Across the upper part of the moon’s disk one night last week flitted a ruddy shadow, tilted about eight degrees to the east. It was an appulse of the moon, visible in most of North America and parts of Europe. Associated Press’s Science Editor Howard Blakeslee compared the sight to “a bandit with a dark cap drawn down over his forehead.”
The total shadow that extends out into space from the night side of Earth is a slightly tapering cone. The partial shadow, or penumbra, is a slightly spreading cone. Earth did not swing directly between moon and sun last week, and the moon slid through the penumbra. Appulses of this kind are astronomical curiosities because an average of 85 occurs in a century, whereas total and partial eclipses (in which the moon passes wholly or partly through the cone of total shadow) happen almost twice as frequently. About twelve appulses in a century are, like last week’s, conspicuous.