That the Moon can never be eclipsed but at the time of her being Full,
and the reason why she is not eclipsed at every Full, have been shewn
already § 316, 317. Let _S_ be the Sun, _E_ the Earth, _RR_ the Earth’s
shadow, and _B_ the Moon in opposition to the Sun: in this situation the
Earth intercepts the Sun’s light in its way to the Moon; and when the
Moon touches the Earth’s shadow at _v_ she begins to be eclipsed on her
eastern limb _x_, and continues eclipsed until her western limb _y_
leaves the shadow at _w_: at _B_ she is in the middle of the shadow, and
consequently in the middle of the Eclipse.
[Sidenote: Why the Moon is visible in a total Eclipse.]
346. The Moon when totally eclipsed, is not invisible if she be above
the Horizon and the Sky be clear; but appears generally of a dusky
colour like tarnished copper, which some have thought to be the Moon’s
native light. But the true cause of her being visible is the scattered
beams of the Sun, bent into the Earth’s shadow by going through the
Atmosphere; which, being more dense near the Earth than at considerable
heights above it, refracts or bends the Sun’s rays more inward § 179,
the nearer they are passing by the Earth’s surface, than those rays
which go through higher parts of the Atmosphere, where it is less dense
according to its height, until it be so thin or rare as to lose its
refractive power. Let the Circle _fghi_, concentric to the Earth,
include the Atmosphere whose refractive power vanishes at the heights
_f_ and _i_; so that the rays _Wfw_ and _Viv_ go on straight without
suffering the least refraction: But all those rays which enter the
Atmosphere between _f_ and _k_, and between _i_ and _l_, on opposite
sides of the Earth, are gradually more bent inward as they go through a
greater portion of the Atmosphere, until the rays _Wk_ and _Vl_,
touching the Earth at _m_ and _n_, are bent so as to meet at _q_, a
little short of the Moon; and therefore the dark shadow of the Earth is
contained in the space _moqpn_ where none of the Sun’s rays can enter:
all the rest _RR_, being mixed by the scattered rays which are refracted
as above, is in some measure enlightened by them; and some of those rays
falling on the Moon give her the colour of tarnished copper, or of iron
almost red hot. So that if the Earth had no Atmosphere, the Moon would
be as invisible in total Eclipses as she is when New. If the Moon were
so near the Earth as to go into its dark shadow, suppose about _po_, she
would be invisible during her stay in it; but visible before and after
in the fainter shadow _RR_.
[Sidenote: PLATE XI.
Why the Sun and Moon are sometimes visible when the Moon is
totally eclipsed.]
347. When the Moon goes through the center of the Earth’s shadow she is
directly opposite to the Sun: yet the Moon has been often seen totally
eclipsed in the Horizon when the Sun was also visible in the opposite
part of it: for, the horizontal refraction being almost 34 minutes of a
degree § 181, and the diameter of the Sun and Moon being each at a mean
state but 32 minutes, the refraction causes both Luminaries to appear
above the Horizon when they are really below it § 179.
[Sidenote: Fig. V.