Selenography: the ‘Geography’ of the Moon

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This is an archive story, published in the September 1943 edition of Geographical.
All facts, figures and statistics were accurate at the time of original publication. The text has been lightly edited solely for house style reasons but otherwise remains unchanged.

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In recent years the townspeople of Britain have taken considerably more interest in the moon than they did in times of peace. The power of the moon, our nearest neighbour in space, to diffuse light is, however, not its chief function, for the tides which cleanse our shores and give great ships access to many of our ports depend for their existence upon the attention of this, our only satellite, and, moreover, observations of the moon provide navigational data for mariners on the seven seas. What sort of an object is then this moon, whose importance is so much enhanced by the black-out? The study of it, which may be compared with the geography of the earth, is usually known as selenography.

The moon’s mean distance from the earth is just over sixty times the earth’s radius, which would make it about 250,000 miles. The sun is about 380 times further from us than is the moon.

The moon has no light of her own, and merely acts as a mirror for the sun’s rays. The phases of the moon, caused by the relative positions of the earth, moon and sun, are well-known, new moon occurring when the moon and sun are both on the same side of the earth, and full moon when the earth is between the sun and the moon. The moon is said to be in syzygy when it is new or full. It follows that the crescent is first seen like a sickle in the eastern sky after sunset, moving further to the east as it gets more full, until, as a full moon, it rises about the same time as the sun sets. At last quarter the moon is high in the heavens in the morning, the crescent becoming smaller and smaller as it draws closer to the sun, finally disappearing in the effulgence of our star. The first appearance of the crescent moon is especially important to the Moslems, particularly for the months of Ramadan and Bairam; from observations made with this in mind it has been found that the new moon may be seen when about twenty-four hours old and twelve degrees from the sun. In 1910, J.K. Fotheringham, the astronomer, dealing with Julius Schmidt’s observations made in Athens, claimed that this was independent of differences in latitude.

The diameter of the moon is some 2160 miles and its mass is 1/81.53 of the earth’s mass. The mean specific gravity of the moon is about 3.4, compared with the earth as a whole 5.5 and the earth's surface 2.65.

THE ORIGIN OF THE MOON

The friction of the tides in the seas of the earth caused by the moon, may be calculated to have the effect of increasing the distance between the earth and the moon by about five feet every hundred years. Sir George Darwin calculated that the initial length of our day would be equivalent to about one-sixth of our present day, and the initial distance of the moon's centre from the earth’s centre about 8,000 miles. If we could go further back would they be united? Most cosmogonists think not, unless an extremely improbable though not impossible thing happened: namely that, during the course of evolution, the tides caused by the sun on the as yet molten earth had a period which exactly coincided with the natural free period of vibration of the mass of the earth, should this molten mass be set pulsating. This would, of course, set up resonance, giving tides sufficiently high to cause rupture.

Why, at the time when something happened to cause our sun to have a planetary system, twin bodies such as the earth and moon came out of chaos into being so close together and of sizes so nearly equal at the same time, is not yet fully understood; but that they did so seems more probable, according to present use of the available evidence, than that at some distant time the moon separated from the earth tidally. The earth may even now not be solid right to the centre. There is a large core which appears to possess the properties of a liquid in that it will not transmit transverse earthquake waves; but it does appear that the moon is now solid throughout its interior. The moon has a bulge towards the earth amounting to about one part in 1,500. This, it has been thought, was caused by tides, for which the earth was responsible, when the moon was still molten; the earth-moon distance being then some 90,000 miles. At that time, the moon became solid and the bulge remained. This bulge on the moon is too great to have been caused since the time when the moon was more than 90,000 miles distant from the earth.

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THE MOON’S SURFACE

The surface structure of the moon, comparable to the geology of the earth, may be called selenology. The selenologist must carry out his studies with a telescope, camera or other optical device, whereas the instrument most usually associated with a geologist is his hammer. The results of selenology' are large-scale phenomena. We learn, for instance, the relative ages of the lunar formations such as that Aristarchus is younger than Kepler, and Kepler than Copernicus. Mr H. G. Tomkins has proposed that the dark substratum seen in various places at full moon especially in the maria, can be considered as a foundation on which all lunar formations are grounded, in order to correlate their ages over wide areas on the moon. He further suggests that the apparent mottling of extensive areas over the lunar surface may be comparable to pumice or volcanic ash, as suggested by Schonberg and Brunn, or that it may be an efflorescence rather than such a crust as is possessed by the earth. Whether or not there are fossils in the lunar strata, or what the mineral formations may be, we have no means of discovering. In 1787 William Herschel was actively engaged in England making observations of the moon with his telescope. We learn from The Herschel Chronicle (Cambridge University Press), edited by Constance A. Lubbock, that on May 20, 1787, he wrote to Mr Ernest, one of King George IV’s Pages:

