ECLIPSE

When one heavenly body enters the shadow of another it is said to suffer an eclipse. With a large source of light like the sun, the shadow cast by a smaller body such as the earth or the moon consists of a long, conical shadow (umbra) where no light at all (apart from light refracted by the atmosphere) reaches, and a_ parcial shadow (penumbra) surrounding the former. An eclipse of the moon can obviously only occur when the moon is in opposition—,e., at the time of full moon, Similarly, an eclipse of the sun can only occur when the moon is in, or nearly in, conjunction—i.e., at the © time of new moon. But since the moon’s orbit is inclined to the ecliptic about 5°, an eclipse of the sun is impo ible unle conjunction of the sun and moon takes place within about 16° of one of the nodes of the moon’s orbit. | The corresponding limit for a lunar eclipse is about 11°. Eclipses of the sun may be total, partial, or annular. Total eclipses can only be seen at places within the umbra of the moon’s shadow, but as this is only about 236,000 m. long, and the mean distance of the moon is about 238,000 m., total eclipses, when such occur, can only be visible over a small portion of the earth’s surface. When the umbra does not reach the earth, as when the moon is near apogee, an annular eclipse is visible at places in line with the point of the shadow. In a total lunar eclipse the whole of the moon enters the umbra of the earth’s shadow, and may then frequently be seen shining with a reddish coppery light, due to light from the sun being refracted by the earth’s atmosphere only the longer red rays being transmitted. In a partial lunar eclipse only a portion of the moon enters the umbra. Succe ive eclipses occur at regular intervals. The eclipse year consists of 346'62 days, being the time occupied by the sun in travelling from either node back to the same. Nineteen eclipse years are almost exactly equal to 223 synodic months, and Constitute the cycle called ‘Saros’ by the Chaldeans. Much of our knowledge of the sun has been obtained from observations only po ible during total eclipses. Thus a f ew seconds before totality, the red light of the chromosphere is seen; and during total it: observations of the pro: ces, the ‘reversing layer,’ and — a noe the corona have yielded much information. The total solar eclipse of May 29, 1919, was remarkable for observations which are said to have overthrown the axiomatic basis of physical thought. It was observed by expeditions at Sobral in N. Brazil, and on the isl. of Principe off the W. coast of Africa. Photographs taken of a number of bright stars in the vicinity of the obscured. sun showed a deflection in close accordance with that Pee by Einstein, as opposed to alf that amount according to the principles of Newton. In 1923 there were two eclipses of the sun and two of the moon. A total eclipse of the sun occured on Sept. 10. For the doctrine that the dimensions of space hitherto believed absolute are relative and shift-