We examine circumstances and expectations for the current apparition of Comet Leonard, which may become the year’s brightest comet.

Comet Leonard Oct 10, 2021
On October 10, 2021, Comet Leonard displayed a compact coma and a 4′ dusty tail pointing north. Now at magnitude 12 and moderately condensed, the comet is bright enough for amateurs to seek before dawn.
Michael Mattiazzo

Predicting a newly discovered comet's brightness is no easy business. Astronomers often use the power-law formula to make visibility forecasts, but an equation can fail to account for a comet's essential unpredictability. Not only do these fragile, icy fossils experience surprise outbursts and disintegrations, but their luster can vary radically depending on something as simple as viewing angle.

Comet Leonard wide-view map
Comet Leonard moves rapidly southeast across the morning sky in early December, disappearing in the solar glow by the 12th. Positions are shown every three days with stars to magnitude 6.
Stellarium with additions by Bob King

With this in mind, let's take a look at the upcoming apparition of Comet Leonard (C/2021 A1). Senior research specialist Greg Leonard at Mt. Lemmon Observatory discovered the 19th-magnitude speck on January 3, 2021, exactly one year before perihelion. Orbital calculations revealed that the object had spent the last 35,000 years wending its way sunward after reaching aphelion at the chilling distance of around 3,500 a.u. Comet Leonard will pass nearest the Sun again on January 3, 2022, at 0.62 a.u. Two weeks before that on December 12th, it makes its closest approach to Earth at 34.9 million kilometers.

Comet Kohoutek
Comet Kohoutek (C/1973 E1) brightened rapidly at a great distance, leading some astronomers to predict it would become a spectacular sight in January 1974. It turned out that the comet was "new," never having passed near the Sun before. Only its volatile surface ices sublimated, which led to a temporary surge that fanned unrealistic expectations.

It's good news that Comet Leonard's been around the block before. That trip and perhaps others in the remote past allowed its most volatile ices to vaporize. First-time comets often become unusually bright even at great distances as fresh ice sublimates in a frenzy. This can artificially inflate their predicted brightness during solar approach and lead to unrealistic expectations. Often, these early outbursts simply fizzle out, and a lackluster apparition follows. Pre-baked as it were, Comet Leonard's brightness predictions may be more reliable.

Last year, Comet NEOWISE (C/2020 F3) blossomed into a magnificent sight, becoming the brightest comet to decorate the sky since McNaught (C/2006 P1) in 2007. This year has seen a dearth of naked-eye objects, the reason so many of us are looking forward to Comet Leonard, which may top out at magnitude 4 or brighter come December.

Comet Leonard detailed map
This map plots Comet Leonard during its slow trek across southern Ursa Major near the 5th-magnitude star 61 Ursae Majoris (61 UMa in the finder map here) nightly through October 29th. Brighter stars are labeled with magnitudes with decimals omitted. Chart details appear at upper left.
Megastar, used with permission

In mid-October, the comet glows around magnitude 12 in southern Ursa Major and stands almost 30° high at the start of morning twilight for observers at mid-northern latitudes. It slowly heads east, crossing into Canes Venatici on November 11th and Coma Berenices later that month, while continuing to brighten. By mid-November the comet may reach magnitude 10, putting it within range of a 6-inch telescope. At month's end it should be easily visible in 50-mm binoculars around 7th magnitude.

Things really heat up in December. Leonard passes about 1.5° west of the bright globular cluster M3 on December 2nd and about 1° to its east on the 3rd, then dashes some 5° north of Arcturus on the 6th. Early in the month it should hover near magnitude 5.5 and become a faint naked-eye object. Having Arcturus nearby will make it easy for less experienced observers to find and follow the comet.

Comet  Leonard on October 10, 2021
In another image taken on October 10, 2021, Comet Leonard glowed at magnitude 12.3 with a 3.8′-diameter coma and 5′ tail pointed in PA 340–330°.
Michael Jäger

Observers may get their last peek at Leonard steeped in morning twilight on December 12th — and near peak brightness — before it transitions into the evening sky. Fortunately, the Moon will be absent throughout the best part of its morning apparition.

From mid-December onward, Comet Leonard fades while remaining stubbornly low in the southwestern sky at dusk for mid-northern observers. Meanwhile, conditions improve for Southern Hemisphere skywatchers as the comet's solar elongation increases. Powering across Sagittarius and Microscopium, it ends the year at 6th magnitude in Piscis Austrinus.

Right now, Leonard sports a small but lush dust tail. If its dust production rate climbs in the coming weeks as the comet approaches the Sun and becomes more active, two special circumstances — an orbital plane crossing and a high phase angle — may boost its brightness above predictions.

Comet Leonard at orbital plane crossing
When the Earth crosses the comet's orbit on December 8th, we may see a brightness enhancement and a faint antitail.
JPL-Horizons with additions by Bob King

Amateur astronomer Michael Mattiazzo, creator of the Southern Comets Homepage, notes that on December 8th Earth will cross the orbital plane of the comet. Northern Hemisphere observers will then view the tail and dusty trail of Leonard edge-on, which should cause the tail to narrow and brighten a bit as the cometary particles stack up across its length, much the same way we see the thick band of the Milky Way from our viewpoint within the galactic plane.

