205. _The Shadows of the Earth and Moon._--The shadows cast by the earth and moon are shown in Fig. 235. Each shadow is seen to be made up of a dark portion called the _umbra_, and of a lighter portion called the _penumbra_. The light of the sun is completely excluded from the umbra, but only partially from the penumbra. The umbra is in the form of a cone, with its apex away from the sun; though in the case of the earth's shadow it tapers very slowly. The penumbra surrounds the umbra, and increases in size as we recede from the sun. The axis of the earth's shadow lies in the plane of the ecliptic, which in the figure is the surface of the page. As the moon's orbit is inclined five degrees to the plane of the ecliptic, the axis of the moon's shadow will sometimes lie above, and sometimes below, the ecliptic. It will lie on the ecliptic only when the moon is at one of her nodes. 206. _When there will be an Eclipse of the Moon._--The moon is eclipsed _whenever it passes into the umbra of the earth's shadow_. It will be seen from the figure that the moon can pass into the shadow of the earth only when she is in opposition, or _at full_. Owing to the inclination of the moon's orbit to the ecliptic, the moon will pass either above or below the earth's shadow when she is at full, unless she happens to be near her node at this time: hence there is not an eclipse of the moon every month. When the moon simply passes into the penumbra of the earth's shadow, the light of the moon is somewhat dimmed, but not sufficiently to attract attention, or to be denominated an eclipse. [Illustration: Fig. 236.] 207. _The Lunar Ecliptic Limits._--In Fig. 236 the line _AB_ represents the plane of the ecliptic, and the line _CD_ the moon's orbit. The large black circles on the line _AB_ represent sections of the umbra of the earth's shadow, and the smaller circles on _CD_ represent the moon at full. It will be seen, that, if the moon is full at _E_, she will just graze the umbra of the earth's shadow. In this case she will suffer no eclipse. Were the moon full at any point nearer her node, as at _F_, she would pass into the umbra of the earth's shadow, and would be _partially_ eclipsed. Were the moon full at _G_, she would pass through the centre of the earth's shadow, and be _totally_ eclipsed. It will be seen from the figure that full moon must occur when the moon is within a certain distance from her node, in order that there may be a lunar eclipse; and this space is called the _lunar ecliptic limits_.