For the most part, across approximately three-quarters of the United States, skies will be clear on Tuesday evening as costumed kids arrive at the door looking for candy or some other Halloween treat.
If you plan to accompany children around the neighborhood, you might want to enlighten them by pointing out some of the objects that will be visible in the sky. Better yet, if you have a telescope, give your visitors a treat of a different kind: Give them a close-up view of some celestial sights, starting with the most beautiful of the planets and Earth's nearest neighbor in space.
The Lord of the Rings
Saturn is the first object to signal for attention as it descends, low in the west-southwest part of the sky. It glimmers into view during twilight, followed a few minutes later by the slightly dimmer star, ruddy Antares, well to its lower right. The rings are now at their widest since April 2003, with the north face tilted nearly 27 degrees to our line of sight. Unfortunately, if you hope to show it off to trick-or-treaters through a telescope, the planet's image likely will be shivering and churning about, due to poor viewing quality near the horizon. Saturn sets about 2 1/2 hours after sunset. [Carve Like an Engineer: Halloween Pumpkin Design Advice from NASA's Jet Propulsion Lab]
Of course . . . there's the moon
The moon will be the most obvious sky sight. Those who may already have started their trick-or-treating in the late afternoon might catch the moon rising above the east-southeast horizon shortly after 4 p.m. local time. It will be highest in the sky right about the time most costumed crusaders have called it a night: around 10 p.m. The moon will still be four nights from full, so it will be gibbous — from the Latin gibbus, meaning "hump." We'll get our first view of the moon in the late afternoon, and it will shine well into the wee hours of the morning, blotting out many of the fainter stars. In rural locations, far from bright lights, the presence of the gibbous moon on Halloween night will likely be a welcome sight to parents, as it will help to illuminate roads and paths for kids.
Interestingly, the gibbous moon is the most-seen phase, occurring for the half-month between first and last quarter (although it looks full for a night or two around the time of full moon). Because it is in the sky for more than half the night, we're more apt to see the gibbous moon compared to any other lunar phase. Yet, it seems there are very few images ever taken of a gibbous moon. We're more apt to see photographers and artists portray either a full moon or a slender sliver of a crescent moon, though a crescent moon is only visible just during early evening or morning hours, and sometimes only briefly.
One good reason to keep a sharp eye on the sky on Halloween night is for the possibility — albeit small — of catching a view of a spectacular fireball meteor. The Taurid meteors, sometimes called the "Halloween fireballs," begin showing up in mid-October and continue into mid-November, but November 5 to 12 is traditionally the time frame when these slow and majestic meteors are at their best. They don't produce a lot of meteors ... maybe a dozen during an hour's watch at the very most, and usually much less. But so far as the Taurids are concerned, it's quality that's important, not quantity.
Meteors — popularly referred to as "shooting stars" — are generated when debris enters and burns up in Earth's atmosphere. In the case of the Taurids, they are attributed to debris left behind by Encke's Comet, or perhaps by a much larger comet that, upon disintegrating, left Encke and a lot of other rubble in its wake. Indeed, the Taurid debris stream contains noticeably larger fragments than those shed by other comets, which is why this particular meteor stream occasionally delivers a few outstandingly brilliant meteors known as "fireballs." If one of these flares across the sky on Halloween night, it might even make the kids temporarily forget about candy!
And here's a "constellation prize"
It's sort of strange that in a night sky, where a variety of strange beasts and creatures proliferate, that no witches, ghosts — or even a creepy spider — are recognized as a constellation. But we do have a gorgon ... or at least the head of one, visible in the northeast part of the sky. The gorgon is Medusa, who had snakes for hair and was legendarily so hideous that if you looked at her, you would instantly turn to stone.
In our current evening sky, ascending in the northeast is Perseus, the Hero, who slayed Medusa by beheading her with his sword. In the sky, one of Medusa's eyes is marked by the star Algol, which has been known since ancient times as the Demon Star and is one of the most famous variable stars in the sky. Its very name, from the Arabic Al Ra's al Ghul, means The Demon's Head, and ancient astrologers considered Algol the most dangerous and unfortunate star in the heavens. This seems to suggest that medieval stargazers were aware of its ability to mysteriously change in brightness. In 1782, amateur astronomer John Goodricke realized that this star was really a pair of stars orbiting a common center of gravity, and that when the dimmer of the two crossed in front of the other, the light from Algol appeared to fade.
Unfortunately, this stellar eclipse, which occurs like clockwork at intervals of 68 hours, 48 minutes and 56 seconds, will not take place on Halloween night. But one is scheduled for next Tuesday night (Nov. 7 —election night in the U.S.).
That night, Algol will be at its minimum brightness at 9:16 p.m. EST (6:16 p.m. PST, or 0216 on Nov. 8 GMT). Normally, Algol appears 3.3 times brighter than it does during the peak of the eclipse. Two hours before the predicted minimum, start checking Algol's brightness. It is at minimum light for about 20 minutes — as the large, dim star passes across the smaller, brighter one — then it gradually returns to normal.
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, New York. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.