Occultations: The Fastest Things in the Sky

Moon occults Spica
A lovely occultation took place when the bright star Spica (upper left) vanished behind the Moon’s north polar region on November 30, 1994.

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Most of these uses for occultations have been superseded by other, more modern techniques.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.

Pleiades and the Moon
The crescent Moon sneaks up on the Pleiades.

Watching the Winking

Half of the Moon’s round edge is usually sunlit and half is dark, but as telescope users soon learn, the dark edge can usually be dimly seen because of earthshine weakly lighting the Moon’s night regions. (The exception is around the time of full Moon.) It’s much easier to watch a star creep up to the dark edge than the bright edge — because stars near the sunlit side generally get swamped by the bright glare in the last minute or so before an occultation.

Fortunately, stars disappear on the dark edge when the Moon is in its waxing phases and thus visible in the evening — when most people do their observing. Stars disappear on the bright edge when the Moon is waning and visible mostly after midnight.

As it travels along its orbit, the Moon takes about an hour to move across the starry background by its own diameter. Therefore, the occulted star pops back into view from behind the Moon’s other side up to about an hour later. But even when a reappearance happens on the Moon’s dark edge (usually in the wee hours of the morning), you have to be watching the right spot at just the right time.

Asteroid Occultations Too

The Moon isn’t the only thing that can pass in front of a star. Anything else in the solar system can too. Occultations sometimes involve asteroids, or “minor planets.” These range in diameter from 600 miles down to just a few miles or less. Thousands of them have orbits that are known very precisely. When a faint asteroid is predicted to cross a star that’s visible in amateur telescopes, many amateurs mobilize to try to time the moments when the star snaps out of view in the seemingly empty sky and snaps back a few seconds later. Asteroid-occultation paths are not perfectly predictable, so watching for these events can be hit or miss. But if people at enough different locations get good timings, their results will reveal the size and shape of the asteroid’s dark silhouette — information that usually can be gained no other way. An example is shown at right.

Asteroid occultations have had a special fascination for me since the first one that was well predicted, when the asteroid Eros was to cross in front of a bright star on the night of January 24, 1975. Over the years I’ve attempted to time 25 such events. I got successful timings for 3 (they really are hit or miss), successful negative observations for 15 (meaning the star was definitely not occulted at my location, valuable information worth reporting) — and 7 blown observations due to equipment failure, clouds moving in at the last minute, the star being too faint to follow, or on one embarrassing occasion because I figured the date wrong and got all set up exactly 24 hours too late.

How are timings done? The standard way is to set up a shortwave radio next to your scope outdoors and tune it to a time-signal station like WWV (in the U.S.) or CHU (in Canada) that beeps each second. You record the sound while you watch the star and shout when you see an event. By carefully listening to the tape later, you can determine the time of your shout between beeps to an accuracy of a few tenths of a second. Recently, amateurs have shifted to using small video cameras with their telescopes to record occultations. With the right timing gear, such as certain GPS receivers, the exact time of each video frame can be determined to just a few hundredths of a second.

Getting Predictions

To coordinate all this activity, a group called the International Occultation Timing Association (IOTA) has been working diligently for decades. Its Web site is full of news and further information.

To get a year’s worth of lunar-occultation predictions generated for your site, e-mail your accurate latitude and longitude and the size of your telescope to Walter Robinson. Predictions of asteroid occultations, along with finder charts and much else about these exciting events, are at Steve Preston’s site.