Don't Miss the Moon Hide the Bright Star Regulus Before Dawn Sunday!

Sunday morning (Oct. 15) should get a big circle on your celestial calendar if you live nearly anywhere in the contiguous United States or southeast Canada. During the predawn hours, a waning crescent moon will temporarily hide the star Regulus in what's known as an "occultation." It's the last good view of this event for North America until 2026!

The 1.4-magnitude star Regulus is known as the heart of the Lion in Leo. The star's name is Latin for "little king." It is also the only first-magnitude star to sit almost exactly on the ecliptic or sun's yearly path in front of the constellations of the zodiac; it lies only 0.46 degrees from the ecliptic line.

As a consequence, Regulus can on occasion be hidden by the moon in an occultation, and early on Sunday morning just such a sight will be available for people living in most of the contiguous U.S. (except the Pacific Northwest and parts of the Northern Plains), as well as over a slice of southeast Canada and parts of the Maritime Provinces. Near and along the West Coast, Regulus will already be hidden behind the moon when it rises shortly after 2:30 a.m. PDT. [The Brightest Stars in the Night Sky]

A spectacular graze!

The northern limit of the visibility zone of the occultation begins over central Oregon and then tracks to the northeast through the middle of Idaho. Then it skims over portions of southern Montana and North Dakota and continues east-northeast through northern Minnesota, cutting straight through the Arrowhead region before heading out over Lake Superior. 

Those who are located above or to the north of the visibility line will lose out, having to settle for a close approach of the moon to the star. Nonetheless, if your area is slated for a miss, go out and watch anyway. It’s breathtaking to watch a bright star "flying" so near to the moon!

A Google map depicting the graze path, courtesy of IOTA – International Occultation Timing Association – can be found here. 

Upon close inspection of that Google map, you will see that the grazing occultation path is composed of three horizontal lines. The green line is the predicted limit if the moon were a perfect sphere. The upper and lower lines encompass the zone where the star will probably disappear and reappear more than once as it passes behind lunar hills and valleys — a spectacular grazing occultation! The path location is uncertain by probably less than a mile. 

Since Regulus is so close to the ecliptic, detailed observations of its grazes have special value for improving knowledge of that part of the lunar profile needed for analysis of solar-eclipse observations. The center of the graze path passes very near to Billings, Montana, and Bismarck, North Dakota. The graze prediction is by Brad Timerson. 

Watching and waiting

Occultations of Regulus happen in cycles of roughly nine years, and each cycle lasts about 18 months. The current cycle began last year on Dec. 18 and will come to an end on April 24, 2018. So each month, the moon will cross paths with Regulus, hiding it from view for various parts of the world.  

And on Sunday, it will be our turn. And it's the last good view for North America until 2026.

In some places, this event will be extraordinarily beautiful. But the quality of the view depends critically on where you are. The luckiest viewers are the ones in the zone from the south-central states to the Great Lakes. If you're there, you will see the Earth-lit moon cover and uncover Regulus in a dark sky well above the horizon. 

As the moon moves east against the background stars, the bright side of the slender waning crescent will first pass in front of Regulus, so lunar glare will prevent this from being a naked-eye event. But in a telescope, it should be spectacular. 

As the occultation approaches, it will be interesting to compare the surface brightness of the star to that of the moon. Normally, the lunar landscape seems overwhelmingly brilliant, but compared to the star, it’s a dull yellow. The better the seeing, the smaller the star will appear and the more intense this effect will be.

Regulus looks like a blazing blue diamond hovering above the day-lit desert of the moon. Whether it can be seen at all, this is definitely a show worth setting your alarm clock for. 

The star will then reappear up to 70 minutes later, depending on your location, from behind the "dark" part of the moon, which likely will be dimly illuminated by earthshine. Of all the things that happen on the celestial sphere, an occultation is one of the most dramatic and perhaps the most abrupt. 

If the Regulus reappearance is predicted to occur at least a few degrees above the eastern horizon in early twilight or darkness at your site, you'll need no equipment at all. Just keep gazing steadily, so you won't miss the instant the star dramatically "pops" back into view.

For those who are situated in the eastern U.S. and southeast Canada, the reappearance of Regulus will take place during morning twilight. The closer you are to the Atlantic seaboard, the closer the reappearance will come to sunrise — hence, the brighter the sky.   

The disappearance and reappearance will be best-viewed in binoculars or a small telescope. They may be essential if the moon and star are very near the horizon and dimmed by a low-lying layer of haze. Binoculars should be on a tripod or some other steady mount.  

An occultation timetable

Here is a listing of 15 selected cities providing local times of when Regulus will wink out behind the bright lunar crescent and reappear from behind the dark lunar limb.

* Denotes morning twilight has begun.

You can also visit this website to find out the occultation times for many hundreds of localities in Universal Time (UT). Remember to convert Universal Time to your local time: Eastern Daylight Time = UT minus 4 hours; Central Daylight Time = UT minus 5 hours; Mountain Daylight Time = UT minus 6 hours; Pacific Daylight Time = UT minus 7 hours.

Star party, anyone?

Despite the very early hour, you might want to consider inviting members of your family or even your neighbors to join you in watching this interesting demonstration of the machinery of the solar system. Who knows? This occultation might make a great impression on them — weather and observing conditions permitting, of course.  

You can explain to them that Regulus is a blue star 4.5 times bigger than (and 160 times brighter than) the sun, and that the starlight they are seeing now actually started on its journey toward Earth some 85 years ago.  

For comparison, the moon is only 1.25 light-seconds distant.  

If you pass on Sunday's opportunity and plan to wait out the next occultation of Regulus that favors the United States and Canada, you're going to have a rather long wait: It will be during the late-night hours of Feb. 2, 2026, and it will involve a bright, waning gibbous moon. 

Editor's note: If you have an amazing night-sky photo you'd like to share with us and our news partners for a possible story or image gallery, send images and comments in to spacephotos@space.com.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers’ Almanac and other publications, and he is also an on-camera meteorologist for Fios1 News in Rye Brook, NY. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.