A rare and fleeting stellar blackout is being hailed by astronomers as the perfect opportunity to ask budding stargazers for help in accurately plotting the brightest object we can see from Earth. A 2.8-mile-wide (4.6km) asteroid called Jurgenstock will pass in front of the double star system Sirius on Monday night, briefly blocking its powerful shine and casting an eclipse-like shadow across Earth for approximately 20 minutes. The so-called occultation will be fleetingly visible over parts of southern Argentina, southern Chile, Central America and the Caribbean at around 10:30pm MST on Monday evening. And the circumstances of the celestial event are so unusual that astronomers are issuing a strange plea to any potential witnesses. Astronomers Bill Merline and Danib Sunham, who published a series of helpful maps illustrating the shadow's path, say anyone in the right position has a very small window to actually see the full occultation – about 1.6 seconds. "If you are the lucky one to