In a few hours' time, sky-watchers in North America will have the chance to see something awesome - the Moon is set to pass over and completely block one of the brightest stars in the night sky in a cosmic event called an occultation. Occultation might sound a little spooky, but it’s actually just a technical term used by astronomers to describe one object being hidden by another object that passes between it and the observer.
In this case, the object being blocked is a star called Aldebaran - the eye of the bull in the constellation Taurus. This ageing, massive, and reddish star is located about 65 light-years away from us, which is just as well, because if it were placed where the Sun is now, its surface would extend almost to the orbit of Mercury.
Aldebaran is approximately 44 times the size of our Sun, and 153 times as bright, and with an apparent magnitude of about 0.85, it’s the 14th brightest star in the night sky. But it’s about to get seriously overshadowed by our Moon from a good portion of the United States, southeast Canada, Mexico, the Caribbean, and Central America.
As Joe Rao explains for Space.com, Aldebaran will disappear behind the lit side of the Moon and reappear again on its dark side:
"As the Moon, three days past full, ascends the eastern sky, Aldebaran will appear to creep up to the Moon's bright limb for many minutes, then hang on the sunlit edge of a few seconds - an eerie orange fire among the lunar hills. You'll need a telescope to see it there through the Moon's glare. Then, in an instant, it will snap out of view.
Aldebaran will reappear from behind the Moon's dark portion (depending on your location) up to 75 minutes later. This time you may be able to see the event with binoculars, since the star will pop back into view farther away from the dazzling lunar surface."
So yep, you’ll need something other than your eyes to see the event properly, so dig out your old binoculars or telescopes for a proper view - even the smallest telescope will be fine.
Here’s a map showing where in the world the event will be visible. Basically, if you live between the white lines you'll be able to see the occultation taking place on the night of October 18-19:
IOTA (international Occultation Timing Association)
And here's the 'graze line' in the United States:
The occultation begins tonight at 10:15pm San Diego PDT (or 1:41am Philadelphia, EDT), and ends at 10:36pm PDT (or 2:48am EDT). Head to EarthSky.org to get a bunch more time conversions if you're in another part of the US - the start and end times fluctuate depending on where you live.
To be clear, the reason this event is special isn't because it's rare. It's because it gives you an awesome opportunity to see objects in our night sky interfering with each other - a reminder that we really do live in a vast, unknowable Universe, where giant cosmic bodies move about us like clockwork, day in, day out. In fact, tonight's occultation is just one of a series that have been taking place each month since 29 January 2015, and will continue on until 3 September 2018. So if you happen to miss this particular event, you've got SO MANY to watch instead.
The next series of occultations will not happen again until 2033. For those of you in the Southern Hemisphere who are missing out, remember - you've still got that hunter's supermoon to look forward to. And be sure to check out the International Occultation Timing Association's website to find out when future occultation events are expected to occur.
Happy sky-watching, guys!