As seen from the eastern United States, there’s a “now-you-see-it, now-you-don’t” event in the early evening sky. The Moon will occult Regulus – passing in front of Leo’s brightest star and blocking it from view. The star will remain hidden for a few minutes. But its disappearance is almost instantaneous:

Regulus is there one second, then gone the next.

It does take a tiny fraction of a second for the Moon to cover the star. Astronomers make precise measurements of that timing. The length of time it takes a star to vanish reveals its apparent diameter – how big it looks in our sky. And that’s how the first good measurement of the size of Regulus came about.

In 1933, a French astronomer recorded an occultation of the star on a rapidly spinning photographic plate. That told him how long it took Regulus to disappear. From that, he calculated the star’s apparent diameter. And he was close to the modern value.

When astronomers combine that number with a star’s distance, they can calculate its true diameter. Regulus is 79 light-years away – and about four times the diameter of the Sun.

Tonight’s occultation is best seen from the eastern U.S. The Moon and Regulus will be in the sky as seen from the rest of the country as well. But at least part of the event will take place during daylight, when Regulus is too faint to see without help. The star will shine close to the Moon after the occultation ends.

Script by Damond Benningfield