Page 579 : ECHO — ECLIPSE
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a stem which sometimes is very long. They are dredged from deep-sea waters in various parts of the world. The sub-kingdom Echinodermata is represented by many fossils in the rocks. The true position of the group among animals is not clear; but some of the larvæ of Echinoderms resemble those of worms, and this serves to relate them in a general way with the worms or to indicate that the remote ancestors of both groups were similar. See [[../Romanes, George John|Romanes]]: Jelly-fish, Starfish and Sea-Urchins.
Echo, in Greek fable, a nymph employed by Zeus to keep the attention of his wife, Hera, by talking to her. Hera changed her from a nymph into a mere echo. She next fell in love with Narcissus, and pined away in grief for him, till nothing but her voice was left.
Echo, a phenomenon which is observed when a train of sound-waves strikes a large and fairly smooth body, such as a hillside or the side of a building. The phenomenon is identical in principle with that which occurs when water-waves strike a solid pier and are reflected, or with the reflection which occurs at a fret when waves travel along a guitar-string. [[../Newton, Sir Isaac|Newton]] employed the echo in a cloister at Trinity College, Cambridge, to measure the speed of sound. Standing at one end of the cloister, he started a group of waves by stamping his foot. These waves were reflected at the far end of the cloister. The interval between the stamping of the foot and the hearing of the echo measures the time necessary for a sound-wave to make one round-trip of the cloister. From this interval and the length of the cloister the speed of sound is directly computed. The most interesting examples of echoes are, possibly, those which occur in the whispering-galleries, such as that in the dome of the Capital at Washington or in the dome of St. Paul’s in London. For further details see [[../Tyndall, John|Tyndall]] on Sound.
Eclipse, in [[../Astronomy|astronomy]], is used to describe the passage of one body through the shadow of another. When any portion of the [[../Earth|earth]]’s surface passes through the shadow of the [[../Moon|moon]], the phenomenon is called a solar eclipse. When the moon passes through the shadow of the earth, the phenomenon is called a lunar eclipse. When a satellite passes into the shadow of its [[../Planets|planet]], we have an eclipse of the satellite corresponding exactly to our lunar eclipse. When, however, the opaque planet or the opaque moon passes between us and the satellite, astronomers call this an occultation.
Conditions Necessary for a Solar Eclipse. The plane in which the moon revolves about the earth does not coincide with the plane in which the earth revolves about the [[../Sun, The|sun]]; consequently there is not an eclipse of the sun once a month, but only when the moon passes through the plane of the earth’s motion. Another condition also must be satisfied, viz., the moon must be “new,” i.e., must lie in the direction of the sun. Now it happens that these two conditions are met at intervals of about six months; so that we expect about two solar eclipses a year.
The moon and the sun subtend nearly the same angle as seen by an observer at the surface of the earth, so that the moon ordinarily barely suffices to cover the face of the sun. When the moon is in that part of her orbit nearest the earth, her angular size is greater than that of the sun and the eclipse is said to be total; when, however, the moon is in that part of her orbit most distant from the earth, her angular size is diminished, so that the face of the sun is not quite covered. An eclipse of this kind is called an annular eclipse. As was well-known to the ancients, the circumstances of any solar eclipse are repeated almost exactly in a period of 6,585 days and 8 hours (approximately 18 years). The result is that any solar eclipse is repeated every 18 years. This is the period called the saros.
Lunar eclipses are of very little interest from a scientific point of view: but the phenomenon is so striking as always to command popular attention. In the case of the solar eclipse, however, we have our only opportunity to study the solar corona, a magnificent appendage of the sun having a diameter not less than 2,000,000 of miles. Previous to 1869 solar eclipses furnished our only opportunity to study the prominences, great outbursts of incandescent vapors which occur constantly on the surface of the sun. Every total solar eclipse is an event of interest and importance to scientists. Long journeys for purposes of observation are made by expeditions sent out by scientific bodies. Thus valuable data, including photographic records of every phase of the phenomena are secured. For details concerning eclipses see the American Ephemeris for two or three years preceding the eclipse. For an excellent account of the entire subject see [[../Young, Charles Augustus|Young]]’s General Astronomy.
Image: LUNAR ECLIPSE
Image: SOLAR ECLIPSE
