54. The continual changing of the Moon’s phases or shapes demonstrates
that she shines not by any light of her own: for if she did, being
globular, we should always see her with a round full Orb like the Sun.
Her Orbit is represented in the Scheme by the little circle _m_, upon
the Earth’s Orbit ⊕: but it is drawn fifty times too large in proportion
to the Earth’s; and yet is almost too small to be seen in the Diagram.
[Sidenote: One half of her always enlightened.]
55. The Moon has scarce any difference of seasons; her Axis being almost
perpendicular to the Ecliptic. What is very singular, one half of her
has no darkness at all; the Earth constantly affording it a strong light
in the Sun’s absence; while the other half has a fortnight’s darkness
and a fortnight’s light by turns.
[Sidenote: Our Earth is her Moon.]
56. Our Earth is a Moon to the Moon, waxing and waneing regularly, but
appearing thirteen times as big, and affording her thirteen times as
much light, as she does to us. When she changes to us, the Earth appears
full to her; and when she is in her first quarter to us, the Earth is in
it’s third quarter to her; and _vice versâ_.
57. But from one half of the Moon, the Earth is never seen at all: from
the middle of the other half, it is always seen over head; turning round
almost thirty times as quick as the Moon does. From the line which
limits our view of the Moon, or all round what we call her edges, only
one half of the Earth’s side next her is seen; the other half being hid
below the Horizon. To her, the Earth seems to be the biggest Body in the
Universe; for it appears thirteen times as big as she does to us.
[Sidenote: A Proof of the Moon’s having no Atmosphere;]
58. The Moon has no such Atmosphere, or body of air surrounding her as
we have: for if she had, we could never see her edge so well defined as
it appears; but there would be a sort of a mist or haziness round her,
which would make the Stars look fainter, when they were seen through it.
But observation proves, that the Stars which disappear behind the Moon
retain their full lustre until they seem to touch her very edge, and
then vanish in a moment. This has been often observed by Astronomers,
but particularly by CASSINI[13] of the Star γ in the breast of Virgo,
which appears single and round to the bare eye; but through a refracting
Telescope of 16 feet appears to be two Stars so near together, that the
distance between them seems to be but equal to one of their apparent
diameters. The Moon was observed to pass over them on the 21st of
_April_ 1720, _N. S._ and as her dark edge drew near to them, it caused
no change in their colour or Situation. At 25 min. 14 sec. past 12 at
night, the most westerly of these Stars was hid by the dark edge of the
Moon; and in 30 seconds afterward, the most easterly Star was hid: each
of them disappearing behind the Moon in an instant, without any
preceding diminution of magnitude or brightness; which by no means could
have been the case if there were an Atmosphere round the Moon; for then,
one of the Stars falling obliquely into it before the other, ought by
refraction to have suffered some change in its colour, or in it’s
distance from the other Star which was not yet entered into the
Atmosphere. But no such alteration could be perceived though the
observation was performed with the utmost attention to that particular;
and was very proper to have made such a discovery. The faint light,
which has been seen all around the Moon, in total Eclipses of the Sun,
has been observed, during the time of darkness, to have it’s center
coincident with the center of the Sun; and is therefore much more likely
to arise from the Atmosphere of the Sun than from that of the Moon; for
if it were the latter, it’s center would have gone along with the
Moon’s.
[Sidenote: Nor Seas.