Sirius, the brightest star in the night sky and the fifth closest star to our solar system, will be briefly occulted Monday by an asteroid hurtling around the Sun. The first time this event has ever been predicted by astronomers, it will be visible as a dimming of Sirius’ bright light for a split-second at around 22:30 p.m. on Monday from a narrow ‘shadow path’ through western Mexico, central U.S. and Canada.
Astronomers call such an event an occultation, which is when a celestial object is hidden from view by another celestial object.
Where is Sirius in the night sky?
You can’t miss it. Really, it’s one of the easiest stars to find in the entire night sky, firstly because it’s the brightest, but also because the three stars in Orion’s Belt point downwards to Sirius at this time of year. If you’re in the northern hemisphere, go outside after dark and look to the southern sky. You will see the famous constellation of Orion the Hunter, with the three bright belt stars, Alnitak, Alnilam and Mintaka. Follow those three stars in a line down to the southeastern horizon and you will see Sirius winking.
What is 4388 Jürgenstock?
It’s a 3.1 mile-wide asteroid from the asteroid belt. It was named after its discoverer, Venezuelan astrometrist Jürgen Stock. It orbits the Sun once every 3 years and 7 months.
When was the last time Sirius was occulted?
“This is the first occultation of Sirius ever predicted,” says Dr. David W. Dunham at the International Occultation Timing Association, Middle East section (IOTA/ME). ”The star catalogs and asteroid ephemerides were not accurate enough to predict such events before 1975 so nobody tried to predict such occultations before those years.” Dunham explains that Sirius is far from the ecliptic, where most asteroids roam, and occultations of stars like Regulus, in Leo, are more frequently occulted.
Where and when will the occultation be observable from?
At precisely 05:21 a.m. Universal Time on February 19, 2019, though it will actually be late on February 18, 2019 in the parts of the world where the occultation is visible from, namely Canada, the U.S. and Mexico. Starting in the South Pacific, the ‘shadow path’ will move through Baja California, Sinaloa and Chihuahua in Mexico, then through New Mexico, Colorado, Nebraska, South Dakota and North Dakota in the U.S., and finally into Manitoba and far western Ontario in Canada. There’s an excellent Google Map here (though you will also need to convert from Universal Time to your local time), while the brilliant Occultation Pages hosts a full and detailed explanation of the event and of the path. It also includes details on how dedicated amateur astronomers within the predicted shadow path can help astronomers reduce the size of the ‘uncertainty zone’, and also how to record the event using a camera or even just a smartphone.
The ‘other’ occultation on February 18
Though rare, it’s actually not the only occultation happening on February 18. “Besides the occultation of Sirius in the evening, there is also an occultation of the 4.2-magnitude. star chi Virginis in the morning that will be visible from southern Texas, near Corpus Christi, and northern Mexico,” says Dunham. “Since the Moon will be nearly full, it will be hard to spot that one by naked eye, but it would be easy with binoculars.”
Bizarrely, there are these are no other occultations of stars brighter than 6th magnitude, the limit of naked-eye visibility, that will be observable from North America during 2019.
What will observers notice?
Occultation Pages suggests that observers in the shadow path will see “the star fade over a period of several tenths of a second, probably will not disappear completely, and then will recover its full brightness over another several tenths of a second.” However, it also suggests that a half-second drop in brightness may be observable for those standing near the edge of the path.
How common is it for a bright star to be occulted by an asteroid?
A bright star is occulted by a reasonably large asteroid about every two or three years. “The last was an occultation of Regulus that was occulted by the large asteroid Adorea on October 13, 2016,” says Dunham, who was in Papua New Guinea to video it with a 10-inch telescope. Fainter naked-eye stars are occulted more frequently, with details on the best ones kept here.
Are such occultations useful to astronomers?
Absolutely. In fact, the ultra-rare occultation of Sirius may help astronomers calculate how far away it is. No space telescope or star camera has ever been able to do that accurately, not even Gaia, the ‘star surveyor’. “The Gaia mission has sensitive cameras that were designed to map some two billion stars down to 20th magnitude,” says Dunham. “Sirius is over 100 million times brighter than those faintest stars, so it was too difficult to design a system that would map both.” Although Gaia does have some filter techniques to get data on some of the brighter stars, Sirius remains too bright for them.
Wishing you clear skies and wide eyes
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