Watch the Moon Meet the Seven Sisters of the Pleiades This Weekend

On Sunday morning, September 22, skygazers across much of the western United States will be able to watch a waning gibbous Moon — 75 percent illuminated by the Sun — pass in front of perhaps the most notable and popular of all the star clusters in the sky: the Pleiades.

The Pleiades cluster (also known as Messier 45 or M45) can be found low in the east-northeast sky this week around 10pm local time. At 4:30am they are literally soaring almost overhead, high in the south.

Sunday’s event, known to astronomers as an occultation, is part of a series of such events in which the Moon interacts with this famous group of stars that will continue over the next few years. We’ll talk more about this upcoming event a bit later, but first let’s talk about the Pleiades themselves.

Legendary Star Pattern

There are few stars as famous as the Pleiades, also known as the Seven Sisters. Anyone who has difficulty recognizing different stars and constellations should start with the Pleiades, because there is nothing like it in the sky, and no one can look at the sky for long on a winter night without noticing them and wondering what they are.

They are located in the zodiacal constellation of Taurus and mark the shoulder of the bull.

“The Bull”, plate 17 in “Urania’s Mirror”, a set of cards with celestial images, accompanying “A Well-Known Treatise on Astronomy”, Sidney Hall and Richard Rouse, London, 1824. (Photo credit: History/Universal Images Group via Getty Images)

To the average eye, this grouping appears at first glance as a twinkling little cloud of light. However, further analysis, aided by good eyesight, will reveal a tight knot of 6 or 7 stars, although some have recorded more under excellent conditions.

Alcyone is the brightest sister. She is accompanied by Maia; Asterope I and II (known as the double star 21 Tauri); Taygeta; Celaeno; and Electra. Finally, there is Merope, a star surrounded by a beautiful cloud of cosmic dust particles that forms a blue reflection nebula. Atlas and Pleione represent the girls’ father and mother.

Eclipse of the Pleiades

As mentioned earlier, the 19-day-old moon will pass through the Pleiades on the morning of September 22. Most of the western United States will be able to see the cluster’s brightest stars pass behind the moon.

During the upcoming occultation, because the Moon is in a waning gibbous phase, the stars will disappear behind the Moon’s bright limb, where a telescope will be needed to see them, and will appear behind the Moon’s dark limb, where binoculars may suffice.

Illustration of the sky just after midnight on September 22, looking east. (Photo source: TheSkyLive.com)

Whether there is an occultation of the moon or not, the moon is always stunning when seen up close. If you want to see, take a closer look at the moon, our guides to the best telescopes and the best binoculars.

And if you want to try taking your own moon photos during this or any other event, we’ve got a guide on how to photograph the moon, as well as lists of the best cameras and lenses for astrophotography.

Where and when to look

Another factor that will be important for residents of the eastern and central United States and Canada is that in those regions the moon will begin to approach the Pleiades after sunrise in the east, while in the central states the morning twilight will be deepening, meaning that spotting the stars in the cluster will become increasingly difficult as the background sky will gradually become brighter, and worse still, both disappearance and reappearance will occur after sunrise.

The situation improves in the Mountain Time Zone, where the occultation begins in a dark sky and ends at twilight, when the sky is still generally dark. In the Pacific Time Zone, the entire occultation takes place in a dark sky from start to finish.

For example: Electra, as seen from near Chicago, will disappear around 5:01 a.m. Central time in the dark sky, but will reappear in mid-twilight at 5:54 a.m. Alcyone will disappear in bright (civil) twilight at 6:08 a.m., but will reappear after sunrise.

For comparison, from Denver, Electra disappears and reappears in the dark sky at 3:43 a.m. MDT and 4:13 a.m., respectively. Alcyone disappears in the dark sky at 4:36 a.m. and reappears in mid-twilight at 5:57 a.m. Finally, Atlas disappears at 5:36 a.m. but reappears in the bright twilight sky at 6:39 a.m. Another Pleiades, Maia, will be occulted but visible only from the southern US.

And that’s it for the bright stars.

To see Universal Times (UT) predictions for many other locations, as well as maps showing the zone of visibility, click on the star name below. Data comes from the International Occultation Timers Association (IOTA).

Alcyone, Atlas, Electra, Have.

Don’t miss our page dedicated to the night sky tonight and our monthly sky calendar for more celestial events!

Joe Rao is an instructor and guest lecturer at New York University Hayden Planetarium. He writes about astronomy for Natural History Magazinethis Farmer’s Almanac and other publications.

Editor’s Note: If you manage to take a great photo of the Moon during its close encounter with the Pleiades and would like to share it with Space.com readers, please send your photo(s), comments, and your name and location to [email protected].