The Moon

by Oliver C. Farrington

1. GRAY PLAINS or "SEAS." These are the darker portions of the Moon's surface as it is seen with the naked eye. They were thought by earlier observers to be seas and were so named. We now know, however, that there is no water on the Moon's surface and that the so-called "seas" are really low-land plains, some of them of vast extent. The Oceanus Procellarum, for instance, covers an area of 90,000 square miles. As seen from the Earth, the plains show a gray-green color, often of varying intensity and sometimes a little bluish in portions. The brightest green color is shown by the area known as Mare Serenitatis. Though appearing perfectly level, a close study shows that these plains have undulating surfaces. They occupy about one-third of the visible surface of the Moon.

2. MOUNTAINS and HIGHLANDS. These constitute the bright portions of the Moon's surface as it is seen with the naked eye.

Although these elevated areas are conveniently called mountains, Shaler has drawn attention to the fact that they are unlike those on the Earth since they lack features due to erosion and there is absence of order in their association. The average declivity of their slopes is also much greater than that of the mountains on the Earth. It has been estimated that the average angle of the lunar surface to its horizon is 52°, while on the Earth it does not amount to more than one-tenth of that figure. This difference is probably due to the lack of water on the Moon, the work of which on the Earth tends continually to reduce slopes to a level. Using the term mountains for convenience, however, those on the Moon may be divided into the following classes:

a. Mountain Chains. These may have a length of 80 to 100 miles and heights of from 5,000 to 17,000 feet. As in the case with the mountains of the Earth, they are usually steeper on one side than on the other. The range called the Appenines, seen near the north pole of the Moon, is a good illustration of such mountain chains. Other ranges are the so-called Alps and Caucasus. These names were applied by Hevelius, an astronomer of Danzig, who made the first map of the Moon in 1647. He gave to the features of the Moon's surface names of localities similar to those on the Earth which they most resembled. His system was largely abandoned by later astronomers, however, the later method being to name the different features of the Moon after celebrated astronomers and philosophers.

b. Highlands Surrounded By Mountains. These are partly with and partly without well-determined directions.

c. Isolated Mountains. These usually occur on the gray plains. They vary from 4,000 to 7,000 feet in height.

d. Vein Mountains. These occur only on the gray plains. They are long, narrow, contorted ridges, usually from 700 to 1,000 feet in height.

e. Circular Mountains. These are the most characteristic and peculiar features of the Moon's surface. They vary in size from the so-called "Walled Plains," 150 to 15 miles in diameter, to crater mountains whose diameters range from 15 miles down to a few hundred feet. Thirty-three thousand of these crater mountains have been counted by one astronomer, the number increasing as the size diminishes.

The form of these craters is that of pits, which generally have ring-like walls about them. These wails slope very steeply to a central cavity and more gently toward the surrounding country. In all these pits, as pointed out by Shaler, except those of the smallest size, and possibly in these, also, there is, within the ring wall and at a considerable though variable depth below its summit, a nearly flat floor, which often has a central pit of small size or, in its place, a steep cone. When this floor is more than 20 miles in diameter, and in increasing numbers as it is wider, there are generally other pits and cones irregularly scattered upon it. Thus, within the ring called Plato, which is about 60 miles in diameter, there are some scores of these lesser pits. On the interior of the ring walls of the pits over 10 miles in diameter, there are usually more or less distinct terraces, which suggest that the material now forming the solid floors they inclose was once fluid and stood at greater heights in the pit than that at which it became permanently frozen. It is, indeed, tolerably certain that the last movement of this material of the floors was one of interrupted subsidence from an originally greater elevation on the outside of the ring wall. The ring wall is commonly of irregular height, with many peaks. In some places there may be seen tongues or protrusions of the substance which forms the ring, as if it had flowed a short distance and then had cooled with steep slopes. It may also be noted: (a) that the pits or craters in many instances intersect each other, showing that they were not all formed at the same time, but in succession; (b) that the larger of them are not found on the plains (seas) but on the upland and apparently the older parts of the surface; and (c) that the evidence from the intersections clearly indicates that the larger of these structures are prevailingly the older and that in general the smallest were the latest formed. In other words, says Shaler, whatever was the nature of the action involved in the production of the craters, its energy diminished with time, until in the end it could no longer break the crust. These features indicate that the surface of the Moon has been subject to forces similar to those which produce volcanoes on the Earth, and it is therefore customary to refer to the crater-like mountains of the Moon as volcanoes. As the parallel cannot be drawn too closely, however, Shaler has urged that the term vulcanoids, meaning volcano-like, be applied to these mountains.

3. RILLS or CLEFTS. These are small, deep, ditch-like furrows to be found over various parts of the Moon's surface. Their course seems quite independent of the surface topography, for they traverse mountains and plains with equal facility. They are without doubt the latest formation on the Moon and some of them may have had their origin in modern times.