it from the west. On that date the Moon will be what many people call a 'new Moon' – a lovely slender crescent, low in the west as dusk deepens. On this evening you’ll only be able to see the most eastern part of the feature. It will take just over 24 hours for the whole of the feature to be lit by the Sun, so by the evening of 22 November the whole of the Sea will be visible, looking like a dirty grey-black thumbprint on the bright, clean face of the Moon.
Mare Crisium will remain fully illuminated until the evening of 3 December, when darkness will start to creep across the Sea from the east. If you have a telescope, this will be a good time to use it to look for the small, young craters on the sea, including 19 kilometre- (11-mile) wide Pierce and 22-metre- (0.01-mile) wide Picard.
If you look at Mare Crisium through a high-magnification eyepiece at this time you might also be able to see other, more subtle features on the lava flooded floor, such as the ghostly, raised outlines of the craters Yerkes and Lick, which were filled and almost almost covered by lava when the basin flooded. Under really good viewing conditions you might also see low ridges of ancient rock here and there on the lava plain – lava ripples that set in place and today look like hairs on a freshly painted wall.
Mare Crisium will have vanished from view on 7 December, plunged into the freezing darkness of the long lunar night once more, not to return to view again until the evening of 20 December.