How to see Comet A3, which could be the best comet in a decade. Neanderthals walked the Earth last time it flew by.

• Comet A3, or Comet Tsuchinshan-ATLAS, is visible in the Northern Hemisphere this October.
• The comet takes 80,000 years to orbit the sun, so Neanderthals were among the last people to see it.
• It could be the brightest comet in a decade. Here's how, where, and when to see it.

A rare comet is gleaming in our skies this October, it's your only chance to ever see it, and it might be visible to the naked eye.

Comet Tsuchinshan-ATLAS, or Comet A3, is a dusty ball of ice from the Oort Cloud that takes about 80,000 years to orbit the sun. That means some of the last people to see it were Neanderthals.

This is the best opportunity in years, and possibly in a decade, to see a bright comet in the sky. Experts are hopeful that Comet A3 will be "highly memorable" and "a real crowd pleaser," according to Dan Bartlett, an astrophotographer who tracks comets.

But you need to be prepared.

"It's just hard to make the time to be in the right place, to have the right experience," Teddy Kareta, a planetary scientist who specializes in comets at the Lowell Observatory, told Business Insider.

When Kareta has talked to people who have seen a truly great comet, though, "there's just nothing like it," he said.

Here's everything you should know to be in the right place at the right time and spot the comet.

How bright is Comet A3?

Nobody is certain about how bright this comet will be, but Kareta says it has the potential to be the brightest comet in a decade.

The last big, bold comet was NEOWISE in 2020. It was visible to the naked eye and produced stunning photos like this:

Astronomers are divided about whether Comet A3 will shine as bright as Comet NEOWISE.

In recent weeks, A3 has at times been brighter than predicted and at other times matched predictions, according to the BBC's Sky at Night Magazine and Robert Massey, the deputy executive director of the Royal Astronomical Society.

From a dark-sky vantage point at Mono Lake, California — tucked into the Sierra Nevadas — Bartlett says he can see the comet with binoculars already.

But comets can change quickly, in a matter of hours. That's because their brightness depends on what type and amount of gas or dust they're emitting, and that can change on a dime.

For example, in 2007 Comet Holmes suddenly flared up and became 500,000 times brighter in just two days because a pocket of gas inside it had breached its surface, according to Space.com.

"Sometimes people will say that predicting comets is kind of like playing the lottery," Kareta said.

When and where to see Comet A3

Weather permitting, everyone in the Northern Hemisphere should be able to see Comet A3 in October.

There are two windows to catch it, one from now through October 2 and another from October 12 to 30, according to a press release from Massey.

During the first window, the comet appears above the eastern horizon during a 15-minute window that begins one hour before sunrise, according to Space.com.

However, the best opportunity to see the comet, when it's closest to Earth, is during the second window. After sunset, it will shine in the west, just to the left of the Big Dipper's handle, making for a more schedule-friendly viewing time.

Where to go and what to bring

For the best viewing, you should drive far from the city and gaze into a very dark sky. Check Business Insider's list of dark-sky spots in each state.

If that's not an option, though, find a relatively dark spot outside the worst of the city lights — maybe on a hill or just outside the city — where you can face away from any major light sources to look toward Comet A3.

"Even if you live in an area with lots and lots of street lights, lots of smog, lots of smoke, lots of whatever, you still ought to go for it," Kareta said.

Check the weather before you go. If it will be cloudy all night, for example, you may want to plan for another evening. Bring warm layers and a chair or blanket to lay back on. Without support, you might be surprised how quickly your neck gets tired of craning back to look at the stars.

If you have binoculars or a telescope, bring them. There's no guarantee of naked-eye visibility. In some cities, you can check out binoculars or a telescope from your local library.

Especially if you're a beginner, you may want to download a skywatching app like Sky Safari ($4.99 in the App Store) or pull up a website like Stellarium (free) on your phone. Once you're on the ground, that will help you pinpoint where the comet is in the sky.

How to look for the comet

Finding a comet in the sky is harder than you might think. You'll need to know a few tips and techniques.

Once you reach your comet-watching spot, it can help to open your skywatching app to identify exactly where the comet is in the sky, near a constellation you recognize — like the Big Dipper.

Then give your eyes at least 30 minutes to adjust to the dark. Try not to look at your phone. Even a small burst of blue light is enough to reset your eyes.

If the comet doesn't immediately jump out to you, try scanning around the area. Your peripheral vision is more sensitive to faint objects, so you might spot it in the corner of your eye.

You're looking for a smudge, slightly bigger than a star. It won't twinkle like the stars do, and it will look a bit blurrier, with less defined borders. It might appear elongated because of the comet's tail, which is dust and gas flowing off its icy body.

"If you look at it and it looks like it's behaving differently than some of the stars nearby, that's a really good sign that you've picked up on the right thing," Kareta said.

If you're still not sure, try taking a long-exposure photo with your phone. This is built into most phone cameras' nighttime settings. (Hint: Turn your brightness down to protect your eyes.)

The long exposure will provide a clearer image of the comet than your eyes can.

If you can't find Comet A3 at all, Kareta recommends trying again a few days later.

"Go one night out, and then take a night off, and go back out. You'll know better how to time it, how to get ready," he said.

How to photograph the comet

If your phone isn't picking up the comet, a DSLR camera might do the trick. Massey recommends taking a series of exposures.

You can also hold your phone camera up to the lens of your binoculars or telescope to take a picture through it. That method can be tricky, so it helps to practice a bit in advance.

After this, Comet A3 won't return for tens of thousands of years. "See it now or lose your chance," Kareta said.