On Tuesday (Sept. 8), skywatchers can first catch a striking predawn pairing of the crescent moon and Jupiter, then — where conditions and location allow — attempt the much harder daytime occultation, when the moon hides the planet from view.
Before sunrise: Find the moon and Jupiter
Set your alarm for 5 a.m. local time on Tuesday and look low toward the east-northeast sky. Check in advance for tall obstructions that could block your view of this delicate predawn tableau. A slender waning crescent moon — just under 10% illuminated and resembling a “celestial smile” — will be immediately apparent. About 5 degrees below it, roughly half the width of your clenched fist held at arm’s length, will be brilliant Jupiter.
Jupiter rises in the east-northeast shortly before 4 a.m. local daylight time, but it does not climb fairly high until just before dawn’s first light. The planet is in the dim zodiacal constellation Cancer; one of my astronomy mentors at New York’s Hayden Planetarium, Dr. Ken Franklin, used to call Cancer “the empty space between the Sickle of Leo and the Gemini Twins.” If you set up a telescope before dawn, you can be among the first to see what became of Jupiter’s tumultuous cloud belts while the planet was hidden in the sun’s glare from early July through mid-August.
Later in the day: The daytime occultation
Another moon-Jupiter show awaits later in the day: a daytime occultation, when the thin waning crescent moon will hide Jupiter from much of North America. Unfortunately, a swath of the western United States — anywhere west and south of a line running roughly from Bellingham, Washington, to Rockport, Texas — falls outside the occultation’s visibility zone.
The occultation occurs during the afternoon, when the moon and Jupiter will appear somewhat lower than the sun and well off to its right. Jupiter will disappear behind the moon’s sunlit limb, with the moon taking roughly a minute or more to inch across the giant planet’s face. Approximately 50 minutes later (give or take ten minutes), Jupiter will gradually reappear from behind the moon’s dark edge.
Jupiter will vanish (going from north to south) between 2:55 and 3:30 p.m. for those in the Eastern Time Zone, 1:45 to 2:20 p.m. for the Central Time Zone, and 12:30 to 1:00 p.m. in the Mountain Time Zone.
To find out when these events occur at your location, use the accompanying map. Interpolate between the ten selected cities to find the local daylight time of Jupiter’s disappearance (top) and reappearance (bottom) for your site.
How to find them in daylight
The chief challenge will be finding the moon and Jupiter at all, so give yourself plenty of time. The easiest method is to locate the pair before sunrise and track them into daylight. Much will depend on the transparency of your local sky — how deep its blue color is. If the day is hazy, the search will be harder.
- Start before sunrise, when the crescent moon and Jupiter are easiest to identify.
- As daylight strengthens, keep tracking the pair or return to the same region of sky about 30 degrees — roughly four fist-widths at arm’s length—from the sun, mostly to the right and, for most locations, somewhat lower.
- Sweep the area with a finderscope or binoculars, remembering that the crescent moon’s low surface brightness may make it hard to distinguish from the blue sky.
Once you have the moon, the next challenge is Jupiter. In a telescope, it will appear far smaller than the moon — about one-sixtieth of the moon’s diameter — and it will have an even lower surface brightness. Watch for Jupiter to return along the moon’s dark limb, a captivating sight as the planet seems to emerge literally from out of the blue.
If you miss it
Sighting the moon and Jupiter in daytime will be challenging, so do not despair if you fail to find them. In less than a month, on the morning of Tuesday, Oct. 6, the moon and Jupiter will have another interaction, but this time about eight hours earlier, before sunrise in a dark or twilight sky. That resultant event should be a spectacular sight, easily visible with the naked eye, binoculars, or a telescope. Space.com will provide details as the date approaches, so stay tuned!
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.
