THE MOON.

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sky during the night will, in fact, be very nearly that of theSun during the day, six months before or after. And there-fore, as the inclination of the Earth ’s axis to the plane of theecliptic causes the Sun ’s path (in such a latitude as our own)to be much longer, and its noontide elevation much greater insummer than in winter , so will the path of the Moon when itis in its full phase be in like manner longer and its elevationgreater in winter than in summer. The duration of daylightin the latitude of London varies from about 7f hours upon theshortest day, to about 164 upon the longest. That of thelight of the full Moon, owing to the deviation of the Moon ’spath from the ecliptic, may, we believe, amount in midwinterto about 18 hours and in midsummer be barely 6 hours. Itis certainly a very interesting fact that we thus receive mostmoonlight at the very season of the year when the long-nights make it most useful.

An eclipse of the Moon is another important phenomenonwhich is very easy of observation, and one which in somerespects may be very well seen without any, or with very little,telescopic aid. At such times the Moon first enters thepenumbra, or outer portion of the Earth ’s shadow, from whichthe Sun ’s light is only partially excluded. Little notice,however, is taken of this ; but, after a while, the dark umbra,into which (except by the refraction of the Sun ’s rays by theEarth ’s atmosphere) no solar light can penetrate, graduallyencroaches upon the disc.

At the distance of the Moon ’s orbit, the diameter of the umbrais about three times that of the Moon , whose speed of motionround the Earth is (as we have mentioned in Lecture III.)such as to take it over a space equal to its own diameterin about one hour. It will therefore, if it pass centrallythrough the umbra, first of all occupy about one hour inbecoming wholly immersed in the shadow. It will thenremain for nearly two hours totally eclipsed before its oppositeedge begins to emerge. Towards the end of the fourth hourit will again be entirely clear.

As an eclipse progresses, it is generally noticed that theedge of the shadow is hazy and more or less tinged with red.