Pluto's Day

Astronomers are cosily familiar with stars quadrillions of miles from the earth, and with galaxies much more distant. But Pluto, a member of the sun's own planetary family, and only 3 1/2 billion miles away, has little personality for them. The outermost member of the solar system, it shines only feebly by reflected sunlight. Even in the biggest telescopes it looks like a faint star; only its motion among the real stars and a slight fuzziness prove it to be a planet. Astronomers are not sure how big it is (probably midway between Mercury and Mars), but recently they have learned how fast it rotates on its axis.

Dr. Merle F. Walker and Robert Hardie, at the Lowell Observatory, Flagstaff, Ariz., measured Pluto's light with photoelectric apparatus. They found that it varied slightly in brightness and that the variations repeated themselves regularly, as if dark markings were passing across Pluto's disk. The period turned out to be 6.390 earth days, so Walker and Hardie concluded that this is the length of Pluto's day.

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