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And when the Moon is totally immersed a considerable amountof such reddish light may often be still detected, so that itswhole disc may remain faintly visible. On other occasions,but more rarely, the totality is nearly absolute, and the Moon is for the time almost invisible. This variable result dependsupon the amount and nature of the clouds and vapours over-lying the bouuding circumference of that hemisphere of theEarth upon which the Sun is shining, which produce importanteffects upon the amount and the tint of the solar rays whichare refracted into its shadow.
Lunar eclipses are, however, sometimes only partial ; atother times, although total, of comparatively brief duration;more frequently still, the Moon is full without sufferingany eclipse. In fact, as we shall presently show, it nevertraverses the umbra of the Earth ’s shadow more than threetimes in a year, while the occasions are comparatively rareupon which its passage is so nearly central that the durationof an eclipse approaches its maximum possible value.*
It is sometimes said that the Moon ’s place, when full, isopposite to that of the Sun as seen from the Earth . But thiscannot in general be strictly true, inasmuch as it wouldinvolve the occurrence of an eclipse at the time of everyFull Moon, and would require the plane of the Moon ’sorbit to be exactly coincident with the plane of the ecliptic,in which the Earth moves round the Sun . These planes areactually inclined to one another at an angle of about 5 degrees.The Moon , when full, will therefore only be exactly in theprolongation of the straight line joining the centres of theEarth and of the Sun , if it happens at the same moment tobe also passing through one of the two points in which theplane of its orbit intersects that of the ecliptic. Otherwise itwill be at a perpendicular distance above or below the straightline in question, which will vary between certain small limits,defined by the above-mentioned inclination.
* The Moon may at times pass through the penumbra of the Earth ’sshadow, when it escapes the umbra. Such penumbral eclipses areoccasionally interesting, although their occurrence is not noticed inthe Almanacs .