Whenever the moon shines near the Pleiades in December and January, it’s nearly full. And on Thursday night, Jan. 9, the 10-day old moon will actually pass through the Pleiades. Most North American observers will be able to watch as the moon passes in front of more than a few stars in this cluster (called an occultation) within a few hours, amid the dazzling lunar glare.
Typically, an occultation of a specific star is not a “hit-or-miss” proposition; each month the moon will pass in front of that star as seen from various parts of our planet. Such a pattern is called a series and can last a few years or more.
For North America, the current spectacular series began Nov. 16, when the Pleiades disappeared at the bright edge and reappeared from behind the dark edge of a waning gibbous moon. As far as the United States and Canada are concerned, the most favorable passages of the moon across the Pleiades during 2025 will come on Jan. 9, and again on Feb. 6 — specifically favoring the central and western US — and during the morning hours of July 20. There will be other opportunities in 2026 and 2027.
The last series of occultations of the Pleiades took place between 2005 and 2010. Since this cluster lies 4 degrees north of the ecliptic — that great circle on the celestial sphere representing the sun’s apparent path during the year — it can be occulted only when the ascending node of the moon’s orbit is in the Pisces-to-Sagittarius part of the ecliptic.
This situation repeats every 18.6 years, the time required for the ascending node to complete one circuit of the ecliptic. Hence a new “season” of Pleiades occultations began in September 2023 and will continue until July 2029. After 2029 the moon will pass south of the Pleiades until the next series commences in 2042.
When to watch
On Jan. 9, skywatchers can watch the dark limb of the 82%-illuminated waxing gibbous moon creep across this beloved star cluster, popularly known as the “Seven Sisters.” The details vary depending on your location, but out in the western U.S. and Canada the moon will already be among the Pleiades stars soon after the sun has set and the twilight sky darkens.
The viewing circumstances markedly improve as you head farther to the east, where the sky will be decidedly darker; near and along the Atlantic Seaboard the moon will not start encroaching upon the cluster until at least a couple of hours after sunset.
A pair of binoculars will readily show the brightest Pleiad (Alcyone) wink out, though a telescope is preferable for viewing the fainter members of this star cluster disappear. Reappearances of stars will occur on the moon’s bright limb, where the glare of the sunlit lunar landscape will likely hide the stars even in a telescope.
From New York City, observers can see the moon occult Electra (magnitude 3.7), Merope (4.2), and Alcyone at about 7:06 p.m., 7:35 p.m., and 8:18 p.m. EST, respectively. More than an hour later, the dark limb of the moon will reach the other side of the cluster and will cover Pleione (5.0) at 9:21 and Atlas (3.6) at 9:33. From New York, the moon will miss two of the seven brightest Pleiades stars, Maia (3.9) and Taygeta (4.3). Soon after 10 p.m. the moon will have moved beyond the cluster.
Specific times and zones of visibility
Courtesy of the International Occultation Timers Association (IOTA), webpages are available that provide timetables for hundreds of locations for four stars in the cluster: Alcyone, Atlas, Electra and Maia.
Each page provides times for the disappearance and reappearance of the star in question. The times are given in Universal Time (UT) which is the same as Greenwich Mean Time (GMT). Since the occultation occurs after midnight UT, the calendar date is Jan. 10. When converting to your local time zone, the times will be before local midnight on the previous date (Jan. 9).
As an example: For the star Atlas, as seen from Chicago, Illinois, this star will disappear at 1:53:25 UT on Jan. 10. Chicago is in the Central Time Zone and is six hours behind Universal Time. So, for the Windy City, Atlas will disappear at 7:53 p.m. CST on Jan. 9. Atlas is predicted to reappear at 2:51:59 UT, which corresponds to 8:51 p.m. CST.
In addition to the timetable, a world map (Mercator projection) is provided, showing the region where the occultation will be visible. The boundaries are in different colors. The Cyan boundaries show the curves of the occultation disappearance or reappearance at moonrise or moonset. A continuous white line marks the nighttime northern and southern limits of the occultation. A continuous blue line denotes the occultation limits occurring during twilight, while a dotted red line depicts the occultation limits occurring in daylight.
For Alcyone, the occultation takes place over much of the US and western Europe. For Atlas, visibility occurs over Canada, western Europe and most of the US, except for the Southeast states. For Electra and Maia, visibility will be confined to the southeast US, Central America, the Caribbean and west Africa.
Of course, many other (fainter) stars will also be occulted during the moon’s 2-to-3-hour passage through the cluster.
Final thoughts
As reported here, this entire spectacle might not sound as arresting as it is in nature, but as a particular star creeps near the moon (in actuality, it is the moon that appears to be drifting eastward in relation to the stars) suddenly, as it reaches the unlighted and unseen part of our nearest neighbor in space, it disappears as though a switch were turned off.
Or as Senior Editor of Sky & Telescope magazine, Alan MacRobert once noted: “Watching the moon cover or uncover bright stars gives a visceral sense of the moon’s orbital motion. Try it!”
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.
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