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National Geographic : 1925 Jan
Contents
INTERVIEWING THE STARS heavens its field reveals, and a loo-inch instrument sees only a small spot at a time. FINDING A CELESTIAL TRAVELER So they found another star, of sufficient magnitude to be seen in the 12-inch find ing telescope, that was very close to the one Dr. Sanford was seeking to locate. When the great Ioo-ton instrument was swung around so as to bring this star on the cross wires of the finder, the pressing of one button here and of another there made slight adjustments in the position of the big instrument until the star to be interviewed as to its observance of the celestial speed laws came into the slit of the spectroscope. Then the photographic plate was put in. It was no bigger than the palm of a man's hand, and the part of it to be used was much smaller. A button was pressed, an arc light of titanium momentarily ap peared, and what is known as the com parison spectrum was photographed on its section of the plate. Then for two and a half hours we sat there, with an astronomer's eye always glued to the eyepiece. The top of Mt. Wilson was every mo ment pointing to a new set of stars, for we were on the rim of the earth, traveling around its axis at the rate of a mile every three and two-fifths seconds to keep our engagement with the morrow's sun on the eastern horizon. But down in a big pier was a huge clock that turned the Ioo-ton telescope west at the identical rate that the earth's surface moved east, so that it always pointed to that star. As the minutes rolled by, the tempera ture would fluctuate a little, the refraction of the air would change slightly, and a few other things that escape every un aided human sense would take place, any one of which would move the star out of its slit in the spectroscope. But the pressing of one button here and of another there would bring it back. Thus the traffic officers of the observatory quizzed the speed king that was quad rillions of miles away. When the interview was over, the star had admitted it was "hitting up a pace" that was bringing it 128 miles closer to us every second. One of the greatest triumphs of modern astronomy has been the discovery, by Dr. Walter S. Adams and his associates at Mt. Wilson Observatory, of spectroscopic methods of fixing the distances of stars. The behavior of electrons in atoms under high pressures and low temperatures and under high temperatures and low pres sures causes them to write peculiar lines on the photographic plates of the spectro scope. Laboratory experiments have revealed the secrets of these lines and the astron omer from their character is able to fix the absolute brightness of almost any star that can register its light in the big tele scope. Knowing the absolute brightness and the apparent brightness of a given star, the difference between the two gives the astronomer the data upon which he can compute its distance. The late Miss Leavitt, of Harvard Ob servatory, worked out another method of fixing the absolute brightness of certain types of stars in the Small Magellanic Cloud, and her results corroborate the Adams method. ETHER WAVES TRAVELING ELEVEN LION MILES A MINUTE, BRING IMPORTANT NEWS MIL- Long before the first Pharaoh had ap peared in Egypt or the Chaldeans or Babylonians had built their first zikkurat, the electrons of a group of atoms in a given star jumped from one orbit to an other. In doing so they created a series of ether waves. These waves swept out into space, hurtling on and on at the tremendous gait of more than eleven million miles a min ute, and are just now arriving in the big spectroscope at Mt. Wilson, not only with a message of conditions in the star from which they started at the time they left, but also with a record of the distance they traveled to bring the news. In fixing stellar distances by the study of spectral lines and light variations, it was necessary to know, by other means, the distances of certain key stars. An ex planation of how this has been and is being done is in order. When a civil engineer wants to find the distance of an inaccessible point, he meas ures a base line of sufficient length, and from the two ends runs lines with his transit to the point whose distance he wants to determine. 111
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