How We Discovered Pluto

Photo: NASA HQ

At National Geographic, Nadia Drake has been writing about NASA’s New Horizons spacecraft reaching Pluto this week, and she’s also written about our history with the former planet, which was discovered by 24-year-old Clyde Tombaugh on February 18, 1930:

Tombaugh spent about a year searching for the missing world, using an instrument called a blink comparator. The noisy machine let viewers flip back and forth between long exposures of the sky, often containing hundreds of thousands of stars, taken several days apart. Anything that traveled a significant distance during that time—a planet or an asteroid, for example—would appear to move as the images flipped.

On that late afternoon—it was February 18—Tombaugh was manning the comparator and squinting at thousands of stars, evaluating each one by eye. Suddenly, in photos taken six days apart in January, he spied a small speck of light that didn’t stay put. In one image, it was to the left of two bright stars. In the next, it had jumped a few millimeters to the right of those stars. Tombaugh flipped back and forth between the images and watched the spot leaping in and out of its original position. He grabbed a ruler and measured the precise difference in the spot’s position. Then he found another photo of the sky, taken earlier in January, and searched for the same spot. Finally, he used a hand-magnifier to confirm the potential planet’s presence in one more set of photos, taken by a different camera. After 45 minutes, Tombaugh was convinced.

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