Watch the Moon cover the Pleiades on Sunday morning

The Moon occulted the Pleiades (M45) on March 20, 2010. This photographer captured the event from San Antonio, Texas. Credit: Ernest D. Adams

Because the Moon is closer to Earth than the planets and stars, it sometimes passes in front of or occults objects that are more distant from our view. One such event will occur early on the morning of September 22: Around 5 a.m. EDT (2 a.m. PDT), you'll find a nearly last quarter Moon near the Pleiades star cluster (M45). Over the next two hours, many stars in the Pleiades will be obscured by the Moon in a series of occultations.

Pleiades occultations are always a pleasure to observe because there are several bright stars, as well as many 7th and 8th magnitude stars, that are occulted during the event. As the Moon slowly moves eastward, its dark edge approaches each star, and suddenly the star disappears from view. Seeing nearly a dozen such events in a short period is exciting.

How to watch the event

You can observe some of the disappearances with your eyes if your vision is sharp enough, but binoculars or a telescope provide better views. You'll see the orb of the Moon nestled among the stars in M45 as your chosen optics reveal each one fading. Twilight interferes with later events for the Eastern time zone, but observers farther west will witness the full sequence of occultations.

M45 spans nearly 1° from Electra to Atlas (17 and 27 Tauri, respectively), and because the Moon moves across its own width (about ½°) every hour, most of the stars between Electra and Alcyone (Eta (η) Tau) occult within the first hour. The two bright stars Atlas and Pleione (28 Tau) occult 40 minutes after Alcyone, extending the entire sequence to nearly 2 hours.

Occultations are best observed through a telescope, which, at high magnification, allows you to see the faint dark edge of the Moon approaching each star, while much of the bright Moon may be out of view. Some telescopes will spot nearly a dozen magnitude 7 stars also occulting, making for a lively evening.

Pleiades occultations do not occur every month, or even every year, because the Moon's orbit is inclined relative to the plane of the Earth's orbit around the Sun, called the ecliptic. Instead, the Moon moves between 6° above and 6° below the plane of the ecliptic over a cycle of approximately 18 years.

The Pleiades are located 4° north of the ecliptic and are currently occurring monthly in a series that began on September 5, 2023, and will continue through July 7, 2029. However, many of these events occur during the day across the United States. Overall, they are quite rare, and that makes this month's event extra special. The last series of Pleiades occultations took place in 1986, 1992, and 2010.

When to see the stars disappear

One factor that particularly complicates predictions of these events is that they depend on geographic location. The Moon's edge is curved, and the Moon is also relatively close to us. Observing from different latitudes means that a different part of that edge will cover a star. So an observer at mid-latitudes might see the Moon's leading edge bulge covering a star at a specific time, while someone farther north might see the same event occur later because the Moon's edge curves away from us and causes the occultation to occur later at higher latitudes.

But don't worry, there is help available. The International Occultation Timing Association (IOTA) has a website that shows the exact times of bright star occultations in many cities in the US and around the world. We've put together a short list of them in the table below. You can access the full list of cities here here — simply scroll down to the correct date and choose the star you want to focus on. Please note that times are given in universal time. (The main IOTA site is located here.)

For most observing sites in the U.S., Electra will be the first star to set. It is the westernmost star and shines at magnitude 3.7. Fifth-magnitude Celaeno (16 Tau) will set within a few minutes of its brighter neighbor. Nearly 30 minutes later (again, timing varies by local latitude) magnitude 4.3 Taygeta will appear, followed a few minutes later by magnitude 3.8 Maia (20 Tau).

The southern edge of the Moon will not reach Merope unless you are north of 38° north latitude in the Rocky Mountain Time Zone. In a narrow region, Merope will skim the southern edge, being hidden by mountain peaks and appearing in valleys. A grazing occultation is visible from an area typically about two miles (3.2 kilometers) wide, depending on lunar topography. The times can then be used to determine details of the lunar profile and give an accurate position of the Moon relative to the star.

Similarly, in some cities north of Chicago, Denver, and Los Angeles, Electra is experiencing a grazing concealment. If your city is close to the grazing area, the timing of the disappearance can vary significantly, so you should be prepared in advance.

Alcyone, the brightest Pleiad at magnitude 2.8, will set about 45 minutes after Maia. Last but not least, Atlas (magnitude 3.6) and Pleione (magnitude 5.0) will disappear about 45 minutes later. In parts of the southwestern United States, Atlas will not be visible to the Moon.

Hidden stars reappear behind the bright Sunlit edge of the Moon up to about an hour after they disappear. Reappearances at the bright edge are much harder to see.

Occultation times for various cities