Fig. 66. Proof of earth's rotundity
"You know that the cause of day and night is the rotation of the earth on its own axis. It shows a large portion of its surface to the sun continually, or in other words, the sun is always shining on some portion of the earth's surface. You are also aware of the earth and its satellite, the moon, both being held in their orbits by the sun's attraction, the moon being further kept in her orbit by the attraction of the earth. Now the earth is composed of three main elements, air, water, and land, and if you consider, for a moment, that the daily rotatory movement of the earth is something like 1,000 miles an hour, this rapid speeding through space must have some effect on air and water in assisting or retarding their flow.
Fig. 67. Phases of the moon
"Nature has divided time, and man has named and subdivided it into years, months, and days. The natural month, however, does not consist of four weeks, nor is the natural year made up of the twelve calendar months given us by the almanac. A natural, or lunar, month is the time the moon takes to perform her journey round the earth, which is 27 days 7 hours, and 43 minutes; this is called the periodical month, while the average calendar or synodical month consists of 29 days 12 hours and 44 minutes. The light of the moon is borrowed from the sun, for if it were her own light, she would shine all the time and not be subject to her present phases. The moon is seen by means of the light which comes to it from the sun being reflected from it. Its changes, or phases, depend upon its relative position to the earth and the sun. When the moon is in opposition to the sun at A ([Fig. 67]) the lighted side is turned toward the earth, as A, and it appears full. When the moon is in conjunction at E with the sun, its dark side is turned toward us, and it is invisible, as at e. As it proceeds in its orbit, as at F, a small part of the light side is seen, and then we have what is called a new moon; and we continue to see more and more of the light side, as the moon approaches at G and H, to the state of opposition or full moon. The waning or decreasing of the moon takes place in the same manner, but in a contrary order. The earth must perform the same office to the moon that the moon does to us; and it will appear to the inhabitants of the moon (if there be any), like a very magnificent moon, being to them about thirteen times as large as the moon is to us and it will also have the same changes or phases. Hence it is evident, that one half of the moon is never in darkness, the earth constantly affording it a strong light, during the absence of the sun; but the other half has a fortnight's light and darkness by turns.
"The moon's orbit is elliptical, and she also rotates on her axis and takes the same time to circle the earth, consequently every part of the moon is successively presented to the sun, yet the same hemisphere is always turned to the earth. This has been discovered by observation with good telescopes. The length of a day and night in the moon is more than twenty-nine and a half days of ours; and while her year is the same length as ours, being measured by her journey around the sun with us, so she has but twelve days and a third in a year. Another remarkable circumstance is that the moon's hemisphere next the earth is never in darkness, for when it is turned from the sun, it is illuminated by light reflected from the earth in the same manner as we are lighted by a full moon. The other hemisphere of the moon however, has a fortnight's light and darkness by turns. If there are inhabitants in the moon, which is doubtful, the satellite will appear to them to be about thirteen times as large as the moon does to us, and when it is new moon to the earth, it is full earth to the moon.
"There are many things regarding our relationship to the moon that would be of interest, if I had time to explain them, such as eclipses, the moon's surface as seen through telescopes, its supposed influence on the weather, etc., but I fear too much moon might prove tiresome. Beside I have shown you sufficient to enable you to understand the relationship existing between the moon and the tides, generally accepted as the true theory.
"If we agree that the tides are occasioned by the attraction of sun and moon, more particularly the latter, we can readily understand their dependence on some known and determinate laws. Our almanacs published long in advance give the exact time of high water at any prominent port in the United States on the morning and afternoon for every day in the year; and seafaring men can tell you when the tide will be high or low, notwithstanding the fact that these movements are not fixed. They know from experience that the time of ebb and flow varies about three quarters of an hour each day.