earth and sun will be gained from the figure, by imagining
the sun to be moved towards the left, and placed at a distance
of 20 ft. from the position of the earth, as represented at the
right-hand end of the figure. We have here eight positions of
the moon, M1, M2, &c., as it moves round the earth E. The
general average distance of the sun is somewhat less than four
hundred times that of the moon. We have next to conceive
that, as the earth performs its annual revolution round the
sun in an orbit whose diameter, as represented on the diagram,
is nearly 40 ft., it carries the orbit of the moon with it. Conceiving
the plane of the earth’s motion, which is that of the
ecliptic, to be represented by the surface of the paper, the orbit
of the moon makes a small angle of a little more than 5° with
this plane. Conceiving the line NN′ to be that of the nodes
at any time, and the earth and lunar orbit to be moving in
the direction of the straight arrows, the earth will be on one
side of the ecliptic from M2 to M5, and on the other side from
M6 to M1, intersecting it at the nodes. The absolute direction
of the line of nodes changes but slowly as the earth and moon
revolve; consequently, in the case shown in the figure, the line
of nodes will pass through the sun after the earth has passed
through an arc nearly equal to the angle M1 N. Six months
later the direction of the opposite node will pass through the
sun. Actually, the line of nodes is in motion in a retrograde
direction, the opposite of that of the arrows, by 19·3° per year,
thus making a revolution in 18·6 years, or 6,793·39 days. (See
Eclipse.)
The varying phases of the moon, due to the different aspects presented by an opaque globe illuminated by the sun, are too familiar to require explanation. We shall merely note some points which are frequently overlooked: (1) the crescent phase of the moon is shown only when the moon is less than 90° from the sun; (2) the bright convex outline of the crescent is then on the side toward the sun, and that the moon is seen full only when in opposition to the sun, and therefore rising about the time of sunset. In consequence of the orbital motion the moon rises, crosses the meridian, and sets, about 48 m. later every successive day. This excess is, however, subject to wide variation, owing to the obliquity of the ecliptic and of the lunar orbit to the equator, and therefore to the horizon. The smaller the angle which the orbit of the moon, when near the point of rising, makes with the horizon the less will be the retardation. Near the autumnal equinox this angle is at a minimum; hence the phenomenon of the “harvest moon,” when for several successive days the difference of times of rising on one day and the next may be only from 15 to 20 minutes. Near the vernal equinox the case is reversed, the interval between two risings of the nearly full moon being at its maximum, and between two settings at its minimum. Generally, when the rising is accelerated the setting is retarded, and vice versa.
The moon always presents nearly the same face to the earth, from which it follows that, when referred to a fixed direction in space, it revolves on its axis in the same time in which it performs its revolution. Relatively to the direction of the earth there is really no rotation. The rate of actual rotation is substantially uniform, while the arc through which the moon moves from day to day varies. Consequently, the face which the moon presents to the earth is subject to a corresponding variation, the globe as we see it slightly oscillating in a period nearly that of revolution. This apparent oscillation is called libration, and its amount on each side of the mean is commonly between 6° and 7°. There is also a libration in latitude, arising from the fact that the axis of rotation of the moon is not precisely perpendicular to the plane of her orbit. This libration is more regular than that in longitude, its amount being about 6° 44′ on each side of the mean. The other side of the moon is therefore invisible from the earth, but in consequence of the libration about six-tenths of the lunar surface may be seen at one time or another, While the remaining four-tenths are for ever hidden from our view.
It is found that the direction of the moon’s equator remains nearly invariable. With respect to the plane of the orbit, and therefore revolves with that plane in a nodal period of 18·6 years. This shows that the side of the moon presented to us is held in position as it were by the earth, from which it also follows that the lunar globe is more or less elliptical, the longer axis being directed toward the earth. The amount of the ellipticity is, however, very small.
Two phenomena presented by the moon are plain to the naked eye. One is the existence of dark and bright regions, irregular in form, on its surface; the other is the complete illumination of the lunar disk when seen as a crescent, a faint light revealing the dark hemisphere. This is due to the light falling from the sun on the earth and being reflected back to the moon. To an observer on the moon our earth would present a surface more than ten times as large as the moon presents to us, consequently this earth-light is more than ten times brighter than our moonlight, thus enabling the lunar surface to be seen by us.
The surface of the moon has been a subject of careful telescopic study from the time of Galileo. The early observers seem to have been under the impression that the dark regions might be oceans; but this impression must have been corrected as soon as the telescope began to be improved, when the whole visible surface was found to be rough and mountainous. The work of drawing up a detailed description of the lunar surface, and laying its features down on maps, has from time to time occupied telescopic observers. The earliest work of this kind, and one of the most elaborate, is the Selenographia of Hevelius, a magnificent folio volume. This contains the first complete map of the moon. Names borrowed from geography and classical mythology are assigned to the regions and features. A system was introduced by Riccioli in his Almagestum novum of designating the more conspicuous smaller features by the names of eminent astronomers and philosophers, while the great dark regions were designated as oceans, with quite fanciful names: Mare imbrium, Oceanus procellarum, &c. More than a century elapsed from the time of Hevelius and Riccioli when J. H. Schröter of Lilienthal produced another profusely illustrated description of lunar topography.
The standard work on this subject during the 19th century was long the well-executed description and map of W. Beer and J. H. Mädler, published in 1836. It was the result of several years’ careful study and micrometric measurement of the features shown by the moon. The volume of text gives descriptive details and measurement of the spots and heights of the mountains.
In recent times photography has been so successfully applied to the mapping of our satellites as nearly to supersede visual observation. The first photograph of the moon. was a daguerreotype, made by Dr J. W. Draper of New York in 1840; but it was not possible to do much in this direction until the more sensitive process of photographing on glass was introduced instead of the daguerreotype. The taking of photographs of the moon then excited much interest among astronomical observers of various countries. Bond at the Harvard observatory, De la Rue in England, and Rutherford in New York, produced lunar photographs of remarkable accuracy and beauty. The fine atmosphere of the Lick observatory was well adapted to this work, and a complete photographic map of the moon on a large scale was prepared which exceeded in precision of detail any before produced. The most extended and elaborate work of this sort yet undertaken is that of Maurice Loewy (1833–1907) and Pierre Puiseux at the Paris observatory, of which the first part was published in 1895.
The broken and irregular character of the surface is most evident near the boundary between the dark and illuminated portions, about the time of first quarter. The most remarkable
