Set your alarm early on Oct. 6 and you can one watch one of the brightest objects in the night sky disappear from view, albeit temporarily.
In the predawn hours on Tuesday morning, a slender waning crescent moon, just 20% illuminated and a little over four days from new phase will rise in the east-northeast at around 2:30 a.m. local daylight time, with brilliant Jupiter shining near its sunlit side.
As this "dynamic duo" ascends, the moon will gradually draw closer to the big planet. Observers in the eastern and central U.S. and adjacent southern Canada can then watch as Jupiter slips behind the moon's bright limb. It will then emerge from behind the moon's dark limb — dimly illuminated by earthshine — roughly an hour or so later.
What is happening?
Unlike a star whose pinpoint of light appears to suddenly "blink out" or "pop on" instantaneously, Jupiter's disappearance and reappearance will each take roughly a minute as the moon's edge advances across the giant planet's disk, whose apparent diameter is ¹⁄₅₈ that of the moon. You're going to need binoculars or better yet a telescope to watch Jupiter disappear behind the moon's bright crescent, but its reappearance will be readily visible to the naked eye. Of course, binoculars or a telescope will provide a far more stunning view.
Clearly, that will be the most memorable part of this sky show.
In October 1968, an occultation of Jupiter occurred over the western and central United States and descriptions of Jupiter's reappearance published in the January 1969 issue of Sky & Telescope magazine were described as "quite dramatic" as seen from Chicago, while an observer at La Pryor, Texas, noted that the most spectacular feature for him was the naked-eye impression of Jupiter ". . . (looking) like a brightening jewel on the dark limb of the moon."
How to watch
Using our specially designed maps you can closely approximate the circumstances for the disappearance and reappearance at your hometown. Do this first by
- Interpolate along the thin horizontal curves for the times listed as Universal Time (UT) of the event. (To convert to Eastern Daylight Time, subtract 4 hours from UT; to get Central Daylight Time, subtract 5 hours; to get Mountain, subtract 6.)
- Then interpolate between the thicker vertical lines to find the altitude of Jupiter and the moon above your east-northeast horizon. Your clenched fist held at arm's length measures roughly 10 degrees. For eastern New York State and New England, Jupiter and the moon will be between 20 and 30 degrees above the horizon. Farther west, their altitude will be lower, especially for the disappearance, so you might want to scout out an observing site in advance that will afford you a low enough view toward the east-northeast.
As an example, what will be the times and altitude for the disappearance of Jupiter as viewed from Memphis, Tennessee? From the disappearance map, Memphis is inside the "8:15" contour. Subtracting 5 hours for Central Daylight Time gives us 3:15 a.m. for when Jupiter goes behind the moon's bright limb. Memphis is roughly midway between the "Jupiter disappears at moonrise" and "Jupiter and moon 10° up" lines, so for Memphis the altitude will be very low; only above 5° above the east-northeast horizon.
For more detailed information, including a map of the occultation zone, as well as Universal Times for 767 locations, including 285 cities in the contiguous U.S. and 71 in Canada, check out these resources courtesy of the International Occultation Timers Association (IOTA).
Watch Jupiter's moons disappear too
A telescope will also show occultations of Jupiter's Galilean moons. Europa roughly 5 minutes before Jupiter itself (depending on your location), Callisto and Io about 3 minutes before, and Ganymede roughly 2 minutes after. Coming out, Europa will be first, about 5 minutes before Jupiter, followed by Callisto and Io about two minutes later. Finally, two minutes after Jupiter fully returns to view, Ganymede will emerge. Except in large telescopes, the satellites might be lost in the glare of the bright lunar crescent just prior to disappearing behind the moon, but their emergence from behind the Moon's dark side should be especially dramatic.
What if you're outside the occultation zone?
While many locations will enjoy the sight of the waning crescent moon occulting Jupiter, a swath of the western U.S. and Canada lies outside of the occultation zone. For those living in the states of Washington, Oregon, California, Idaho, Nevada, most of Arizona and parts of western Montana and western Utah, the main event will have already taken place before the moon and Jupiter rise.
But, as a consolation prize, these regions will be treated to the stunning and rare sight of the planet appearing very closely above the upper edge of the lunar disk shortly after local moonrise; not an occultation but still a head-turning celestial tableau!
For places like Boise and Tucson Jupiter will appear only about one-sixth of the moon's diameter above the moon's dim earth lit rim. The gap widens as you head farther west.
How rare is this?
One question that not a few will probably ask is: "How often does the Moon eclipse Jupiter?" If we consult Jean Meeus' Astronomical Tables of the Sun, Moon and Planets, we find that on a worldwide basis, there are 123 occultations of Jupiter between 1980 and 2040. But if we confine our search to a specific location and only include those instances when the sun is below the horizon, such an event occurs on average once about every 13 years.
Still, there are oddities that occasionally show up. In the year 2037, as seen from Portland, Oregon, the moon will occult Jupiter on the morning after Thanksgiving, Nov. 27 ("Black Friday"). Portland's next dark-sky occultation of Jupiter will come less than a month later, near the break of dawn on Christmas Eve, Dec. 24.
Conversely, for New York City prior to this year, the last dark-sky occultation of Jupiter occurred nearly 22 years ago on Dec. 7, 2004. Incredibly, before that date we must go back over 115 years, to the late 19th century — Sept. 3, 1889 — for New York's previous case of a dark-sky occultation of Jupiter. The New York Times wrote of that late 19th-century spectacle:
"The moon looked like half of a big custard pie, and Jupiter like a firefly getting ready to take a bite out of it. Imperceptibly, Jupiter crowded along, nearer and nearer to the big light of the night. Dimmer and dimmer it grew as it neared the moon, its light paling into insignificance as the light of the tallow candle pales into insignificance before a calcium light."
This month's event is the best Jupiter occultation for North America since 2004. There is no scientific value to this occultation, since there are no abrupt events to be timed, but it certainly will be an unusual observing project for skywatchers as well as a memorable eye-catching spectacle wherever it can be seen.
So clear skies, good luck and make the most of this upcoming opportunity!
Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, Sky and Telescope, The Old Farmer's Almanac and other publications.
