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And thus Tibullus
from wand'ring Stars, The Midwife Nations found from far the Brass. And lastly, Juvenal
, speaking of a loud scolding Woman, says pleasantly, that the alone was able to relieve the Moon out of the Labour of an Eclipse
Forbear your Drums and Trumpets, if you please,
Her Voice alone the lab’ring Moon can ease. And this strange and absurd Piece of Superstition is practised to this Day among the Turks and Heathen Nations with many and various ridiculous Ceremonies more than I have related, as Historians inforın us.
Euphrof. Well, you have entertained me with a pleafant Digression, which thoroughly convinces me how necessary it is for every one to Study Philosophy who would have but a tolerable Notion of Things, and not be absurd and ridiculous in his Sentiments. But to return to the Matter-Pray, in how many Particulars does an Eclipse of the Moon differ from an Eclipse of the Sun?
Cleon. In the following, viz. First, The Moon is really and truly eclipsed by the Shadow of the Earth ; whereas in a Solar Eclipse the Sun is not eclipsed, but the Earth by the Shadow of the Moon.
Secondly; An Eclipse of the Moon begins on the Eaft Side or Limb, and ends on the West; but the con trary happens in a Solar Eclipse.
Thirdly; The Lunar Eclipses are more frequent to any one Place than Solar Eclipses. Because,
Fourthly; An Eclipse of the Moon appears from al! Parts of the Earth to be the same as it really is ; whereas an Eclipse of the Sun does not; but may be total in one Part, partial in another, and none at all to others, at the fame Time.
Fifthly; But Solar Eclipses are more frequent with respect to the whole Earth than the Lunar ones ; because the large Penumbra of the Moon can oftener fall on the broad Surface of the Earth than the small Globe of the Moon can fall into the conical Shadow of the Earth.
Sixthly; The total Darkness of a Lunar Eclipse lasteth It Hour, sometimes more ; but that of a Solar Eclipse not above two Minutes,
These are the principal Differences of Eclipses, Solar and Lunar.
Euphros. I thank you, Cleonicus; you take a great deal of Pains with me; 'but one Question more. Pray, do you know the Dimenfions of the Earth's Shadow at the Diftance of the Moon ?
Cleon. Yes; the Shadow of the Earth, where the Moon passes through it, is about 5900 Miles in Diameter, which is almost three Times the Diameter of the Moon.
Euphrof. How long may an Eclipse of the Moon continue from first to last when greatest?
Cleon. From the Time of the Moon's Immersion into the Shadow at I, to the Time of her Emersion at H, is sometimes 31 Hours, and sometimes more; and the Time of total Darkness is generally 1 Hour, as I told you before.
Euphrof. I have a few more Questions concerning Eclipses that I want resolved; but on Account of the Visit we are to make this Evening, must refer them to another Season,
DIALOGUE VII. Of the Boundaries, and Number of Eclipses, and the Times of the Year when they bappen.
Remember, Euphrofane, you told me, when we last more to ask concerning them; if you'll now propose them I'll endeavour to give you Satisfaction in each Particular.
Euphrof. One thing I have often wanted to know, why our Almanacks of several years present us with different Numbers of Eclipses, sometimes they tell us we shall have two, sometimes four, and at other Times fix: Also, why they happen at some certain new and full Moons and not at others ; and other such like Matters,
Cleon, That I may give you a good Idea of such Things, it is necessary you should conceive it by a little easy Inftrument of two circular Pieces of Pasteboard which I have here prepared, and contrived to fhew the Nature of the Thing. One of these Pieces AFMP is to represent the Plane of the Moon's Orbit.
Euphrof. Very good! and what is the other design'd for?
Cleon. The other Piece UFLP, represents the Plane in which the Sun, or the Earth's Shadow appears to move in at the Distance of the Moon. For in the Firmament, the Sun seems to go in the same Tract very nearly with the Moon; and if the conical Shadow of the Earth were cut through, at the Distance of the Moon, that Section would appear a dark circular Space, and to move in the same Tract with the Sun.
Euphrof. Well; and how then?
Cleon. Then the Edge of the circular Piece AFMP will represent the Orbit of the Moon, and that of the other Piece the Orbit of the Sun, or (in other Words) the Ecliptic, at the Distance of the Moon.'
Euphrof. So far I apprehend you pretty well ; pray, proceed.
Cleon. In the last Place, you must know, that the Plane of the Moon's Orbit does not lie exactly level with the Plane of the Ecliptic, but one Half below it, and the other above it, just 'as you see me put these two Pieces of Pafteboard together.
