Passing shadow

In astronomy, an occultation occurs when one celestial object passes in front of another, temporarily blocking it from sight (from the perspective of an observer on Earth).  Some occultations are entirely unremarkable; the Moon occults thousands of stars every month on its trips around the Earth, and we barely notice it because it's so ordinary.  And technically, solar eclipses are occultations, even though we don't usually think of them that way.

Other occultations, though, are rarer and more interesting.  The rings of Uranus were discovered in 1977 when they occulted the star SAO 158687, making it appear to blink on and off five times.  Pluto occulted stars in 1988, 2002, and 2006, and as it passed in front of the stars the alteration in the stars' light allowed astronomers here on Earth to study the chemistry of Pluto's thin atmosphere, a technique called atmospheric limb sounding.

Of course, because the two objects have to be lined up perfectly, occultations of bright and/or familiar objects are rare occurrences.  Jupiter, for example, is going to occult the planet Saturn -- but not until February 10, 7541.

That's a long time to wait.

Because natural occultations are so uncommon, there's a proposal to create an artificial version of occultation -- and use it to try and find more exoplanets.  A proposed orbiting telescope called BOSS (for Big Occulting Steerable Satellite -- of course it has a cute acronym, because these are astronomers we're talking about) will, if deployed, have a shield that can block the light of stars and allow the much dimmer reflected light of their planets to be detected.  Ordinarily, the glare of even an ordinary star is so bright that it swallows up the signal; shielding it, creating an artificial eclipse, might reveal hidden planetary systems.

The reason this topic comes up is that a loyal reader of Skeptophilia alerted me to an occultation that is going to occur at 1:08 AM, Greenwich Mean Time, tonight.  The bright star Betelgeuse is, for seven minutes, going to be covered by the asteroid 319 Leona -- so for that time, the familiar figure of Orion will appear to be missing his left shoulder.

Unfortunately for me, the occultation won't be visible from upstate New York, but you're in luck if you're in the southern Mediterranean or the southern tip of Florida.  Here's a map of where the occultation will be visible:

The coolest part is that Betelgeuse is so huge -- if it were where the Sun is, its outer edge would be near the orbit of Jupiter, and we'd be inside the star -- that despite its distance of 548 light years, its angular diameter is larger than that of the much closer asteroid that's occulting it.  So through a telescope, it'll be an annular occultation -- the occulting object won't quite be able to cover the entire star up.

Even so, Betelgeuse will for seven minutes dim to near invisibility to the naked eye.

You have to wonder what the ancients would have made of this.  Many cultures believed the heavens to be timeless and changeless, which is why the appearance of comets and events like the supernovae of 1054 and 1604 ("Kepler's supernova") were considered such portents of evil.

Even though we now know better, it still will be a little unnerving to have such a familiar star suddenly appear to vanish from sight.  It's a pity I won't get to see it -- it's a little late to purchase a plane ticket for Spain.  If the path of occultation passed across upstate New York, I'd even get up in the middle of a frigid December night to see one of the brightest stars in the night sky blink off for a few minutes.

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