How to see the rare green comet now swinging past Earth

How to see the rare green comet now swinging past Earth
How to see the rare green comet now swinging past Earth

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If your weather forecast includes some clear nights over the next few weeks, find a dark place to observe the sky and turn your gaze towards the northern constellations. A newfound comet is passing by Earth, and it could become bright enough to be seen with the unaided eye.

Around 50,000 years ago, when Earth was warming up from the last ice age, Neanderthals and early Homo Sapiens may have gazed up into the night and marvelled as a bright comet tracked across the sky.

Fast-forward to March 2, 2022, when astronomers Bryce Bolin and Frank Masci spotted this very same comet while surveying the sky with the Zwicky Transient Facility (ZTF) at Palomar Observatory in California. Now named C/2022 E3 (ZTF), this comet apparently travelled out to the edge of our solar system and back since its last visit.

Between now and early February, C/2022 E3 (ZTF) will first make its closest pass around the Sun on January 12. Then it will swing past Earth for a relatively close encounter on February 1.


The path of comet C/2022 E3 (ZTF) past Earth, with its closest approach on Feb 1, 2023. Credit: NASA/JPL-Caltech

At that time, it will come as close as 42 million kilometres from us, or over 100 times the distance to the Moon.

Even that far away, the comet will be bright enough to be visible using binoculars or a backyard telescope. It's also possible that, for roughly 2 weeks, it could be visible to the unaided eye.

Watch below: Andreas Gada shares his views of Comet ZTF from Central Ontario

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How to see Comet C/2022 E3 (ZTF)

Locating this comet in the night sky is relatively simple. Based on its path around the Sun, astronomers have already mapped out its exact path as it moves through the constellations, night by night.

To start, look to the northeast in the hours after midnight, local time. The comet will be found in the space between the constellations Bootes, Corona Borealis, and Hercules. Night by night, towards the end of January, the comet will rise earlier and climb higher in the sky, eventually passing the Little Dipper. Then, into early February, it will continue on this path into the northwest sky.


This sky chart shows the position of comet C/2022 E3 (ZTF) in relation to the constellations from January 19 to February 10, 2023, at around 10:30 p.m. in each local time zone. The comet's apparent magnitude (in brackets) dips below 6.5 (the limit for viewing with the unaided eye) from Jan 23-Feb 9. Note that the positions of the constellations in the sky will differ slightly night to night, and the Moon may make viewing a bit more challenging for the last few days of January and first few days of February. Credit: Stellarium/Scott Sutherland

While the comet's path and position in the night sky are very well known by this point, its brightness is another matter.

Comets are famously unpredictable. They can suddenly brighten and put on a spectacular display as they pass by. Or, they can just as suddenly fizzle out and dim to the point where only the most powerful telescopes can find them. The predictions shown in the sky chart above are based on observations as of January 19. This prediction may change, so check back for updates.

Anyone with access to a pair of binoculars or a backyard telescope should be able to see C/2022 E3 (ZTF) on clear nights during the comet's entire pass by Earth. The bigger the binoculars or telescope, the longer it will be visible and the more details the observer will pick out.

There is even a period during the flyby when the comet is expected to be visible with the unaided eye — no binoculars or telescope needed!

Astronomers rate the brightness of an object in the night sky using a measurement called apparent magnitude. The brighter an object is, the lower its apparent magnitude is. For something to be seen with the unaided human eye, its apparent magnitude needs to be less than 6.5 (although this will vary some from person to person).

As shown in the above sky chart, the comet's apparent magnitude is expected to be around 6.42 starting on January 23. It should gradually brighten for the next week or so, reaching its brightest at its closest point to Earth, around January 31-February 1. It will then grow dimmer, and be lost from sight (at least for the unaided eye) after February 9.

Note that the visibility of the comet will be affected by any light pollution in your area. Also, the comet is easier to spot when it is higher in the sky, as we have to look through less atmosphere to see it then, as opposed to when it is closer to the horizon.

Read more: Bright 'evening star' Venus dazzles this winter in two planetary conjunctions

C/2022 E3 (ZTF)'s final visit to Earth?

With an estimated 50,000-year orbit, there's very little chance that anyone alive on Earth today will still be around if the comet comes back. However, from all indications, C/2022 E3 (ZTF) won't ever be swinging past Earth again because it is currently on a trajectory to be ejected from our solar system for good.

The dust, asteroids, comets, and even planets in our solar system trace elliptical paths as they orbit around the Sun. The shape of the ellipse any particular object follows is measured by its "eccentricity", which has a value between greater than zero and less than one. The closer it is to zero, the more circular the orbit is, and the closer to one, the more stretched out the ellipse is.

Orbit types
Orbit types

*The shapes of different orbits are shown here, from the bound circular (e=0) and elliptical (0 1) trajectory. Credit: NASA/Robert Simmon/Scott Sutherland

From NASA's database, C/2022 E3 (ZTF) has an eccentricity of roughly 1.00027.

With an eccentricity greater than 1, that means the comet is no longer bound to the Sun. It is following an 'escape trajectory' — one that's either parabolic or hyperbolic — so, once it's gone, it is never coming back.

One way or another, we should all get out to see C/2022 E3 (ZTF) as it swings past. Unfortunately, we're not going to get another chance!

(Thumbnail image courtesy Andreas Gada. The colour levels of the image have been adjusted to make the comet stand out more against the background.)

Watch below: What's that in the sky? How to identify that flash of light you just saw

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