Time of her Opposition to the Sun, or at full Moon, (which falls out about the autumnal Equinox,) there is then the leaft Difference in the Time of her rifing from one Night to another, after the Full, that there is any Time of the Year. And if the Moon be at the fame Time afcending from South to North Latitude, that will reduce her Path nearer to a parallel Pofition to the Horizon, by about 5 Degrees, than if the moved in the Ecliptic; and fo make the Difference in the Time of her rifing to be still less.

This full Moon is called the Harvest Moon; and it is remarkable for its varying fo little in the Time of its rifing for feveral Nights to.gether, immediately after the Full.

If the full Moon happens juft at the Equinox, or presently after it, the Ecliptic becomes more perpendicular to the Horizon, at the Time of the Moon's rifing, every Day than other for many Days after; and fo long she loses more and more in the Time of her rifing. But if the full Moon falls out before the Equinox, the Ecliptic grows more and more oblique to the Horizon at the Time of the Moon's rifing; and the lofes lefs and lefs in the Time of her rifing every Day than other, till fhe arrives at the first Point of Aries. Now as that Week in the Summer or Winter, in which the Sun rifes every Day nearest to the fame Time, does not begin at the Solstice, but before it, and has the folfticial Day in the

Middle

Middle of it; fo that Week in the Year, next after a full Moon, when the rifes, every Evening nearest to the fame Time, muft begin not with the Moon's arriving at the first Point of Aries, (when the Ecliptic is in the moft oblique Pofition of all to the Horizon at the Time of her rifing,) but two or three Days before. Therefore it is, generally, that the full Moon in August, when the comes to her Oppofition in Pifces, has most remarkably the Property of the Harvest Moon. And the next full Moon after, viz. in September, which has the fame Property of rising nearly at the fame Time, for feveral Nights after the Full, tho' not quite fo remarkably as the former, is, by fome, called the Hunter's Moon.

That the Surface of the Moon is very uneven and mountainous, is apparent from the Irregularity or Unevenness of the Line which separates the enlightened from the dark Part of its Disk, at any Time when the Moon is not full; which Line, when viewed with a Telescope, appears, as it were, toothed, and cut with innumerable irregular Notches. And, befides, feveral fhining Points may be seen in the dark Part of the Moon, quite feparated, tho' not far diftant from the enlightened Part. These are concluded to be the Tops of Mountains, which are fooner reached by the Sun's Beams than the lower Parts, that lie about them. The bright Top of one of these lunar Mountains has been obferved on the 4th

Part II. Day after the new Moon, to be distant from the Confines of the enlightened Surface about a 16th Part of the Moon's Diameter; from whence (fuppofing the true Measure of the Moon's Diameter to be known) it is eafy to compute the Height of that Mountain; which is accordingly computed to be nine Miles high, that is, three times higher than the Top of any Mountain on the Earth.

Some Parts of the Moon's Surface, even when she is full, appear of a duskish Colour. These are supposed, by fome, to be Seas and Lakes. But Dr. Keill afferts (vid. Aftron. Lectures, Lect. 10. " that when they are "looked at with a good Telescope, they

appear to confist of an Infinity of Caverns "and empty Pits, whofe Shadows fall with"in them; which can never be in a Sea, "or liquid Body. These black Spots there"fore cannot poffibly be Seas: but they "confift of fome darker and fad coloured "Matter, which does not reflect the Light "fo ftrongly as the folid and shining Moun"tains do.'

There seem to be no Clouds about the Moon for when there are no Clouds in our Air, fhe conftantly appears to us with the fame Luftre.

CHAP.

CHA P. V.

Of the Eclipfes of the Sun and

A

Moon.

N Eclipfe of the Sun is occafioned by the Moon's coming betwixt the Sun and the Earth, and thereby hiding the Light of the Sun from the Earth; which can only happen when the Moon is in Conjunction with the Sun, or at New-Moon.

An Eclipfe of the Moon is occafioned by the Shadow of the Earth falling upon the Moon; or by the Moon's paffing thro' the Shadow of the Earth, whereby fhe is neceffarily darkened, and lofes the Light of the Sun. This can only happen when the Moon is in Oppofition to the Sun, or at Full-Moon.

The Reason why the Sun is not eclipfed every New-Moon, and the Moon every Time fhe is full, is because of the Inclination of the Moon's Orbit to the Plane of the Ecliptic; fo that an Eclipfe can never happen, but when the Moon is either at or near one of the Nodes.

If the Moon happens to be exactly in the Node at the Time of her Conjunction, it occafions a central Eclipfe of the Sun, which is

common

commonly total; but fometimes, as when the Earth is in its Peribelion, and the Moon in her Apogee, the Eclipfe will be annular; that is, a luminous Circle of the Sun will appear round the Body of the Moon.

If the be near the Node, the Eclipfe may be total, tho' not central. If fhe be any where within 12 Degrees of the Node, she will hide at least some Part of the Sun from the Earth, and produce a partial Eclipfe. If the be further from the Node than 12 Degrees, there will be no Eclipfe at all; but the Light of the Sun will pass to the Earth, either under or over the Moon, without Obstruction.

So if the Moon be exactly in the Node at the Time of her Oppofition, the Eclipfe of the Moon, by the Shadow of the Earth, will be central and total. If near the Node, total, tho' not central. If at a greater Distance, but within 16 Degrees, the Shadow of the Earth will fall upon Part of the Moon, and occafion a partial Eclipfe. But if at a greater Distance than 16 Degrees, there will be no Eclipfe at all.

The Line of the Nodes being carried nearly parallel to itself, round the Sun in a Year, would twice in the Year, if produced, pafs thro' the Sun; and the New and Full Moons which happen nearest to fuch a Pofition of the Nodes will, moft commonly, produce Eclipfes. So that there are ordinarily four Eclipfes in a Year, viz. two of the Sun, and two of the Moon; which fall out in Pairs,

thus,