Timocharis

Go on a sightseeing tour of a little crater with a very violent past

Up near the top of the lunar disc, nestled in the curve of the rugged Montes Apenninus, is a small crater called Timocharis. It really doesn’t look like anything special; the Moon is spattered with countless tens of thousands of craters just like it, each one proof that our satellite has been absolutely pummelled and pulverised in its distant past by asteroids and comets that were left circling the Sun after it was born 4.6 billion years ago. Some of these impact craters are enormous, and lunar observers gaze at them again and again, ignoring the smaller holes surrounding them. But some of these small craters have fascinating stories behind them too, and deserve more than just a fleeting glance, such as Timocharis.

The crater was named in 1651 by the skilled lunar cartographer and observer Giovanni Battista Riccioli to honour the Greek astronomer and philosopher Timocharis, who not only worked in the famous Great Library of Alexandria, but very accurately measured lunar star occultations and observed a planetary occultation by Venus.

Timocharis lies in the Sea of Showers, Mare Imbrium, a vast, dark plain of ancient frozen lava which represents one of the eyes of the ‘Man in the Moon’. It is just 34 kilometres (21 miles) across and three kilometres (1.86 miles) deep, and forms a tight triangle with – but is dwarfed by – its near neighbours, enormous 96-kilometre (59.6-mile) Copernicus to the southwest and 59-kilometre (36.6-mile) Eratosthene directly to its south.

In lunar terms Timocharis is a relatively young feature, as is proven by the fact that it is not surrounded by any major bright beams or pale splashes of debris. At full Moon you can see some subtle rays spreading away from Timocharis, but they are pale imitations of those which surround its bigger, older neighbours.

If you take the time to hunt it down with your telescope, a high-power eyepiece reveals Timocharis to be a surprisingly complicated feature. It is roughly polygonal in shape, with sharp, rugged outer walls, the inner slopes of which are broken up into multiple ledges and terraces. In some places these have slumped down towards the crater’s floor. When sunlight is striking Timocharis from a steep angle – for example when it is near the terminator, it can look like a stony bullseye in the eyepiece.

On still evenings of really good seeing, a high-power eyepiece will show Timocharis has a central mountain peak – or rather it used to. Once, long ago, a towering mountain rose above Timocharis’ floor, but now there is a small, deep crater there. Some time after Timocharis was formed, an asteroid came barrelling in from deep space and slammed into the mountain at its centre, obliterating it and scattering its shattered pieces across the crater floor, leaving behind a hole which now stares out from the heart of Timocharis like an empty eye socket. Many of the countless tiny craters you’ll see spattered across Timocharis’ floor and slopes were probably formed by that impact.

At the start of our observing period on 25 March, the Moon will be in its waning gibbous phase, just a couple of days away from full, so Timocharis will be in full sunlight. At this time, seen through a telescope the crater will be a pale-grey ring with a dark centre. It won’t be until 1 April, when the Moon is approaching its first quarter phase, that the crater will begin to show any kind of detail. With the setting Sun’s rays coming in at a lower angle, the crater’s walls and internal features will cast shadows that will make them stand out more clearly.

By 4 April the crater will stand out starkly in an eyepiece, looking like a round pit with a jumbled floor, and subtle details on its surrounding slopes will be more obvious too. On the evening of 5 April the terminator will be almost upon the crater, and this is when it will look most dramatic through a telescope – even a small one will show it standing out very clearly. By the next evening Timocharis will have been enveloped in darkness, and it will remain in darkness until the evening of 20 April, when the Sun rises above its local horizon and sunlight splashes over its walls and down onto its jumbled floor once more.