Moon tour

This month we bow down before the ‘Monarch of The Moon’

Ask any Moon observer to name their favourite lunar crater and chances are most will nominate the same feature: Copernicus. Copernicus has a nickname – the Monarch of the Moon – and if you’ve only ever seen photographs of the crater but never seen it with your own eyes, you would be forgiven for thinking that sounds a bit pompous. But it is entirely justified.

Just as every amateur astronomer remembers their first view of Saturn and its rings through a telescope, or their first display of the northern lights, they never forget the first time they see Copernicus through a telescope. In fact, the crater is one of the first things a beginner will see through a pair of humble binoculars because it stands out so starkly on the Moon, even when viewed under 10x magnification through a shaking pair of binoculars on a chilly October night.

Copernicus was formed in fire and fury. Some time between 800 million and 1 billion years ago the Moon collided with a large asteroid. The explosion caused by the impact blasted an enormous hole out of the area of the Moon we now know as Oceanus Procellarum, the Ocean of Storms. The gaping pit left behind was almost four kilometres (2.5 miles) deep and more than 93 kilometres (58 miles) wide, which means its opposite walls are as far apart as London and Clacton-on-Sea.

The impact didn’t just excavate a hole; it sent an enormous debris cloud of dust and rocks billowing up into the sky and across the lunar landscape. Some of that material fell back down to the Moon, spraying across it and leaving bright rays of ejecta splashed on the landscape. These ‘debris rays’ are so bright and so long – the longest stretches for over 800 kilometres (497 miles), the distance between London and John o’ Groats – that they can even be seen with the naked eye when the Moon is full, looking like white chalk lines drawn on the Moon’s grey-blue face.

The crater itself lies just south of Montes Carpatus, the Carpathian Mountains, a short range of mountains on the southern shore of Mare Imbrium. Copernicus is roughly circular in shape and its walls are terraced on all sides, with a wide shelf dropped down on the western side and landslides in several places. The

crater’s floor is quite flat, a plain of ancient lava pocked here and there with much smaller, much younger craterlets. In the centre of the crater a trio of mountains protrude from the lava plain.

So how can you see this celebrity crater this coming month, and when will it look its best? Copernicus is fully illuminated at the start of our observing period, when the Moon will be a waning crescent glowing in the east before dawn. With the Sun’s light slanting in at a low angle from the west the crater will stand out starkly from its surroundings and will be an impressive sight in a telescope. Magnified 100x or more it looks like an empty eye socket staring back at you from across space. However, it will only be visible until the 13th, when the terminator will sweep over it and plunge it into darkness. The crater will then be hidden from view until 27 October, when it emerges into the sunlight again. As the nights pass the crater’s appearance will change, its sharp outline and interior details blurring away as the Sun climbs higher in the lunar sky.