Illustration showing asteroid 163 Erigone about to cover Leo’s brightest star Regulus around 2:07 Eastern Daylight Time Thursday morning March 20, 2014. As the asteroid’s shadow passes over the ground, observers will see Regulus disappear for up to 14 seconds. Illustration: Bob King with help from photos by the ESO/NASA
How does a tiny asteroid make one of the brightest stars in the sky disappear? By passing directly in front of it. Upwards of 20 million people will have the opportunity to watch asteroid 163 Erigone occult the bright star Regulus for up to 14 seconds next Thursday morning March 20. Billed as the best and brightest occultation ever predicted for North America, the sight of Regulus vanishing in plain sight should be a jaw-dropper.
Predicted path of the asteroid shadow. Shortly after 2:06 am EDT on March 20 observers between the red lines have the best chance of seeing the bright star Regulus temporarily disappear as asteroid 163 Erigone passes in front of it. Times shown are EDT. Click for detailed map. Credit: IOTA
Only thing is, you just have to be in the right place to see it. Check the map. If you live within the band, which cuts a swath some 45 miles (73 km) wide from northern Ontario to New Jersey, you’re in! While the northern reaches of the occultation occur over sparely populated tundra, the southern half encompasses all of New York City plus a few nibbles of New Jersey and Connecticut. All the rest of us will have to travel to the centerline much as we would to see a total eclipse of the sun.
To find Regulus, face southwest shortly before 2 a.m. The star will be about 40 degrees high (four ‘fists’ held at arm’s length against the sky). Brilliant Jupiter shines well to its lower right. You may also notice a ‘coathangar’ or ‘backwards question mark’ shape of stars above Regulus called the Sickle of Leo. Stellarium
It’s hard to imagine Regulus casting rays of starlight on Earth, but it really does just as all the stars do. During the occultation, Erigone (eh-RIG-uh-nee), a smallish asteroid about 45 miles (73 km) across, will block the star’s light, casting a shadow for up to 14 seconds for those situated along the centerline. The farther from the centerline, the less time Regulus will vanish from view. If you live just outside the error limits (shown in red on the map) you’ll miss the shadow (and occultation) entirely.
We don’t know a whole lot about Erigone. It was discovered in 1876 by French astronomer Henri Joseph Perrotin and based upon its spectrum (what “colors” of light it absorbs and reflects), it’s classed as a C-type asteroid rich in water-containing minerals. Like the largest asteroid Ceres, it resembles meteorites that fall to Earth called carbonaceous chondrites. At 45 miles in diameter and dark as a charcoal briquette, that’s about it. During the occultation it will shine at magnitude 12.4 and remain hidden in the glare of Regulus.
But here’s where amateur and even beginning astronomers can help. If lots of observers across many locations make accurate timings of the disappearance and reappearance of Regulus, we can construct an outline of the asteroid’s shape with a resolution of 0.6 miles (1 km) at its distance of 110 million miles (177 million km). That’s incredible.
On July 19, 2011, amateur astronomers discovered through multiple timings – each observer represented by a color line – that asteroid 90 Antiope was not a single object but two co-orbiting asteroids! We might be in for similar surprises with Erigone. Notice that the “yellow line” observer recorded only a brief disappearance of the background star; the green line observer saw a much longer occultation since the full width of Antiope blocked the star. Credit: IOTA
For one observer, only the tip of Erigone will pass in front of Regulus, causing the star to blink off and on again very quickly. Another may see the full bulk of the asteroid block the star for many seconds. Plotting all these chords of visibility on a graph will yield a clear profile of Erigone’s silhouette as well as refine its orbit with great precision.
Dedicated occultation watchers have been doing this for years with mostly fainter stars, sometimes traveling great distances to secure a key data point. They use video cameras with a GPS video time inserter to make timings accurate to 1/10 of a second or better and contribute their results to the International Occultation Timing Association or IOTA.
Click to see the video on how to use the free iPhone occultation app to time the disappearance and reappearance of Regulus.
Many in the organization will be out there at 2 a.m. Thursday March 20, and you can too. If you’d like to make and submit your observation, it’s super easy. Just download the free iPhone app Occultation 1.0 and tap the screen when Regulus winks out and again when it reappears. For instructions on how to use it, check out the video. A similar free app called Time the Sat is available for Android devices.
A potential unknown moon of Erigone might be found by timing Regulus’ disappearance and reappearance well outside the central swath between the two gray bands. Credit: IOTA/Ted Blank/Google Earth
By the way, “misses” are as important as “hits” when it comes to occultations. Misses help define the edges of the asteroid. You can’t determine a true shape unless you know where Erigone ends and empty space begins. Other discoveries could also be made. Anyone living from northern Minnesota to Kentucky to Nova Scotia could conceivably see a brief disappearance from an unknown moon of the asteroid.
Regulus has an end-to-end diameter of 4.3 times the size of the sun due to its rapid rotation. Because of its large size, it won’t blink out instantaneously. Observers making very careful timings may be able to “see” the shape of the star. Illustration: Bob King
Even Regulus gets into the act. It’s a hot, blue star 79 light years from Earth and spinning so rapidly (once every 16 hours compared to the sun’s leisurely once every 27 days) that it’s stretched into an ellipse.
Multiple precise timings by careful observers could very well refine the shape of the star. How often do we get that kind of opportunity? Awesome in the extreme!
Not that you have to “do science” during the event, though I highly recommend it. Of course there’s also nothing wrong with simply enjoying one of the rarest sights you’ll ever see.
For complete information on everything from weather to software, do stop by IOTA’s Regulus occultation site. It’s loaded with good stuff. Click HERE for a detailed Google Earth Map with path.