March 27, 2006
Alan MacRobert, Senior Editor
855-638-5388 x151, amacrobert@SkyandTelescope.com
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A special celestial event to watch is coming up on Saturday evening, April 1, 2006, for anyone who lives in the eastern or central part of North America. That evening, if the sky is clear, you can watch the waxing Moon eclipse, or "occult," a number of stars in the Pleiades cluster in the western sky during and after dusk. You'll have a decent view with your unaided eyes if your vision is sharp, but binoculars will do much better. And if you have a telescope, now is certainly the time to get it out.
Just keep watch on the Moon from twilight on April 1st until the Moon sinks too low in the west to follow. You'll notice right away that the Moon is next to, or among, the stars of the Pleiades cluster. Optical aid will also show the Moon's dark, night side dimly visible by earthshine — the light of Earth's daylit face lighting up the Moon's night landscape.
As time goes on, you'll see that the Moon's dim earthlit edge is creeping toward the stars it's facing. Eventually, with a little luck, you'll see the edge approach a star until the star seems to hang right on the edge, like a tiny white fire on the Moon. Then suddenly — instantly — the star will snap out of view. You've just witnessed a lunar occultation (from the Latin word occultare, to hide).
The Moon moves by about the width of its own diameter per hour against the background stars. So the occulted star reappears out from behind the Moon's other, sunlit edge up to an hour or so after disappearing. But the reappearances are much harder to see, since they happen in the bright glare of the Moon's daylit side. For these you really need a good telescope.
More on this beautiful event appears in the April 2006 issue of Sky & Telescope and the April/May 2006 issue of Night Sky magazine. Included in each is how to get time predictions of when individual stars will be covered up by the Moon's dark edge at your particular location.
The Value of Occultations
Astronomers have tracked occultations for centuries. Aristotle told of the Moon covering Mars on April 4, 357 BC — proof 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 the Moon could have no more than 1/2,000th as much air as Earth.
For many years, precise timings of occultations gave the most accurate fixes available on the Moon's orbital motion around the Earth. 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 stars are so close together that they may look single even in the most powerful telescopes.
Most of these scientific uses for occultations have been superseded by other, more modern techniques. But amateur astronomers still go on expeditions to time grazing occultations — when the Moon's edge barely skims a star sideways. During such an event, the star may flash in and out of view several times as lunar hills and valleys slide silently across it. Timings of grazing occultations are still valuable for mapping the Moon's profile very accurately.
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