Why 2023 Is A Big Year For The Only Naked Eye Comet That Can Appear Twice In One Human Lifetime

Have you seen the “green comet” yet? If not, you had better get your skates on because this weekend presents the last good opportunity to easily see comet C/2022 E3 (ZTF) in a dark moonless sky from the northern hemisphere (though do use binoculars and don’t look directly at it!).

After all, comet C/2022 E3 (ZTF) won’t be back for about 55,000 years. It’s what’s called a long-period comet. It comes from a distant region around the edge of the solar system called the Oort cloud, where it hangs out with billions of other relics left over from the formation of our solar system.

If you’re struggling to remember the name of the “green comet,” you’re not alone. Comets are cataloged when they’re first spotted, typically taking the name of the telescope or observatory they were spotted with. However, there is one comet that’s extremely well-known—and for which 2023 is a significant year.

Halley’s comet—officially known as 1P/Halley—is a monster. With a peanut-shaped nucleus measuring a whopping 9×5 miles/15×8 kilometers wide, this ball of frozen water and gases and dust was last seen in the solar system in 1986. It’s due back in 2161 when it’s predicted to make a relatively close pass to Earth and be as bright as the brightest stars in the night sky. Cue easy, naked eye, observing that will put the elusive “green comet” to shame.

1P/Halley’s 75-year orbit makes it a short-period comet, but it also puts it in a class of one. It’s the only comet ever observed that can be seen twice in one human lifetime. Observations of it date back to 467 AD, though the first person to work that out it was the same comet revisiting was English astronomer Edmund Halley. In 1705 he calculated that his comet would return in 1758. He was right, though by then he’d died.

2023 is the year that Halley’s comet reaches its aphelion—it’s farthest point from the Sun—before making its return journey to loop around our star. Now in the southern hemisphere constellation of Hydra it will reach aphelion at 35.1 au (beyond the distance of the orbit of Neptune and closer to Pluto) in December 2023.

Just over 37 years later it will wow another generation of comet-watchers.

If you can’t wait that long for the return of Halley’s comet, there is an indirect way of momentarily glimpsing its grandeur.

In April/May and October each year Earth moves through streams of particles left in the inner solar system by Halley’s comet from that last pass in 1986.

The Eta Aquariids is a meteor shower that last from April 19 to about May 28 and peaks on May 5/6, 2023. It produces about 10-30 “shooting stars” each hour as seen from the northern hemisphere and up to 60 from the southern hemisphere. Sadly, this year, the Eta Aquariids peak during a full Moon, which will make it harder to see these bits of Halley’s comet collide with Earth’s atmosphere.

Thankfully another chance comes from October 2 through November 7 with the Orionids, which peak on October 20/21, 2023 when observers can expect to see about 20 “shooting stars” per hour after midnight. The Moon will have set by midnight, which is when the show gets going.

Wishing you clear skies and wide eyes.

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