THE MOON.

85

The two points of which we have been speaking, in whichits monthly orbit intersects the plane of the ecliptic, lie upona straight line very approximately passing through the centreof the Earth , which is technically termed the line of Nodes.

F ig. XX.—Showing that the direction of the line of Nodes of the Moon ’s orbit passes throughthe Sun at two opposite periods of the year.

F ig. XX.—Showing that the direction of the line of Nodes of the Moon ’s orbit passes through

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The Sutl

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The most important question involved in determining thefrequency of lunar eclipses consequently is:—How often willthis latter line also pass through the centre of the Sun ? Itcan only do so at intervals of about six months, as may be

Earth s Shadow

Line of! Nodes

to the\Suv

Fig. XXI.—Showing how the Moon may suffer a Total Eclipse of thelongest possible duration.

seen in Fig. XX., in which the Earth , as it travels round theSun , is supposed to carry the lunar orbit with it, the line ofNodes remaining parallel to itself, and only passing throughthe Sun iu the two opposite positions M^q, and m.,n 2 . In the