On July 20, The US And Canada Will Witness The Little-Known Seven Sisters Eclipse

People who like to gawp at celestial events (as we all should) are in for a real treat at the moment. As well as the best meteor shower of the year – courtesy of Comet 109P/Swift-Tuttle – there is the upcoming lunar eclipse, visible to around 60 percent of the world's population.

But the Moon offers up some tasty treats in between these bigger events, with regular appearances of the Lunar X, and who could forget the sinister Eyes of Clavius? One such treat, visible this month, is the "Seven Sisters Eclipse".

On July 20, for viewers in the US and Canada, the waning crescent Moon will pass in front of the Pleiades, eclipsing them from view.

"Commonly called the Pleiades or Seven Sisters, M45 is known as an open star cluster. It contains over a thousand stars that are loosely bound by gravity, but it is visually dominated by a handful of its brightest members," NASA explains of the group of stars.

"The Pleiades cluster has been observed since ancient times, so it has no known discoverer. However, Galileo Galilei, the Italian scientist best known for discovering the largest moons of Jupiter and championing a heliocentric model of the solar system, was the first to observe the Pleiades through a telescope. M45 is located an estimated distance of 445 light-years from Earth in the constellation Taurus, though this number is not universally agreed upon. It has an apparent magnitude of 1.6 and can be seen with the naked eye."

Though only six stars are bright enough to be visible to the naked eye, the cluster is referred to as "seven sisters", with myths around the world consistently referring to one of the "sisters" having gone missing. Previous investigations have suggested that in the ancient past, a seventh star may have been visible, but it is now too close to another in the sky to be distinguished. Given how far back that star would have been visible, astronomers have even suggested the myth of the Seven Sisters may be the oldest surviving story in human history.

This particular eclipse is actually seen pretty regularly. In fact, it has been visible every month since September 2023 and will continue to be so until July 2029, according to timeanddate.com.

"The Moon changes its position in the sky relative to the background stars, completing one orbit every 27.3 days, so the two will next pass each other in the sky on [August 16]," Anna Gammon-Ross, astronomer at the Royal Greenwich Observatory, explained to Discover Magazine.

"However, the Moon’s altitude in the sky varies as it orbits, so sometimes the Moon will pass above the Pleiades, sometimes below it, and sometimes, like in the case of [July 20], it will pass right in front of it."

The Seven Sisters eclipse will be visible (or invisible) with the naked eye. Or in other words, you will still be able to see the Moon as it passes in front of the Pleiades in the early hours of Sunday morning. Look towards the horizon and identify the Seven Sisters, and watch as the Moon makes them, temporarily, disappear from view.