October comet Tsuchinshan-ATLAS: Last chance to see it for 80,000 years

A celestial spectacle not seen since the Stone Age graces our night sky and offers stargazers a unique opportunity. Comet Tsuchinshan-ATLAS, also known as C/2023 A3 or simply Comet A3, has proven to be one of the most impressive comets visible to the naked eye this year.

When the sun disappears below the horizon on Tuesday evening, October 15th, sky enthusiasts have something to look forward to. It is the second of two premier viewing opportunities for Northern Hemisphere observers to experience this cosmic visitor at its brightest.

How long will the comet be visible?

Comet A3 will technically be visible until the end of October, but its brightness will decrease each day the further it gets from Earth. So if you go out on Tuesday night, you'll have the best chance of watching it.

The comet made its closest approach to Earth on Saturday, at a dizzying distance of 44 million miles. However, it was difficult to detect as it remained close to the sun.

Its perihelion – the closest point to the Sun – occurred on September 27, according to EarthSky. After comets orbit the sun, they are often brighter and have longer tails because the sun's heat vaporizes ice from the surface and ejects it into space.

How fast is the comet moving?

Comets move quickly. Really fast. According to NASA's Jet Propulsion Laboratory, Comet A3's maximum orbital speed is a whopping 150,000 miles per hour.

The comet reached its maximum speed when it was closest to the sun, as our star exerts the greatest attraction on the dirty snowball.

As it moves away from the Sun, the comet slows down slightly, but is still moving at considerable speed. Nevertheless, the orbital period of Comet A3 is 80,000 years. That's because it has to travel an awfully long way – to the Oort cloud, which is at least 2,000 astronomical units (AU) from the sun. For comparison, one AU is approximately 93 million miles.

Where to look for the comet tonight

To catch a glimpse of our heavenly guest, you'll need to look west at your location about 45 minutes after sunset. The comet will be about 46 million miles from Earth and should be visible just above the western horizon.

The comet will set about an hour and 40 minutes after it is first seen.

Because the moon is nearly full on Tuesday, the sky will be slightly brighter, which may make it difficult to see the comet. However, the moon should shine in the southern part of the sky so that it does not interfere too much with the visual spectacle.

To see the comet you need a clear sky. So check your local weather forecast to see if you have a chance.

Here's how to see Comet A3 tonight

According to Forbes, the comet will have a magnitude of +0.9 tonight, meaning it will be visible to the naked eye. To illustrate, the North Star has a magnitude of about 2, meaning the comet is more than twice as bright as the famous cardinal direction.

To get the best view, use binoculars or, if you're lucky enough to have one, a telescope.

Finding high ground will help the comet stay above the horizon longer, and it also helps to move to a dark place away from the light pollution of city lights.

What is a comet?

Comets are icy remnants from the early days of our solar system, made up of dust, rock and frozen gases. While comets are often imagined as fiery objects streaking across the sky, they only develop their iconic glowing tails as they approach the sun.

As they approach the sun, the heat causes their ice to vaporize, releasing gas and dust that form a glowing coma and a long tail that can stretch millions of kilometers into space.

In their original state, before heating, comets can range in size from just a few kilometers to dozens of kilometers. But once their materials begin to sublimate and expand near the Sun, they can appear larger than most planets.

NASA estimates billions of comets orbit in the Kuiper Belt, a region of icy objects beyond Neptune, and even further out in the Oort Cloud, home to the most distant and oldest objects like Comet A3.

New comets are discovered every year. However, many either burn up when they get too close to the sun or remain too far away to be seen without special telescopes.

Well-known comets from recent history include Neowise in 2020 and Hale-Bopp and Hyakutake in the 1990s. In 2023, a rare green comet that had not visited Earth in 50,000 years passed close to our planet, providing skywatchers with a breathtaking display.

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