If you often look at the Moon with a telescope, you’ve probably seen a star or two sometimes shining quite close to it in the background. If you kept watching, maybe you noticed the Moon moving with respect to the star after just a few minutes — dramatic evidence of the Moon’s rapid orbital motion around Earth.

But what happens when a background star is directly in the Moon’s way? In this case, the Moon appears to plow right over the star and black it out, like a tanker ship running over a candy wrapper. This event is called an occultation, from the Latin occultare, “to hide.”

Occultations happen often, and they’re fascinating to watch. A star appears to creep up to the Moon’s edge minute by minute, hangs right on the edge for a number of seconds, and then abruptly snaps out of view. For decades amateur astronomers have made a project of timing the exact moments when occultations occur, producing data with real scientific value.

Sky Collisions

Records of occultations go way back. Aristotle told of the Moon covering Mars on April 4, 357 B.C. — proving that Mars was farther away than the Moon. The suddenness of star occultations offered the first proof that the Moon has no air and therefore cannot support life. If the Moon had an atmosphere, stars would gradually dim as the Moon’s edge approached them, the same way the setting Sun dims before it reaches Earth’s horizon. Scrutinizing an occultation in 1843, Friedrich Wilhelm Bessel found that a star’s light rays did not bend at the Moon’s edge by any amount he could measure, a sign that any lunar air could have no more than 1/2000 the density of Earth’s atmosphere.

More recently, occultations have been used for several other scientific purposes. For many years, occultation timings gave the most precise fixes that anyone could get on the Moon’s orbital motion. Also, many close double stars were first discovered by their stepwise occultations. In such an event the star drops out of sight on the Moon’s edge in two distinct steps, as first one star of the double is covered, then the other — even though the star may look single in the largest telescopes.

Most of these uses for occultations have been superseded by other, more modern techniques. But amateurs still gang up to go on expeditions to time grazing occultations: when the Moon’s edge barely skims a star sideways. During a graze, the star may flash in and out of view several times as lunar mountains and valleys slide in front of it. Timings of grazing occultations can map the Moon’s profile very accurately.