The Moon

Part 1

Transcriber Note

Text emphasis is denoted as Italics. Whole and fractional parts of number as 1234-56/789.

THE MOON

OLIVER C. FARRINGTON

CURATOR OF GEOLOGY

GEOLOGY

LEAFLET 6

FIELD MUSEUM OF NATURAL HISTORY

CHICAGO

1925

LIST OF GEOLOGICAL LEAFLETS ISSUED TO DATE

No. 1. Model of an Arizona Gold Mine $ .10

No. 2. Models of Blast Furnaces for Smelting Iron .10

No. 3. Amber--Its Physical Properties and Geological Occurrence .10

No. 4. Meteorites .10

No. 5. Soils .10

No. 6. The Moon .10

D. C. DA VIES, DIRECTOR

FIELD MUSEUM OF NATURAL HISTORY CHICAGO. U.S.A.

PHOTOGRAPH OF MODEL IN RELIEF OF THE VISIBLE HEMISPHERE OF THE MOON. HALL 35.

The model is 19 feet in diameter.]

FIELD MUSEUM OF NATURAL HISTORY

DEPARTMENT OF GEOLOGY

CHICAGO, 1925

------------------------------------------------------------ LEAFLET NUMBER 6 ------------------------------------------------------------

Table of Contents

The Moon 1 General Observations. 7 Description of Individual Features. 7 Sources of Additional Information About the Moon. 13

THE MOON

Part 2

Except for occasional comets and meteors, the Moon is the celestial body nearest the Earth. Its mean distance from the Earth is 237,640 miles, but as it moves in an elliptical orbit, it has at one point a remoteness of 253,263 miles and opposite to this one of 221,436 miles. The diameter of the Moon is about one-fourth that of the Earth, or 2,160 miles, and its volume is 1/49 that of the Earth. The mass of the Moon (volume multiplied by density) is 1/81 and the density ⅗ that of the Earth. The period of the Moon's revolution about the Earth is 27 days, 7 hours, 43 minutes and 11½ seconds. As its period of rotation on its axis is the same, only one side of the Moon is ever seen from the Earth. Since, however, the Moon's axis is inclined about 83° to the plane of its orbit, we sometimes see a little distance beyond each of its poles, and, since the rate of motion of the Moon in its orbit varies slightly, we sometimes see a little beyond the eastern and western edges of the hemisphere. The total result of these librations, as they are called, is to make four-sevenths of the Moon's surface visible to us. Of the remaining three-sevenths, nothing is known. So far as is known, the Moon is not flattened at the poles.

Owing to its slow rotation on its axis, the Moon's day has a length of 29½ of our days. Each portion of its surface is therefore exposed to or shielded from the light of the Sun for a fortnight continuously.

The Moon has no atmosphere. Hence, it can have no diffused light, and nothing can be seen on it except where the Sun's rays shine directly. "If a man stepped into the shadow of a lunar crag," says Todd, "he would instantly become invisible. For a similar reason, no sound, however loud, can be heard on the Moon. The rolling of a rock down the wall of a lunar crater, will be known only by the tremor it produces." Moreover, changes of temperature on the Moon are rapid and violent. Where the Sun's rays strike, a temperature about that of boiling water is believed to be reached, while in unilluminated portions it is thought to go as low as 100° below zero.

The force of gravity upon the surface of the Moon is only ⅙ of that on the Earth. Therefore, a man weighing 150 pounds on the Earth, would weigh only 25 pounds on the Moon, and the same muscular energy by which he could jump 6 feet on the Earth would carry him a distance of 36 feet on the Moon. On the Earth a body falls 16 feet in one second; on the Moon only 2.6 feet in the same time.

The surface of the Moon is made up of mountains, valleys and plains, resembling in general appearance those of the Earth. As a whole, however, the surface of the Moon is much more uneven than that of the Earth. Some of the mountains of the Moon have a height of over 20,000 feet. As there is no sea-level to measure from, this figure expresses height above the surrounding surface, it being determined by the length of the shadows cast by the mountains.

In order to represent in a vivid and accurate manner the character and appearance of the Moon's surface, the construction of a large model of the Moon was undertaken a number of years ago by Th. Dickert, Curator of the Natural History Museum of Bonn, Germany and Dr. J. F. Julius Schmidt, Director of the Observatory of Athens, Greece and an eminent selenographer. The model was presented to the Museum by the late Lewis Reese of Chicago, and is installed at the west end of Hall 35 of the Museum. The model is 19.2 feet in diameter, and is by far the largest and most elaborate representation of the Moon's surface ever made. Its horizontal scale is 1:600,000, one inch on the model equaling 9-47/100 miles on the surface of the Moon, and its vertical scale is 1:200,000, one inch equaling 3-15/100 miles on the Moon.

Some characteristic features of the Moon's surface which are especially well illustrated on the model are the following:

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.

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