In view of the generally held opinion that the moon is now quite solid throughout, it appears unlikely that Herschel actually did view an eruption. Even at the time there were sceptics, for Laland in a letter to Herschel dated May 21, 1788, wrote: ‘Mt Aristarchus which is naturally very brilliant might well reflect the light of the earth in such a manner as to produce this bright appearance across the pale light of the moon’. Maybe if there were radioactive materials near the surface of the moon, and the heat could accumulate, there might be some form of volcanic activity—but this possibility may be remote and it depends on a concatenation of circumstances any one of which may be absent.

HOW, WHEN AND WHAT TO SEE

The physical features of the moon are remarkable and it would be no exaggeration to say that the moon is the most interesting of heavenly bodies for a small telescope. With a fair-sized telescope it is better to use a low power and a dark eye-piece cap rather than reduce the aperture, which affects the sharpness of the definition.

The moon is only three-quarters as bright in apogee (point of orbit most distant from earth) as in perigree (point of orbit nearest to earth), and should we not wish to see a feature which would then be in darkness, best viewing conditions are about the time of first quarter and last quarter, since the features are in greater relief especially near the terminator (dark-light boundary), on account of their shadows, than nearer to full moon. In the northern hemisphere the most favourable viewing conditions, on account of the moon’s altitude above the horizon, are at vernal equinox for the first quarter and autumnal equinox for the last quarter, and vice versa in the southern hemisphere.

More than a hundred years ago John Russell, R.A., spent some twenty years making a careful drawing of the moon, though since that time photography has played a much more important part in this study. A catalogue of over 6,000 named lunar formations was presented by Mary A. Blagg and K. Muller to the International Astronomical Union in 1932. Since the moon is approximately a quarter of a million miles from us, on looking through a telescope which magnifies, say, 1,000 times, we should still see objects only as they would appear at a naked-eye distance of 250 miles. Thus only the most pronounced features are visible to us. At full moon contrast is lost and prominent objects such as Maginus disappear for two or three days before and after. Craters appear brighter than their surroundings. Linne shows some variation, and in the south-west portion of the moon the rays or streaks may be observed. Altogether about six-tenths of the moon’s surface may be observed from time to time, while the other four-tenths have never been observed. The one-tenth is due to the apparent swaying of the moon, called the moon’s libration, which is due to the inclination of its axis to its orbit. Owing to libration we rarely see a lunar object and its shadow in the same place twice, the maximum variation amounting to over twenty degrees. Objects near the centre of the moon (approximately equidistant from the three craters, Herschel, Schroter and Triesnecker) may be seen in their true shape, but nearer the limb more and more foreshortening occurs. Objects near the limb are in profile.

Early telescopists, using low-powered instruments, imagined they had discovered extensive seas on the moon, but more perfect and higher-powered telescopes have shown these features to be vast plains, by no means level or smooth, and possibly once the beds of lunar oceans. The Sinus Iridum, bounded by great cliffs rising to peaks over 16,000 feet high, is one of the finest objects and is best viewed when the moon is eight or nine days old.

Lunar mountains and mountain ranges are much more pronounced than terrestrial ones and some attain a height of five miles. These lunar mountains may be divided roughly into two classes. The first class consists of ordinary mountain peaks, ridges, hills and chains. Possibly the most conspicuous range is the Apennines in the northern hemisphere of the moon, which rises from the Mare Imbrium. It is about 600 miles long and the highest peaks reach a height of three and a half miles. The shadows from these mountains attain a length of a hundred miles as measured with a micrometer attached to a telescope. This may be verified by measurement of photographs.

