Moon tour One of the Moon’s most incredible but elusive features peeks around its limb
One of the Moon’s most incredible but elusive features peeks around its limb this month
This month’s tour destination is, in the opinion of many Moon enthusiasts, the most spectacular feature on the surface of Earth’s satellite. It resembles nothing less than an enormous archery target bullseye on the Moon – but frustratingly, we can’t really see it.
We can see part of it – a hint of it – but this lunar feature is located just around the western limb of the Moon, so we only see its closest edge, and we can only see that when the Moon’s wobble – or ‘libration’ – briefly tips it towards us. A few privileged people have seen it in all its glory: the Apollo astronauts flew over it and stared down at it with awe and wonder, but for the rest of us the only views we can enjoy are the images taken by satellites orbiting or space probes passing the Moon on their way to a distant world.
Mare Orientale – or the Eastern Sea – is a 327-kilometre (203-mile) wide dark sea, or mare, of frozen lava, surrounded by three concentric rings of crater-pocked mountains which make the whole feature more than 900 kilometres (560 miles) across. It is one of the youngest impact features on the Moon, blasted out of it around 3.8 billion years ago by an enormous asteroid more than 60 kilometres (37.3 miles) wide. In contrast, the asteroid that helped kill off the dinosaurs was only ten kilometres (6.2 miles) wide. It’s hard to imagine the enormous amount of energy released in that event. If time travel is ever invented, surely one of the most popular Moon destinations will be the day Mare Orientale was formed, so people can see the asteroid plummeting into the Moon, exploding with a blinding flash and blasting a gaping hole out of the lunar crust, leaving behind the enormous bullseye target we see today.
It’s fascinating to imagine how different things would have been if that asteroid had struck the Moon on its Earth-facing side. Some think it would have affected the development of many of our cultures and religions. Today, during a full Moon, instead of us looking up at the gently smiling face of the ‘Man in the Moon’ we would instead see a huge eye, staring down at us from the sky, never blinking, never closing. And during a total lunar eclipse that eye would appear red, bloodshot. How terrifying would that be? Surely we would now fear the Moon and cower under its gaze, rather than writing poetry or songs about it.
Because Mare Orientale is only ever visible for a brief time, any opportunity to catch even a fleeting glimpse of it is grasped by lunar observers. This month the Moon’s libration will woozily swing the western side of the Moon towards us most favourably on 10 February, when the Moon is full. That’s when Mare Orientale and its surrounding rings of mountain will come into view, but only through a good pair of binoculars or preferably a telescope, and even then only as a mottled, light-anddark area close to the lunar limb.
To find Mare Orientale, imagine the Moon’s face as a clock face. Around 10 February when the Moon is full, Mare Orientale will be tilted towards us because the lunar libration will be at its most pronounced. Focus your binoculars or telescope towards the darkfloored crater Grimaldi, which can be found at the eight o’clock position. To the crater’s lower left, right on the limb, you will see a mottled area – this is Mare Orientale and its mountains.
If you use a high-magnification eyepiece on Mare Orientale when it is presented to us at its very best you can see a surprising amount of detail around and within it, and there are lots of lunar charts and maps available online to help you identify them. Unfortunately, no matter how much you magnify the view you won’t be able to see the feature’s true ‘bullseye’ nature, but that’s not the point. Just enjoy seeing something most sky-watchers, and many Moon observers, never manage to see.