Euphrof. I see your Meaning plain ; the Half FAP lies below the Ecliptic, and the other Half FMP above it,
Cleon. Well, then the Distance between these Planes is called the Latitude of the Moon ; that below the Ecliptic is the South Latitude, the other above it the North Latitude; because the Moon in describing the lower Part FAP is Southward of the Sun ; but in the o her Half FMP she is Northward of it.
Euphrof. I understand you so far very well ; pray go on.
Cleon. Also the Points F and P, where the Planes cross each other, are called the Nodes ; and F the Afcending Node, because there the Moon rises above the 3
Ecliptic, Ecliptic, and is thus marked 83 ; the other, P, is the Descending Node, marked thus 8: And now having fufficiently explained the Instrument, it will be very easy to understand the Things you enquire farther of Eclipses. Euphrol
. If so, I shall be very glad; and pray let me know when, or in what Part of the Moon's Orbit there may, and when there may not be an Eclipse?
Cleon. I shall satisfy your Enquiry firft of Solar Eclipses; and therefore from what I have already faid, you will easily conceive, that a Spectator at the Earth, abc, will view the apparent Faces of the Sun and Moon very nearly equal at the Distance of the Moon; and consequently the Moon journeying round every Month in her Orbit AFMP, and the equal Solar Disk moving round the Ecliptic UFLP, which two Orbits interfect each other at F and P; it must happen that in the Course of a Year the sun will be seen in the Nodes F and P, at two different, and almoft opposite Seasons of the Year; and will be for some Time so near them on each side, that when the Moon pafies that Part of her Orbit, she must necessarily hide or cover either the Whole or Part of the Sun's Disk or Face, and so produce an Eclipse of the Sung total or partial : For since the Inclination or Distance between the two Orbits grows less and less from AU, where it is greatest, towards the Nodes where it is nothing; so there must be a certain visible Latitude or Distance as at C, B, which is just equal to the Sum of half the Diameters of the Moon B, and Sun C; (because the greateft Latitude, AU or IM, far exceeds that Sum.) Again, since the visible Latitude BC is equal to the half Diameters of the Sun and Moon, the Moon in passing along will just touch the lower or Southern Limb of the Sun, but cover no Part of his Surface : Confequently any new Moon before the Point B, as at d, will have such a Latitude from the Sun at e, as will exceed the Sum of their half Diameters, and so will be seen to pass at some Distance below the Sun, and not touch it. But if the new Moon appears nearer to the Node F, as at D, where her Latitude from the Sun at E is less than the said Sum of the half Diameters of the Sun and Moon, then the Moon will be seen to pass over a Part of the
Sun's Disk, and so cause a partial Eclipse of the Sun where it is visible.
Once more; if the new Moon happens in the very Node itself, as at P, the Sun being there also, the Moon then having no Latitude must necefiarily pass over the whole Dirk of the Sun, and so produce a central and total Eclipse of the Sun.
Lastly, New Moons on the other Side of the Node F, to the same Distance GH, will produce Eclipses, more or less, on the upper or northern Part of the Sun; but at H the Sum of the half Diameters and visible Latitude being again equal, the Luminaries will there but just touch each other; and in all Parts farther from the Node, as at K, there will be no Eclipfe poffible; for the Moon will then pass above the Disk of the Sun.
Euphrof. It is ali very evident; and therefore I presume the Points B and H in the Moon's Orbit are what you call the ecliptic Boundaries, or Limits of Solar Eclipses.
Cleon. Yes, they are so; and now with Respect to Lunar Eclipses, we must turn our Eye to the opposite Parts of the Vibits on each side the Node P, where we shall view the Full Moon in her Orbit, and the Section of the Earth's Shadow at the Distance of the Moon's Orbit in the Points O, P, Q, S.
Now, in the first Place, let us consider that if the Full-Moon happens at Nor R, where the Earth's Shadow passing by, just touches it, then in any Point between N and R, the said Shadow will more or less involve the Moon, and so cause a Lunar Eclipse in a greater or lefler Degree; and therefore those two Points, N and R, in the Lunar Orbit, where the Sum of the half Diameters of the Moon and Earth's Shadow is equal to the true Latitude of the Moon, are the Boundaries or Limits of Lunar Eclipses, on each side the Node P.
Again, in the second Place; the nearer the Moon is to the Node P, the greater will be the Eclipse, and therefore greatest of all, and central, in the Node itself, where the Shadow of the Earth is near three Times greater than the Moon, as I formerly told
you. Euphrol. The Manner of explaining the Boundaries ef a Lunar Eclipse I fee is the same nearly as that of the