The second class is composed of features conventionally called craters. These so-called craters may be walled plains, ring plains or craters proper. Walled plains such as Albategnius, Clavius and Schiller have a diameter, approximately, of between 40 and 150 miles. They are usually surrounded by a complex succession of walls, the floor being comparatively level, usually not much lower than the outside, and the central mountain is often absent. Plato is probably the best example. The ring plains such as Kepler, Archimedes and Tycho, of diameter usually between twenty and sixty miles, form the majority of the so-called lunar craters. They are more uniform and circular than walled plains and are, more often than not, surrounded by a single mountain range. The outer slope is small and the terraced interior often steep. The comparatively level floor of the ‘crater’ is nearly always much lower than the outside; the deepest of this type is Newton with a rim 23,800 feet above the interior. Wargentin, however, which must be included in the group, has a floor which is practically level with the top of the wall. The craters which most nearly resemble terrestrial volcanic craters usually on the moon have a diameter of from four to twelve miles and a small floor with a volcanic cone. They are approximately circular with a steep outer slope. Examples are Messier, Bessel and Linne. These craters proper are usually characteristically bright which enables them to be recognized at full moon, and is a feature which probably caused Herschel to imagine they were actually in eruption.

Of the valleys, perhaps the most notable is the Great Alpine Valley, though the deep narrow winding rill of Ariadaeus, like the bed of a dried-up stream, can be seen with the aid of a two-inch telescope. The cleft of Hyginus, which may be seen with the aid of a similar telescope, is just east of the rill of Ariadaeus and is more like a crack in the smooth surface than a river valley. In a small telescope it is like a hair, but such markings are often from fifty to a hundred miles long and up to two and a half miles in width.

Faults or closed cracks in the moon’s surface are also sometimes visible because one side is higher than the other.

Lunar rays are features peculiar to the moon. They are bright streaks which are best seen about the time of full moon (unlike other lunar features) and radiate from some of the principal craters. These rays are never above or below the general surface of the moon and traverse without a break all other features such as crater walls, valleys and ‘seas’. No complete explanation of their existence has yet been given. Possibly the finest system of rays radiates from the lunar crater Tycho, in the southern hemisphere of the moon, though some other radiant points for rays are Kepler, Messier, Timocharis, Proclus and Aristarchus. There are others. The craters Euclides and Landsberg A are surrounded by a bright patch sometimes called a nimbus.

In addition to the darkness of the ‘seas’ and the brightness of the rays and patches, and also the depth of lunar shadows, the variation of brightness and colour in different parts of the moon is most interesting to a careful observer with a small telescope. The brightness varies from place to place, Aristarchus being the brightest object on the moon and Grimaldi and Riccioli the darkest. The brightness also varies from time to time as on the floor of Plato, where it has undoubtedly something to do with the altitude of the sun. The floors of the ‘seas’ are also tinted with various colours, such as Mare Crisium grey-green, Lacus Somniorum bright grey, Palus Somnii bright yellow-brown, Mare Frigoris yellowish-green, and so on.

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MOON MAPS

To enable lunar features to be picked out on a map, the map is divided into quarters by the lunar equator, which is drawn very nearly through Rhaeticus and Landsberg, and lunar longitude 0° which is drawn through the centre of Walter and the east side of Aristillus. These lines intersect near the centre of the moon’s disc in mean libration and form axes of coordinates, the unit of reference then being one-thousandth of the semi-diameter of the disc. A less accurate though sometimes convenient method is to divide these quarters numbered I to IV (NW, NE, SE and SW) further into quarters by NS and EW lines, and the 16 approximately equal areas thus formed are lettered A B C D from west to east and abed from south to north.

It is usual to draw the map with west to the left and east to the right, south at the top and north at the bottom, since this is the way the moon is viewed in an ordinary inverting telescope.

The principal features have names of their own. Features near (or inside) a larger one, when not separately named, are denoted by the nearest named feature with letters alter. Eminences are usually given small Greek letters only after their names, and depressions capital Roman letters only. Double letters are used to indicate small features near larger ones. The larger feature is indicated by the first letter. Rills have Roman numbers followed by the letter r. Landsberg A is a depression to the east of Landsberg.

NO ATMOSPHERE – AND THE CONSEQUENCES

The moon has no atmosphere. When it passes in front of a distant star, the star disappears from view for the whole width of the moon even though only part of the moon’s width may be illuminated. The process is like an eclipse of the star but in this case is called occultation. Occultations of stars are frequent. For example, on March 12, 1943, at 16h. 40.2m. U.T. a Tauri disappeared behind the moon. It reappeared on the same evening at 17h. 51.8m. U.T. Times of occultation are given in the nautical almanac. The disappearance or immersion of the star always takes place on the east side of the moon and the reappearance or emersion always takes place on the west side. The immersion and emersion are always instantaneous and there is no gradual falling-off of brightness as there would be if the moon had an atmosphere. Occasionally a star seems to hang for an instant on the limb as though it may have chanced on an irregularity of the moon’s surface, though this is exceedingly rare.

