Once-in-a-Lifetime Comet Is Visible Now in Pre-Dawn Sky

Engraving of the Great Comet of 1577 over Prague.  (Jiri Daschitzky)

If you thought 2020’s surprises were over, hold on. There’s one more winging toward us before we can finally close — and shred — this year’s calendar. But this one is a welcome surprise: a comet you may be able to see with your own eyes.

How to See the Comet

Through the end of November, comet “C/2020 S3,” or Erasmus, can be found low in the sky over the southeastern horizon in the hour or so before dawn. Though it is not very bright, and you may need a pair of binoculars to see it, you can use the bright nearby planet Venus as a guide.

By about 5:30 a.m., Venus and Erasmus will have risen high enough above the horizon to spot, assuming there are no obstructions such as trees or buildings to your east. If you can see Venus above the skyline, then you have a shot at the comet. And Venus is hard to miss; it’s the brightest thing in the sky at this time — in fact, only the sun and moon are brighter than Venus.

The positions of Venus, the bright star Spica (up and right from Venus), and Comet Erasmus (red crosshair) around Nov. 23. Over the days to follow, the comet will gradually move downward and to the left as it heads toward its close encounter with the sun on Dec. 12. (Stellarium)

Facing southeast, start with Venus, and then look about 15 degrees to its right — about the width of your hand spread fully open, from thumb to pinky tip. That’s about where comet Erasmus will be.


If you don’t see it with your eyes — light pollution or haze can make this difficult — and if you have binoculars, try scanning the area with them.

Under good dark conditions, you should be able to spot a faint smudge of light like a cotton ball against the night sky. If you still can’t see it, relax your eyes a bit, and try averting your gaze to either side. Your eyes are more sensitive to faint light when you look at something off center, indirectly.

Twilight will seep in as it gets closer to 6 a.m., and eventually the comet will become lost in the glow. With each passing night, as the comet approaches the sun, it will sink lower on the horizon, rising later and eventually departing the night sky completely.

Once In 2,500 Years

C/2020 S3 Erasmus was discovered on Sept. 17 by Nicolas Erasmus, from an observatory on Mauna Loa, Hawaii. Since then it has been traveling closer to the sun, and will reach its nearest point, called perihelion, on Dec. 12, just inside the orbit of Mercury. We won’t see it then, since it will be on the far side of the sun and completely lost in the glare of daytime.

Diagram shows the orbit of comet C/2020 S3 Erasmus as it makes its closest approach to the sun on Dec. 12, after passing a comfortable distance away from Earth. (NASA/JPL)

Erasmus likely originated in the distant “halo” of dust, ice, and cometary bodies called the Oort Cloud, which surrounds our solar system and extends halfway to the nearest star.

Estimates of Erasmus’ orbital period vary, but NASA’s Jet Propulsion Laboratory Solar System Dynamics system pegs it at 2,512 years, plus or minus 105 years. The uncertainty is not uncommon for long-period Oort Cloud comets with highly eccentric orbits — in any case, the chance of witnessing Erasmus’ passage is literally a once-in-a-lifetime opportunity.

Another Disaster for 2020

In ancient times, comets were considered bad omens foretelling cataclysmic events, the deaths of rulers, or other calamities. The  word “disaster” comes from "dis," a pejorative, and "astro," star, or “bad star.”

A painting of the Great Comet of 1680, at a time when comets were still seen as bad omens and portents of disaster. (Lieve Verschuier)

Were a comet to collide with Earth — and they have in the past — it would certainly be a disaster, so maybe the ancients were onto something. Fortunately, Erasmus will not come close to Earth, and after December will be heading away, not to return for more than two thousand years.

So, relax.