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Days.

A TABLE of the Equation of Time, shewing

how much a Clock should be faster or slower
than the Sun, every Day of the Year, at Noon.

The third Year atier Leap-Year.
July. | Aug. Sept. Oct. Nov. Dec.

M. S. M. S. M. S. M. S.M. S. M. S. 1 3 2015 541 0 * 15 10 23/16 1310 33 2 3 311 5 500 34 10 42 16 14110

10 31 3 425 346 0 5311

00/16 141 97 46 5741 1211 1816 13 9-22 4 5 35 1 3211 36 16 11 8 57

Clock

Clock

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* A concise EQUATION-TABLE, adapted to the Second Year af

ter Leap-Year, and which will be within a Minute of the Truth for every Year; shewing, to the nearest full Minute, how much a Clock should be faster or slower than the Sun. By Mr. SMEATON.

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This table is near enough the truth for regulating common clocks and watches. It may be easily copied by the pen, and, being doubled, may be put into a pocket-book.

CHAP. XV.

The Moon's Surface mountainous: Her Phases de

scribed: Her Path, and the Paths of Jupiter's Moons delineated: The Proportions of the Diameters of their Orbits, and those of Saturn's Moons, to each other; and the Diameter of the Sun.

VII.

252.

Y looking at the Moon through an ordinary PLATE

telescope, we perceive that her surface is diversified with long tracts of prodigious high moun- The tains and deep cavities. Some of her mountains, by Moon's comparing their height with her diameter (which is mountain. 2180 miles,) are found to be three times as high as ous. the highest mountains on our Earth. This ruggedness of the Moon's surface is of great use to us, by reflecting the Sun's light to all sides: for if the Moon were smooth and polished like a looking-glass, or coa vered with water, she could never distribute the Sun's light all round: only, in some positions, she would shew us his image, no bigger than a point, but with such a lustre as might be hurtful to our eyes.

253. The Moon's surface being so uneven, many have wondered why her edge appears not jagged as well as the curve bounding the light and dark parts. But if we consider, that what we call the edge of the Why no Moon's disc is not a single line set round with mountains, in which case it would appear irregularly in- her edge. dented, but a large zone, having many mountains lying behind one another from the observer's eye, we shall find that the mountains in some rows will be opposite to the vales in others, and fill up the inequalities, so as to make her appear quite round; just as when one looks at an orange, although its roughness be very discernible on the side next the eye, especially if the Sun or a candle shines obliquely on that side, yet the line terminating the vi sible part still appears smooth and even.

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254. As the Sun can only enlighten that half of the

Earth which is at any moment turned toward him, The Moon and being withdrawn from the opposite half, leaves it

in darkness; so he likewise doth to the Moon; only twilight.

with this difference, that the Earth being surrounded
by an atmosphere, and the Moon, as far as we know,
having none, we have twilight after the Sun sets;
but the Lunar inhabitants have an immediate transi-
tion from the brightest sunshine to the blackest dark-
ness, § 177. For, let trks w be the Earth, and A,

B, C, D, E, F, G, H, the Moon, in eight different
Fig. I. parts of her orbit. As the Earth turns round its

axis, from west to east, when any place comes to
t, the twilight begins there, and when it revolves
from thence to r, the Sun S rises; when the place
comes to s, the Sun sets, and when it comes to w,
the twilight ends. But as the Moon turns round
her axis, which is only once a month, the moment
that any point of her surface comes to r (see the
Moon at G) the Sun rises there without any pre-
vious warning by twilight; and when the same point
comes to s the Sun sets, and that point goes into

darkness as black as at midnight.
The 255. The Moon being an opaque spherical body
Moon's
phases. (for her hills take off no more from her roundness

than the inequalities on the surface of an orange take
off from its roundness), we can only see that part of
the enlightened half of her which is toward the Earth.
And therefore when the Moon is at A, in conjunction
with the Sun S, her dark half is toward the Earth,
and she disappears, as at a; there being no light on
that half to render it visible. When she comes to
her first octant at B, or has gone an eighth part of
her orbit from her conjunction, a quarter of her en-
lightened side is seen toward the Earth, and she ap-
pears horned, as at h. When she has gone a quarter
of her orbit from between the Earth and Sun to C,
she shows us one half of her enlightened side, as at
c; and we say, she is a quarter old. At D she is in
her second octant, and by shewing us more of her

enlightened side she appears gibbous, as at d. At E her whole enlightened side is toward the Earth, and therefore she appears round as at e; when we say

it is full Moon. In her third octant at F, part of her dark side being toward the Earth, she again appears gibbous, and is on the decrease, as at f. At G we see just one half of her enlightened side, and she appears half-decreased, or in her third quarter, as at g. At H we only see a quarter of her enlightened side, being in her fourth octant, where she appears horned, as at h. And at A, having completed her course from the Sun to the Sun again, she disappears; and we say, it is new Moon. Thus, in going from A to E, the Moon seems continually to increase; and in going from E to A, to decrease in the same proportion; having like phases at equal distances from A to E; but as seen from the Sun S, she is always full.

256. The Moon appears not perfectly round when The she is full in the highest or lowest part of her orbit, disc nut because we have not a full view of her enlightened always side at that time. When full in the highest part of quite her orbit a small deficiency appears on her lower when full. edge; and the contrary, when full in the lowest part of her orbit.

257. It is plain by the figure, that when the Moon The phachanges to the Earth, the Earth appears full to the Earth and Moon; and vice versa. For when the Moon is at Moon conA, new to the Earth, the whole enlightened side of trary. the Earth is toward the Moon; and when the Moon is at E, full to the Earth, its dark side is toward her. Hence a new Moon answers to a full Earth, and a full Moon to a new Earth. The quarters are also reversed to each other.

258. Between the third quarter and change, the An agreeMoon is frequently visible in the forenoon, even when able phethe Sun shines; and then she affords us an opportunity of seeing a very agreeable appearance, wherever we find a globular stone above the level of the

eye,

nomenon.

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