Page images

the preceding cycle, and the day will be the same as it was then. The Oolden Number, a term still used in our almanacs, denotes the year of the lunar cycle. Seven is the golden number for 1868.

Eclipse Of The Moon.—This is caused by the passing of the moon into the shadow of the earth,

Fig. 52.

[graphic][merged small]

and hence can take place only at full moon—opposition. As the moon's orbit is inclined to the ecliptic, her path is partly above and partly below the earth's shadow; thus an eclipse of the moon can take place only at or near one of the nodes. In the figure, the umbra is represented by the space between the lines K c and I b; outside of this is the penumbra, where the earth cuts off the light of only a portion of the sun. The moon enters the penumbra of the earth at a,—this is termed her first contact with the penumbra; next she encounters the dark shadow of the earth at b,—this is called the first contact with the umbra; she then emerges from the umbra at c,—which is called the second contact with the umbra; finally, she touches the outer edge of the penumbra at d,the second contact with the penumbra. Since the earth is so much larger than the moon, the eclipse can never be annular, as, however, the eclipse may occur a little above or below the node, the moon may only partly enter the earth's shadow, either on its upper or lower limb. From the first to last contact with the penumbra, five hours and a half may elapse. Total eclipses of the moon are rarer events than those of the sun, since the lunar ecliptic limit is only about 12°; yet they are more frequently seen by us, (1) because each one is visible over the entire unillumined hemisphere of the earth, and also (2) because by the diurnal rotation during the long duration of the eclipse, large areas may be brought within its limits. So it will happen that while the inhabitants of one district witness the eclipse throughout its continuance, those of other regions merely see its beginning, and others only its termination. The moon does not completely disappear even in total eclipses. The cause of this fact lies in the refraction of the solar rays in traversing the lower strata of the earth's atmosphere; they are analyzed, and purple our moon with the tints of sunset. The amount of refraction and the color depend upon the state of the air at the time.

Historical Accounts Of Eclipses.—The earliest account of an eclipse on record is in the Chinese annals; it is thought to be the solar eclipse of October 13, 2127 B. c On May 28, 584 B. c, one occurred which was predicted by Thales, as we have before mentioned. In the writings of the early English chroniclers are numerous passages relating to eclipses. William of Malmesbury thus refers to that of August 2, 1133, which was considered a presage of calamity to Henry I.: "The elements manifested their sorrows at this great man's last departure. For the sun on that day, at the 6th hour, shrouded his glorious face, as the poets say, in hideous darkness, agitating the hearts of men by an eclipse; and on the 6th day of the week, early in the morning, there was so great an earthquake, that the ground appeared absolutely to sink down; an horrid noise being first heard beneath the surface." The same writer, speaking of the total eclipse of March 20, 1140, says: "During this year, in Lent, on the 13th of the kalends of April, at the 9th hour of the 4th day of the week, there was an eclipse, throughout England, as I have heard. With us, indeed, and with all our neighbours, the obscuration of the Sun also was so remarkable, that persons sitting at table, as it then happened almost every where, for it was Lent, at first feared that Chaos was come again: afterwards learning the cause, they went out and beheld the stars around the Sun. It was thought and said by many, not untruly, that the king [Stephen] would not continue a year in the government." Columbus made use of an eclipse of the moon, which took place March 1,1504, to relieve his fleet, which was in great distress from want of supplies. As a punishment to the islanders of Jamaica, who refused to assist him, he threatened to deprive them of the light of the moon. At first they were indifferent to his threats, but "when the eclipse actually commenced, the barbarians vied with each other in the production of the necessary supplies for the Spanish fleet-w^"


Description.—Twice a day, at intervals of about twelve hours and twenty-five minutes, the water begins to set in from the ocean, beating the pebbles and the foot of the rocky shore, and dashing its spray high in air. For about six hours it climbs far up on the beach, flooding the low lands and transforming simple creeks into respectable rivers. The instant of high-water or flood-tide being reached, it begins to descend, and the ebb succeeds the flow. The water, however, falls somewhat slower than it rose.

Cause Of The Tides.—The tides are caused by a great wave, which, raised by the moon's attraction,

Fig. 53.

[graphic][merged small][graphic][merged small]

follows her in her course around the earth. The sun, also, aids somewhat in producing this effect; but as the moon is 400 times nearer the earth, her influence is far greater. As the waters are free to yield to the attraction of the moon, she draws them away from C and D and they become heaped up at A» The earth, being nearer the moon than the waters on the opposite side, is more strongly attracted, and so, being drawn away from them, they are left heaped up at B. As the result, high-water is produced at A by the water being pulled from the earth, and at B by the earth being pulled from the water. The influence of the moon is not instantaneous, but requires a little time to produce its full effect; hence high-water does not occur at any place when the moon is on the meridian, but a few hours after. As the moon rises about fifty minutes later each day, there is a corresponding difference in the time of high-water. While, however, the lunar tidewave thus lags about fifty minutes every day, the solar tide occurs uniformly at the same time. They therefore steadily separate from each other. At one time they coincide, and high-water is the sum of lunar and solar tides; at other times, high-water of the solar tide and low-water of the lunar tide occur simultaneously, and high-water is the difference between the lunar and solar tides.

We should bear in mind tha philosophical truth, that the tide in the open sea ioes not consist of a progressive movement of the water itself, but only of the form of the wave.

Causes that modify the tides.—At new and full moon (the syzygies) the sun acts with the moon (as in Fig.

« PreviousContinue »