For centuries, Horsetail Falls trickled the remnants of snowpack down the majestic El Capitan cliff in Yosemite National Park in obscurity. It remained a secret among the region's first residents who comprised the aboriginal nation of Ahwahnechee, while subsequent explorers never let on about a secret surrounding the mini-waterfall.

But when outdoor photographers and, more recently, digital shutter-clickers and social media mavens stumbled upon Horsetail Fall, word got out about one seemingly supernatural element of the once-clandestine attraction.

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A few times a year, Horsetail Fall glows. Its luminescence, always seen before sunset during the last two weeks in February, is usually orange, but on a good night, it can turn into a raging crimson, as if the Yosemite gods are telling its visitors to beware.

Fortunately, there's been no account of any tourists turning to stone at the sight of Horsetail Fall in its colorful finery, but among more curious bystanders, there exists one perplexing question: Why does it glow?

It's like Dante's Inferno at dusk

Most of the 2,000 people who visited 1,500-foot-high Horsetail Fall in 2019 already know the answer. They're the ones who make reservations at all the hotels in Yosemite well in advance for another glimpse of that mysterious, fiery glow, which resembles lava spewing out of the side of a deformed volcano.

This year, park officials promise that if everything is just right, the fiery effect could be bigger and brighter than ever. Since Horsetail Fall isn't connected to any river system, it exists entirely on snowpack melt that pours down the cliff during the winter and spring. Because there's been more snow than usual, the waterfall should be much bigger to create a more dazzling show.

OK, kids! Whip out them protractors!

As for how what causes the effect, there's nothing mystical about it. The waterfall is simply a reflection of the sun's rays beaming directly at the waterfall. How it happens, however, requires a bit more explanation.

Because the earth rotates on its axis while revolving in a solar orbit, the sun's position in the sky changes throughout the year. In February, it's low enough on the horizon to shine directly onto El Capitan. Taking into account the sun's position and the steep angle of the cliff, the only way for that effect to take place is if the sun's rays make contact with the waterfall at just the right angle. It's like a gigantic projector beaming directly at a movie screen from 93 million miles away.

And since water is a natural reflector of light, the glow is the result of the sun's rays bouncing off the waterfall. But there's more. A smooth water surface will give you more of a static and motionless glow. However, since the waterfall surface is a lot rougher, it distorts the reflection, creating the illusion of lava cascading down the cliff.

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But conditions have to be just right

If you're expecting a marathon light show, you'd be sadly disappointed. The effect lasts only about 10 minutes each of the dozen or so evenings in February it's slated to perform that weird light display. Because the sun is constantly moving, it doesn't take long for it to be out of position, thus ending the effect for the night. And since Horsetail Fall runs out of water in the summer, there's no way for the effect to take place when the sun is in that same spot on the horizon in October.

Factors that could hamper the effect include cloud cover, haze, snow and other weather conditions capable of obscuring the sun. People tend to make reservations for up to two weeks to catch the firefall take the changing weather into account. Weather in the mountains is always unpredictable, meaning there's always a chance that clouds one day might result in sunshine the next.

One thing that is predictable is when the firefall starts each day. If you're on a quest to catch Horsetail Falls in its glowing glory, make sure to get yourself a good spot before 5:40 p.m. local time, just when the sun is about to set.

Man, but you gotta lava that illusion

Because of increasing interest in the firefall of late Yosemite park officials have taken measures to control traffic flow and prevent any damage to the environment caused by trampling visitors and the accumulation of litter. According to the Natural Yosemite Firefall Guide, the best way to catch the phenomenon is by heading to the Yosemite Lodge, which will have a parking area available, although a shuttle service can also take you there.

From that point, you'll have to hike a mile and a half to the El Capitan picnic area, which is about as close you'll get to the fall. You'll have to take into account traveling in deep snow and it's highly recommended to dress warmly. As some of Horsetail Fall's hardier fans will tell you, it's worth the effort to catch one of the most mysterious natural light shows in the world.

Next: Horsetail Falls Opens This Weekend In Yosemite National Park And Here’s What You Should Know