The sight of a relatively bright star suddenly vanishing as a faint asteroid crosses in front of it, and then just as suddenly reappearing several seconds later is an exceedingly rare and startling celestial occurrence. But it's happening in the predawn hours on Tuesday and lucky skywatchers across parts of Canada and the western United States may have a chance to see it.
Astronomers refer to such an event as an "occultation" a temporary hiding of one celestial object by another such as a planet moving in front of a star as seen from Earth. In this particular case, however, the occulting body will be a minor planet: a tiny asteroid known as 824 Anastasia. ?
The star in question is Zeta Ophiuchi, the third brightest star in the constellation of Ophiuchus, the Serpent Holder. At magnitude +2.6, it's a star that's bright enough to easily see with the naked eye; you don't even need binoculars, although they would give a better view. ?
The projected path of this stellar eclipse just 25 miles wide will run from Southern California (encompassing a part of the Los Angeles metro area) then goes north-northeast through central Nevada, west-central Idaho, northwest Montana and on up into Alberta, Canada, passing almost directly over this province's two largest cities, Calgary and Edmonton.
The International Occultation Timing Association (IOTA) is encouraging as many people as possible to try to see and time the event.
"Anyone who can see and count, lives in or near the eclipse path, and is willing to get up in the middle of the night and go outside for about ten minutes, can help us measure the size and shape of this asteroid," said IOTA member Brad Timerson. "We want as many as possible to try to observe the eclipse since the detail of Anastasia's shape that we can derive is proportional to the number of places from which the eclipse is observed."
What makes this event so special is that this is the brightest asteroid occultation (eclipse) ever predicted for North America involving an asteroid this large.
During a similar asteroid-star occultation event that was visible from China in 1991, 3,000 skywatchers attempted to see the eclipse. But only four observers actually saw it, since the predictions regarding the eclipse track were somewhat uncertain.
Today, thanks primarily to the European Space Agency's star-tracking spacecraft Hipparcos, predictions of such events have greatly improved.
How to locate Zeta Ophiuchi
If you are novice to astronomy and think you'll have trouble in locating Zeta Ophiuchi, here is a reliable way of identifying it. You can try it during the early evening hours of Monday, April 5, which will be many hours before the occultation. ?
First, locate the familiar seven stars that make up the Big Dipper (around 11 o'clock, local time, the Dipper will be high in the northern sky, almost directly overhead).
The stars in the handle of the Dipper make a curve that is easily translated into a smooth arc. ?
Continue that imaginary arc about the length of the Big Dipper and you will come to the brilliant yellow-orange star, Arcturus in the constellation Bootes. Then, follow that same arc past Arcturus for about the same distance, and you will come to Spica, a bright bluish-white star in the constellation of Virgo.
Now line Spica up with a nearby landmark, such as a telephone pole or the top of a chimney. ?Make note of the time and exactly where you are standing. ?
Now wait about 3 hours and 13 minutes, go back outside and stand where you were when you were
looking at Spica and look toward your landmark. Thanks to the rotation of the Earth, Spica will no longer be there, but another star, shining about one-quarter as bright as Spica will be there instead.
That's your target star: Zeta Ophiuchi.
The Eclipse Track
Here is a view of the shadow track of 824 Anastasia as it moves on a south-to-north path across western North America. The solid parallel lines define the "best" estimate of the path, estimated to be 25 miles wide. But there is still some uncertainty which is why an additional set of dotted lines are placed on either side of the path, suggesting that the star conceivably might appear to briefly wink out just outside of the main eclipse viewing zone.
Between 3:34 and 3:35 a.m. Pacific Daylight Time, the 25-mile-wide path is expected to pass over Orange County and extend north from there, over West Covina and Ontario, Calif.
However, as already has been noted, the location of the path is quite uncertain. Here is a close-up of the track as it passes over southern California. The light green line is the predicted central line, the blue lines are the eastern and western edges of the predicted path, the red and gray lines are the edges of two zones of uncertainty. ?
As it stands, the path could instead pass over downtown Los Angeles or the San Fernando Valley (with a small chance that it could pass over even Ventura and Bakersfield), or it could shift east, possibly passing over San Bernardino or, rather less likely, over San Diego and Palm Springs.
"Even an observer on the predicted central line has only a 26% chance to have an occultation," Timerson said.
After leaving the vicinity of Los Angeles, the shadow of the asteroid will take five minutes to race through Nevada, Idaho and Montana, crossing into Alberta, Canada just before 4:40 a.m. Mountain Daylight Time. Between 4:40 and 4:41 a.m., the shadow will sweep over Calgary and Edmonton, but it might pass east or west of those cities.
Timing this rare event
One of the best methods for timing this event is to use a tape recorder in conjunction with a the time signal ticks from shortwave radio station WWV, which can be best heard at frequencies of 5.0 or 10.0 megahertz.
Here's what to do: With the tape recorder continuously running and WWV on the radio in the background, call out "D" when the star disappears and "R" when it reappears (or, thinking it like a light, "off" and "on"). ?
Reporting your observation
If you did watch at the right time, IOTA wants to know whether or not you saw an occultation.
"If you had no eclipse of the star, just let us know that fact and send your location in an e-mail message to email@example.com," wrote IOTA President David Dunham. "We prefer that you use a GPS receiver or Google Earth, to determine your longitude, latitude, and elevation above sea level."
Anyone having trouble can send the address of their observation point, including approximate distance from the center of the nearest road, and from the nearest cross street. The time an observer starts and stops observing the star Zeta Ophiuchi is also valuable, Dunham added.
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.