CONVERSATION XXXVIIL Of the Harvest Moon. Tutor. From what we said yesterday, you will easily understand the reason why the moon rises about three quarters of an hour later every day than on the one preceding.. Charles. It is owing to the daily progress which the moon is making in her orbit, on which account any meridian on the earth must make more than one complete rotation on its axis, before it comes again into the same situation with respect to the moon that it had before. And you told us that this occasioned a difference of about 50 minutes. Tutor. At the equator this is generally the difference of time between the rising of the moon on one day and the preceding. But in places of considerable latitude, as that in which we live, there is a remarkable difference about the time of harvest, when at the season of full moon she rises for several nights together only about 20 minutes later on the one day than on that immediately preceding. By thus succeeding the sun before the twilight is ended, the moon prolongs the light, to the great benefit of those who are engaged in gathering in the fruits of the earth: and hence the full moon at this season is called the harvest moon. It is believed that this was observed by persons engaged in agriculture, at a much earlier period than it was noticed by astronomers; the former ascribed it to the goodness of the Deity, not doubting but that he had so ordered it on purpose for their advantage. James, But the people at the equator do not enjoy this benefit. Tutor. Nor is it necessary that they should, for in those parts of the earth, the seasons vary but little, and the weather changes but, seldom, and at stated times: to them, then, moon-light is not wanting for gathering the fruits of the earth. Charles. Can you explain how it happens, that the moon at this season of the year rises one day after another with so small a difference of time? Tutor. With the assistance of a globe I could at once clear the matter up. But I will endeavour to give you a general idea of the subject without that instrument. That the moon loses more time in her risings when she is in one part of her orbit, and less in another, is occasioned by the moon's orbit lying sometimes more oblique to the horizon than at others. James. But the moon's path is not marked on the globe. Tutor. It is not; you may, however, consi der it, without much error, as coinciding with the ecliptic. And in the latitute of London, as much of the ecliptic rises about Pisces and Aries in two hours, as the moon goes through in six days; therefore while the moon is in these signs she differs but two hours in rising for six days together; that is, one day with another, about 20 minutes later every day than on the preceding: There is a time well known to husbandmen, When in this sign she dwells; and most protracts LOFFT. Charles. Is the moon in those signs at the time of harvest? Tutor. In August and September you know that the sun appears in Virgo and Libra, and of course when the moon is full, she must be in the opposite signs, viz. Pisces and Aries. James. Are there then two full moons that afford us this advantage? Tutor. There are, the one when the sun is in Virgo, which is called the harvest moon; the other when the sun is in Libra, and which, being less advantageous, is called the hunter's moon. You ought to be told that when the moon is in Virgo and Libra, then she rises with the greatest difference of time, viz. an hour and a quarter later every day than the former. Charles. Will you explain, sir, how it is that the people at the equator have no harvest moon? Tutor. At the equator, the north and south poles lie in the horizon, and therefore the ecliptic makes the same angle southward with the horizon when Aries rises, as it does northward when Libra rises: but as the harvest moon depends upon the different angles, at which different parts of the ecliptic rise, it is evident there can be no harvest moon at the equator. The farther any place is from the equator, if it be not beyond the polar circles, the angle which the ecliptic makes with the horizon, when Pisces and Aries rise, gradually diminishes, and therefore when the moon is in these signs she rises with a nearly proportionable difference later every day than on the former, and this is more remarkable about the time of full moon. James. Why have you excepted the space on the globe beyond the polar circles? Tutor. At the polar circles, when the sun touches the summer tropic, he continues 24 hours above the horizon; and 24 hours below it when he touches the winter tropic. For the same reason the full moon neither rises in the summer, when she is not wanted; nor sets in the winter, when her presence is so necessary. These are the only two full moons which happen about the tropics; for all the others rise and set. In summer the full moons are low, and their stay above the horizon short: in winter they are high, and stay long above the horizon. A wonderful display this of the divine wisdom and goodness, in apportioning the quantity of light suitable to the various necessities of the inhabitants of the earth, according to their different situations. Charles. At the poles, the matter is, I suppose, still different. Tutor. There one half of the ecliptic never sets, and the other half never rises; consequently the sun continues one half year above the horizon, and the other half below it. The full moon being always opposite to the sun, can never be seen by the inhabitants of the poles, while the sun is above the horizon. But all the time that the sun is below the horizon, the full moons never set. Consequently to them the full moon is never visible in the summer; and in their winter they have her always before and after the full, shining for fourteen of our days and nights without intermission. And when the sun is depressed the lowest under the horizon, then the moon ascends with her highest altitude. James. This indeed exhibits in a high degree VOL. I.T 2 |