A sunspot four times the size of Earth is lined up with the Sun right now and it's so big that an astronomer said it could be seen with the naked eye – provided you've got the right equipment to observe it safely.

Astronomer Bum-Suk Yeom from South Korea advised amateur astronomers to reach for eclipse glasses that have lenses that block out 100 percent of the Sun's UV and infrared rays to protect the eyes.

"A giant sunspot is crossing the Sun's disk, and I could see it clearly with solar glasses," said Yeom, per spaceweather.com, adding: "Caution! You must use eclipse glasses or solar filters to protect your eyes."

Sunspot circled in red, with arrow pointing to it.
A picture of the Sun on May 23. A sunspot is passing on the Sun's surface, so big that it can be seen with the naked eye, provided you've got the right equipment. (SDO/NASA)

Don't stare straight at the Sun

Observers in New Jersey, New York, Pennsylvania, and Nebraska were able to snap pictures of the sunspots without any glasses on Monday, as the smoke from nearby wildfires provided a natural filter for the setting Sun, per spaceweather.com

But looking straight at the Sun without appropriate eyewear is extremely dangerous as the UV light from the Sun can burn your retinas and leave permanent damage.

Sun enthusiasts can buy solar glasses online, though make sure to source these through an official website like the American Astronomical Society's list of suppliers of safe solar filters and viewers to make sure the glasses have been appropriately tested.

The Sun is gearing up to a peak of activity

Experts have been keeping a watchful eye on this particular sunspot, called AR3310, while it is facing Earth.

Sunspots are areas where the Sun's magnetic fields can be particularly active. As this one rolled around the side of the Sun, it let off a substantial solar flare, a giant explosion that sends energy, light, and high-speed particles into space.

That flare was registered as a high-level M-flare, the second highest level on the scale that classes solar flares by strength.

The National Oceanic and Atmospheric Administration, which monitors space weather, has said that there's a 20 percent chance that the spot could let off a powerful X-class flare while it's still facing Earth, per spaceweather.com.

A flare of this size could trigger radio blackouts – which impact aviation – and long-lasting radiation storms.

These radiation storms can bring beautiful aurora, but they can also affect power grids and pipelines if the flare is pointed straight at us.

This is the latest in a series of remarkable sights seen on the Sun in the past months. The Sun is currently gearing up to peak activity in its 11-year cycle, during which sunspots, like this one, are more likely to appear.

Graph showing fluctuations in the number of sunspots over time.
A chart shows the number of sunspots on the Sun against years. The Sun is reaching a peak of activity that happens about every decade, that's prompting it to display more sunspots. (NOAA)

Over the past months, we've seen auroras visible all the way down to New Mexico, solar plasma waterfalls, solar tornados, powerful coronal mass ejections, and geomagnetic storms. More activity is likely to come in the coming months.

This article was originally published by Business Insider.

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