345. Having been so prolix concerning Eclipses of the Sun, we shall drop that subject at present, and proceed to the doctrine of lunar Eclipses; which, being more simple, may be explained in less time. [Sidenote: Eclipses of the Moon. Fig. II.] 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.]