6. ZODIACAL LIGHT. This is one of the most extensive phenomena in the solar system; but of such feeble light as to be barely visible in the darkest nights. It is disposed somewhat excentrically about the Sun, and is flattened out nearly in the plane of the ecliptic. In that plane it has a radius of about 90 millions of miles, and at right angles to that plane, about 20 millions of miles. Occasionally it reaches as far as the Earth's orbit, and the separate particles of it are then distinguished as the so called "shooting stars." They revolve about the Sun as planetary bodies, and are recognised by the enormous velocity which they are observed to have, and which nothing but revolving motion at that distance from the Sun could possibly give. (See PLANETARY TABLE). On their approaching nearer to the Earth still, they occasionally fall upon it, as "meteoric" stones; and are always found to have a very remarkable chemical composition. They become inflamed and explode on entering our atmosphere; having previously shone only by light reflected from the Sun, like the Moon and all the planets. On the whole, the Zodiacal Light is supposed to be a remnant of the great nebulous atmosphere about the Sun, out of which the planets may have been formed by condensation. |