If this lack of atmosphere on the moon were not at once apparent by direct observation, including the occultations of stars by the moon, we could have deduced it from observations of the moon’s gravitation and the velocities of molecules of the gases in possible atmospheres. For example, having obtained the mass and dimensions of the moon it is possible to calculate, using the known gravitational laws, how fast any object at its surface would have to be moving away from it in order to leave the surface and never return. This is called the velocity of escape and for the moon it is 2-4 kilometres per second. The dimensions of the moving object do not matter. Gravity has no favourites. The mean molecular velocity for hydrogen is 1-84 km./sec. at 0°C. and since even if the escape velocity is four times the mean velocity of the molecules the atmosphere would be almost completely lost in 50,000 years, the moon now would have no hydrogen in its atmosphere even if it had any initially. Moreover, the mean molecular velocity of a gas is proportional to the square root of its absolute temperature, and thus, on account of the sun’s rays warming up the atmosphere, the hydrogen would be doubly sure of escaping the moon’s gravitation. Further, as the mean molecular velocity is inversely proportional to the square root of the molecular weight of the gas, the moon would also lose nitrogen, oxygen and water vapour, but would retain carbon dioxide unless at some time in the past it were much hotter to increase the speed of the carbon dioxide molecules. Escape, of course, could be hindered by collisions with other objects and other molecules but would scarcely be prevented from taking place eventually. Thus we deduce by this indirect method the conclusion that the moon has no atmosphere.

Mr H. G. Wells in his phantasy of the moon imagined an underground atmosphere in caves and tunnels but we have no means of observing this. As there is no atmosphere there can be no wind, and with no water vapour there can be no rain, no ice and no snow. Ordinary denudation and the morning ‘stone showers’ known in mountainous districts on the earth would be absent on the moon. There can be no rivers on the moon, no lakes and no real seas. In the mountainous districts there may be evening ‘stone showers’, as these are caused by the variations in temperature. At the time of high noon on the moon the temperature may attain some 120°C. as there is no atmospheric protection from the sun’s rays, and this temperature is also suggested by telescopic bolometric and thermopile measurements. At the time of the moon’s night the temperature may fall well below 0°C. Should particles of rock high on one of the mountains of the moon become dislodged by the differences in temperature working on crystals of different expansibility, the particles would fall to join others on a scree at the foot of the slope, but only gravity would then act to move them further, for there would be no wind, rain or river action. The angle of rest for the scree would be quickly attained. Changes on the moon’s surface might thus be expected to be very, very slow and slight, and certainly would not be noticeable at our distance for perhaps thousands of years.

There being no gaseous envelope on our satellite, there is unlikely to be any plant or animal life at its surface. Professor Turner thought that there might have been life on the moon at some distant time, though what grounds he had for this belief I do not know. Jules Verne wrote an acknowledged fantasy on the moon, but the great lunar hoax of which the New York Sun published 60,000 copies in September 1835 must rank as one of the greatest of all time and is still talked about in America. It is not so well known in Great Britain, though an English edition of the paper was published in 1836. The author is unknown, though it may have been Nicollet, and it was possibly translated from the French by Richard Alton Locke, who may have added parts of his own, since there appear to be passages unlikely to have been written by an able astronomer. The hoax concerns a telescope alleged to have been invented by Sir John Herschel (son of William) and Sir David Brewster and first turned on the moon on January 10, 1835. This instrument is stated to have enabled the two astronomers to see everything on the moon, including the vegetation and animals. Vegetation is fully described, including rose poppies and trees. The animals are also described and include brown quadrupeds like bison. There was stated to have been seen a large amphibious creature rolling on a beach, good large sheep and even Vespertilio-homo or bat-men, four feet tall, who could fly or walk erect. They were alleged to be covered with glossy copper-coloured hair and were seen near the shores of Lake Langrenus. All was described as by an eye-witness had he been with Sir John Herschel on the night of January 10, 1835. Of course it was all false and Sir John Herschel and Sir David Brewster knew nothing about it, but the New York Sun sold the whole edition of 60,000 copies and perhaps Nicollet had a laugh over the supposedly credulous Arago, who was obnoxious to him. Whatever the true story, we believe that there can be no life on the moon.

It is said that the sun gives us 570,000 times more light than the moon – also that the average slope of the lunar mountains is 47º, thus giving much more light to us by reflection at full moon than at other times, even allowing for the area visible. But for little or much moonlight on black-out nights we echo the words of Hippolyta in Shakespeare’s Midsummer-Night’s Dream: ‘Well shone, Moon’.

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