The New Student's Reference Work/Eclipse

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Eclipse, in astronomy, is used to describe the passage of one body through the shadow of another.  When any portion of the earth’s surface passes through the shadow of the 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 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.

Solar Eclipse

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; 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 Eclipse

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’s General Astronomy.