Diminished Planet

Pluto, the outermost planet of the solar system, is so small, dim and far away (3.7 billion miles) that astronomers have seen it only as a point of light like a star, have had to estimate its size by calculating the apparent effect of its gravitation upon the motion of Neptune. Measured in this indirect way, Pluto was thought by some to be almost as big as the earth. Last week Astronomer Gerard P. Kuiper of Yerkes Observatory, having measured Pluto's diameter with the 200-inch telescope on Palomar Mountain, announced that those estimates were probably wrong.

Using the great telescope visually (it is normally used as a camera), Dr. Kuiper caught Pluto on a night of unusually good "seeing." The disc was clear enough and steady enough to be measured with a special instrument. It proved to be only 3,600 miles in diameter. So Pluto has less than half the earth's diameter (7,920 miles) and is about one-tenth its mass. It is slightly larger than Mercury and considerably smaller than Mars, less than one five-hundredth the size of its neighbor Neptune (over 30,000 miles in diameter).

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