Michelson A. A. Light waves and their uses (1903)

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Light Waves and Their Uses

By th© telescope we have discovered that all the planets, including many of the minor planets, have discs of appreciable size. W© have found markings on the planets, have discovered the satellites of Jupiter and the rings of Saturn, and have observed various interesting details concerning the structure of these rings. The strange markings on the planet Mars, which bear such a remarkable resemblance to the works of intelligent beings, are among the most interesting of the recent revelations of th© telescope.

It is hard to realize that such observations concern bodies that are distant millions of miles from us; in fact, the distance is so great that it can be more readily expressed by the time light takes to reach us from these bodies. In some cases this may be as much as several years. We can compare this distance with the circumference of the earth, by considering that light or a telegram will go around the earth seven times in a second, while from these bodies it would take several hours for light to reach us. Yet these are our nearest neighbors, or, rather, members of our immediate family. Our farther neighbors are so remote that probably the light from many of them has not yet reached us. To these more distant bodies our own little family of planets is probably invisible; even the sun itself is a second-rate star. If, however, Jupiter were sufficiently bright, then the sun and Jupiter together would form what is called a “ double star,” and to an inhabitant of a distant planet which might be traveling about this distant star it would appear as a double star with a separation of about one second, which may be expressed as th© angle subtended by two luminous points about one-half inch apart when at a distance of three miles. They would therefore be entirely invisible to th© naked eye as separate objects.

One of the most serious difficulties in the way of further progress in the investigation of the telescopic characteristics

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of the planets and of the constitution of star systems, is what is called bad “seeing.” It must be remembered that light, in order to reach a telescope, must pass through from forty to one hundred miles of atmosphere. This atmosphere is not homogeneous. If the atmosphere were homogeneous, there would not be any very serious objection. The intensity of the light from the object would be practically as great as if there were no air present. But the air is unequally heated, and therefore has unequal densities in different portions. Hence the different portions of a beam of light which have passed through different parts of the atmosphere and reached different parts of the objective of the telescope would be differently retarded, and these differences in retardation would not be constant, but would vary, sometimes rapidly and sometimes slowly, producing what is technically called “ boiling.”

This unsteadiness of the image is the most serious difficulty with which astronomers have to contend; there is no instrumental remedy. The best that can be done is to choose an appropriate site, and it seems to be tlio general opinion of astronomers that such a site is best chosen on some very high plateau or tableland. By some it is considered that a high mountain top is a desirable location, and there is no question that such a site possesses very marked advantages in consequence of the rarity of the air. If the air were very rare,, 4 boiling” would have less effect than it has in dense air. But to compensate this advantage we have the very bad effect of currents of heated air traveling up the side of the mountain. As a matter of fact, however, even in the worst locations, there are occasional nights when the astronomer has almost perfect seeing — when even the largest instruments attain almost their theoretical limit of accuracy. This theoretical efficiency may be most conveniently tested by observations on double stars.