The Far Side Of The Moon.

LOVE TO THE MOON AND BACK

Look at the Moon, even with the naked eye, and you will see the obvious features such as the principal maria. The positions of these features on the disk are always much the same, because of the synchronous rotation. 

Yet there are slight shifts, due to the effects known as librations. All in all we can examine a grand total of 59 per cent of the lunar surface, and only 41 per cent is permanently averted, though of course we can never see more than 50 per cent.

 The most important libration – the libration in longitude – is due to the fact that the Moon’s path round the Earth is elliptical rather than circular, and it moves at its fastest when closest to us (perigee). 

However, the rate of axial rotation does not change, so that the position in orbit and the amount of axial spin become periodically ‘out of step’; we can see a little way round alternate mean limbs. There is also a libration in latitude, because the Moon’s orbit is inclined by over 5 degrees, and we can see for some distance beyond the northern and southern limbs.

Finally there is a diurnal or daily libration, because we are observing from the surface, not the centre of the globe. All these effects mean that the ‘libration regions’ are carried in and out of view. They are so foreshortened that it is often difficult to distinguish between a crater and a ridge, and before 1959 our maps of them were very imperfect.

About the permanently hidden regions nothing definite was known. It was reasonable to assume that they were basically similar to the familiar areas – though some strange ideas had been put forward from time to time. 

The last-century Danish astronomer Andreas Hansen once proposed that all the Moon’s air and water had been drawn round to the far side, which might well be inhabited! The first pictures of the far side were obtained in October 1959 by the Russian space probe Lunik 3 (also known as Luna 3). 

It went right round the Moon, taking pictures of the far side and later sending them back by television techniques. 

The pictures are very blurred and lacking in detail, but they were good enough to show that, as expected, the far side is just as barren and just as crater scarred as the areas we have always known. Later spacecraft, both manned and automatic, have enabled us to draw up very complete maps of the entire lunar surface.

There is a definite difference between the near and the far sides, no doubt because the Moon’s rotation has been synchronous since a fairly early stage in the evolution of the Earth–Moon system; the crust is thickest on the far side.

 One major sea, the Mare Orientale, lies mainly on the hidden regions; only a small part of it can be seen from Earth, and then only under conditions of favorable libration. The spacecraft pictures have shown it to be a vast, multi-ringed structure which is probably the youngest of all the lunar seas.

 Otherwise there are no large maria on the far side, and this is the main difference between the two hemispheres. One very interesting object is Tsiolkovskii, 240 kilometres (150 miles) in diameter. 

It has a dark floor which gives the impression of being shadowed in many photographs, though the real cause of the darkness is the hue of the floor itself; there is no doubt that we are seeing a lake of solidified lava, from which a central peak rises. 

In many ways Tsiolkovskii seems to be a sort of link between a crater and a mare. It intrudes into a larger but less regular basin, Fermi, which has the usual light-coloured interior. Many of the familiar types of features are seen on the far side, and the distribution of the craters is equally nonrandom; when one formation breaks into another, it is always the smaller crater which is the intruder. 

Valleys, peaks and rays systems exist. Though the Moon has no overall magnetic field that we can detect, there are regions of localized magnetism here and there; one of these lies near the rather irregular far-side crater Van de Graaff. 

It has been suggested that the Moon used to have a definite magnetic field which has now died away. On the original Lunik 3 picture a long, bright feature running for hundreds of kilometres was shown, and was thought to be a mountain range which was promptly named in honour of the Soviet Union. 

Alas, it was later found that the feature is nothing more than a surface ray, and the Soviet Mountains were tactfully deleted from the maps. However, it was surely right to name the most imposing far-side feature in honour of Konstantin Eduardovich Tsiolkovskii, the great pioneer who was writing about spaceflight almost a hundred years ago.

  NEED TO KNOW

Diurnal libration. We are observing from the Earth’s surface at A, not its centre, so that we can see a little way alternately round the northern and southern limbs

Van de Graaff is a large but rather irregular formation, perhaps compound, notable because of the amount of remnant magnetism in and near it. Its floor contains several smaller craters. The wall is broken (top right-hand corner) by Birkeland, a well-formed crater with a prominent central peak.

 Tsiolkovskii is exceptional in many ways. It is 240 km (150 miles) in diameter, with terraced walls and a massive central mountain structure. The darkness is caused by lava; in fact Tsiolkovskii seems to be intermediate in type of lunar feature, falling somewhere between a crater and a mare, or sea.