Watch a Bright Star Wink Off and On

People who caught the Pleiades occultation on April 8th know how exciting it is to watch a star disappear behind the dark limb of the Moon. Much rarer and more exciting are times when a star is blotted out by a passing asteroid. And rarest of all are cases where an asteroid occults a star that's visible to the unaided eye. That's what's going to happen early on the morning of Thursday, April 17th.

This is a good-news/bad-news story. The good news is that 22 Scorpii, the star being occulted, is not only fairly bright (magnitude 4.8), but also extremely easy to locate. It's just a finger's width from Antares, one of the brightest and most colorful stars in the sky. (See the finder chart below.)

The bad news is that 1988 EB, the occulting asteroid, is quite small — about 10 miles across — so it will take it just one or two seconds to pass in front of the star. You could miss this event if you sneeze at the wrong time!

The yellow line shows the most likely path of the occultation, but the uncertainty in the asteroid's position is quite large. So the event could actually happen anywhere within the shaded area. And there's even a one-in-three chance that it will fall outside (though fairly near to) the shaded area.

S&T Illustration. Sources: Steve Preston / Derek C Breit

The good news is that the occultation occurs over the most densely populated part of the United States, making it potentially visible to nearly 100 million people.

The bad news is that the path of the occultation will be narrow (the same width as the asteroid), and the uncertainty of the asteroid's position is quite large. So even if you're on the predicted centerline, there's a 95% chance that the asteroid's shadow will actually pass to the north or south, and you won't see a thing.

Moreover, the event happens at a time when most people would prefer to be fast asleep, around 1:55 a.m. EDT in New York City, 1:56 a.m. EDT in Cleveland, 12:57 a.m. CDT in Chicago, and 12:58 a.m. CDT in North Dakota, at the western end of the path. How many people will wake up in the middle of the night for an event they're only 5% likely to see? Nobody knows! The last comparable event that we know of was when the asteroid 433 Eros occulted the medium-bright star Kappa Geminorum in 1975.

S&T Illustration

If you do stay up or wake up to watch this event, it's advisable to use binoculars, or even a telescope. Even though 22 Scorpii is easy to see with the unaided eye in a dark sky, it will be pretty close to the horizon, especially near the western end of the occultation path. And when a disappearance is predicted to be just one or two seconds long, you don't want to have any doubt at all that you can see the star clearly and consistently.

Make sure that you start observing with plenty of time to spare. Several S&T editors have missed occultations because it took them longer than they expected to locate the target star. Please report any sightings — positive or negative — as comments to this article, or e-mail them to [email protected]. Include your location, the sky conditions, and the time accurate to one second (if possible).

So if you live in the potential occultation path, why not set your alarm clocks for the early morning of April 17th? This could easily be a once-in-a-lifetime opportunity.