Besides bright lines, the corona shows also a faint continuous spectrum, in which have been observed a few of the more prominent dark lines of the solar spectrum.
This shows, that, while the corona may be in the main composed of glowing gas (as indicated by the bright lines of its spectrum), it also contains considerable matter in such a state as to reflect the sunlight, probably in the form of dust or fog.
V. ECLIPSES.
Fig. 235.
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.
Fig. 236.