Based on the latest national forecast, skies will be mainly clear on Saturday evening (Oct. 31) across about 90% of the contiguous United States, as costumed kids arrive at the door looking for candy or some other Halloween treat.
A storm moving across Lake Superior will drag a cold front across parts of the Northern Plains and western Great Lakes. It's not likely to bring any precipitation, but perhaps will spread some cloudiness across parts of North Dakota, Minnesota, Wisconsin and the Upper Peninsula of Michigan.
The rest of the country, however, will be in fine shape with a bright full moon lighting up the evening sky. More on that in a moment.
If you plan to accompany children around your neighborhood, you might want to enlighten them by pointing out some of the objects that will be visible in the sky. Of course, in this year of the coronavirus pandemic, you will need to take extra precautions. Be sure that youngsters are wearing face masks; they can help curb the spread of the coronavirus and to protect other people. And remember physical distancing of at least 6 feet (2 meters) from others. It can make a big difference!
"King of the planets" and the "Lord of the Rings"
Jupiter and Saturn will be the first objects to vie for attention as the sky darkens; about a half-hour after sunset both will be near their highest point in sky, about one-third of the way up from the south-southwest horizon. You really can't miss them; Jupiter — the largest planet in our solar system — appears to shine with a very bright, silvery-white light, while dimmer Saturn, shining sedately with a yellowish tinge will also be evident slightly above and to the left of the "king of the planets."
With even a small telescope using low magnification you'll be able to see Jupiter's disk as well as all four of the famous Galilean moons, so named because Galileo Galilei was the first to see them with his own crude telescope in 1610. On this Halloween night, you'll see one satellite all by itself on one side of Jupiter — that will be Europa — while the other three reside on the other side of Jupiter.
Going outward from Jupiter will be Io, Ganymede and finally Callisto.
But as interesting as Jupiter is with its obvious disk and four bright moons, many will consider Saturn as the most beautiful of all the planets. Saturn's rings are still wide open, with the north face tilted some nearly 21 degrees to our line of sight.
But if you hope to show off the moon, Jupiter and Saturn to trick-or-treaters through a telescope, do it as soon as it gets dark because later in the evening their images likely will be shivering and churning about, due to poor viewing quality near the horizon. Jupiter will set around 10:20 p.m. local time and Saturn about a half hour later.
The god of war
There's another very bright and more colorful planet that will attract the attention of most folks: Mars, a fiery-colored object hovering in the eastern sky as twilight ends. Its reddish-orange tinge suggested the color of blood, which is why this planet was named for the Roman god of war.
Almost four weeks ago, Mars made the closest approach it will make to Earth until 2035. Its distance from us on Halloween night has since receded from 38.6 million miles (62.1 million kilometers) to 43.4 million miles (69.8 million km) and is virtually a match with Jupiter for brilliance at magnitude -2.1. Through even a small telescope it will show up as an orange-yellow disk with some darker markings streaked across its surface. Admittedly not as impressive as Jupiter's moons or Saturn's rings, but still, some of your neighborhood ghosts and goblins will likely want to see it nonetheless.
The Halloween "Blue Moon"
Of course, the main celestial object that will attract everyone's attention will be the moon, which will be full on Halloween. I suppose that next to Christmas Eve night, the holiday that is most associated with a full moon is Halloween. Who can forget, for example, that scene in the annual Halloween classic, "It's the Great Pumpkin Charlie Brown," when the silhouette of Snoopy — not the "Great Pumpkin" — rises up out of the pumpkin patch, backdropped by a bright full moon?
This will also be the second full moon in October 2020, so it will also be branded a "Blue Moon." The phrase "Once in a blue moon" was first noted in 1824 and refers to occurrences that are uncommon, though not truly rare. Yet, to have two full moons in the same month is not as uncommon as one might think. In fact, it occurs, on average, about every 32 months.
But to have a full moon coincide with Halloween is truly quite rare.
I did a survey of the dates of all full moons occurring from 1900 to 2100 and found that — across all United States time zones — there are only eight Halloween full moons that take place: in 1906, 1925, 1944, 2020, 2039, 2058, 2077 and 2096.
Notice that there is an interval of 19 years running in between most of these years. That's the Metonic Cycle, defined as 235 synodic lunar months, a period which is just 1 hour 27 minutes and 33 seconds longer than 19 tropical years (a tropical year is equal to 365.2422 days). So, most of the time, a full moon recurs on the same date after 19 years has elapsed.
But unfortunately, the cycle is not exact, and as a result there are occasions where a full moon will recur one day later. That was the case in 1963, 1982 and 2001 — when the full moon fell not on Halloween, but the following day (Nov. 1). Our Halloween full moon this year breaks a 76-year drought going back to 1944. However, after this year, the Metonic Cycle will work to bring a Halloween full moon back to us at 19-year intervals four more times, through the rest of the 21st century.
Still, over a span of two centuries, a full moon occurring on Halloween only happens about 4% of the time.
Or truly, once in a blue moon!
Helpful hints for young telescope viewers
In the September 2019 issue of The Astronomical League Magazine "Reflector," author Richard W. Schmude Jr., offered some tips for those who plan to do a public outreach for astronomy on Halloween:
"Firstly, children sometimes grab or touch the eyepiece, so use an inexpensive one. I gently warn children not to touch the telescope. In my area, parents have learned to tell their children not to touch the telescope. In some cases, a child will grab the eyepiece, causing the telescope to shift. For this reason, a Dobsonian telescope with a good finderscope is a good choice for Halloween outreach. A small stool or booster ladder may help very small viewers. Sometimes, parents hold their children up to look through the eyepiece. One may also place a monitor-connected video camera in the telescope and people can easily see the object on a screen." (I would add that in this year of the pandemic, that might be the solution of all). "Finally," adds Mr. Schmude, "I have my bag of goodies next to my telescope so that the children get two treats!"
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. Follow us on Twitter @Spacedotcom and on Facebook.