Comet Lulin antitail
Comet Lulin (C/2007 N3) exhibits an antitail (left) on January 31, 2009, around the time that Earth passed through the comet's orbital plane. An ion tail extends to the right.
Joseph Brimacombe

Observers should also be keen to look for an antitail, a narrow, spike-like appendage pointing in the direction opposite that of the tail. Antitails form when we view larger cometary dust particles deposited along the comet's orbit edge-on.

Phase angle explanation
Forward scattering by dust particles in a comet increases as the phase angle (β) increases. This can cause a surge in the object's brightness. The more closely the comet is aligned with the Sun the greater the potential increase. The related scattering angle, called Theta (θ), decreases as β increases.
Bob King

Tantalizingly, Comet Leonard's phase angle (Sun-comet-Earth angle) may play a crucial role in elevating its brightness during much of December. The larger the angle — up to a maximum of 180° — the more nearly in line the comet is with the Sun. Sunlight shining through a cloud of fine particles like comet dust is scattered forward toward the observer. We see the same effect in steamy breath on a cold day, contrails, and cloud edges.

Forward scattering examples
Here are three examples of forward scattering or backlighting. From left: the shining edges of clouds covering the Sun; breath on a cold day, and Comet McNaught by daylight on January 13, 2007, when its phase angle was 145°.
Bob King (clouds, breath) / Mark Vornhusen, CC BY-SA 3.0

In 2006, Comet McNaught's phase angle reached 149°, and the comet swelled in brightness by more than two magnitudes, rendering it visible in daylight. Comet Leonard's phase angle (β) will be greater than 120° from December 9th to 22nd, with a maximum β of 160° on December 14th. This could result in significant forward scattering and a subsequent surge in brightness. At its peak phase angle the comet will be burrowed deep in evening twilight just 15° from the Sun, but favorable geometry may briefly improve its visibility.

Comet Leonard light curves
This light curve chart lays out three possible scenarios for Comet Leonard. The four blue and multiple black data points are observations from the Comet Observation Database (COBS) and the Minor Planet Center (MPC), respectively. The gray curve is calculated based on the original Minor Planet Electronic Circular (MPEC) for the comet or the latest values from the MPC. The red curve derives from recent COBS/MPC observations, while the green spike includes the effects of forward scattering.
Gideon van Buitenen / astro.vanbuitenen.nl/comet/2021A1

Below are Mattiazzo's predictions for a possible phase-angle-related brightness surge based on similar circumstances during McNaught's apparition. Remember, there are no guarantees. While Leonard's current appearance gives cause for optimism, we really don't know yet how dusty the comet is or will become. And dust production makes all the difference when it comes to phase angle. Further, McNaught's surge coincided with perihelion when it was most active, while Leonard will still be a couple weeks shy of closest approach.

December 10th and 20th: β = 130° / +1.0-magnitude enhancement to third magnitude
December 12th and 17th: β = 145° / +2-magnitude enhancement to second magnitude
December 14th: β = 160° / +3.5-magnitude boost to first magnitude

While we hope to see a fine apparition here on Earth, Venusians may witness a rare meteor shower. Leonard crosses the planet's orbit on December 17th. Two days later, Venus passes just 4 million kilometers from Leonard's dust trail, close enough to plow through it!

Comet Leonard won't be the second coming of NEOWISE, but I expect it has surprises in store. Watch for more reports and maps in the weeks ahead.

Resources: Weekly Bright Comets, C/2021 A1 (Leonard) Twitter, Visual Comets, and COBS.



Image of Anthony Barreiro

Anthony Barreiro

October 13, 2021 at 4:18 pm

Thanks very much Bob. I greatly appreciate all the diagrams and explanation of the geometry of this comet's apparition. Is there any idea how condensed vs. diffuse Comet Leonard may appear? Observing from the city where stars fainter than about third magnitude are invisible to the unaided eye, I could see a moderately condensed fourth or fifth magnitude comet through binoculars, but if it's very diffuse it would be washed out by the light pollution.

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Image of Bob King

Bob King

October 13, 2021 at 9:40 pm

Hi Anthony,

You're welcome. We still don't know how dusty or how condensed Comet Leonard may become. The last estimate I saw was magnitude 12 with a DC = 3. I've been eager to observe it myself so I can get a sense of the very thing you want to know, but we've had more than a week of overcast skies here. It should clear by the weekend, and I'll add my observation to the post.

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Image of Anthony Barreiro

Anthony Barreiro

October 14, 2021 at 2:24 pm

Thanks Bob. Clear skies!

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Image of Rasalgehu


October 28, 2021 at 6:31 am

The distance between the comet and M3 is not 1°.5 but much less. On December 3rd it will pass only about 6' from the center of the globular cluster, practically the comet's nucleus will be in the edge of the cluster.

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Image of Warren-Odom


December 3, 2021 at 8:19 pm

Obviously depends on what time (and what time zone) one is talking about. Unfortunately neither you nor the author specify. My first guess is that the author was referring to 0:00 UTC on the respective days. My 2nd guess is that he's referring to predawn viewing time in the U.S., about 6:00 AM EST as shown on a couple of the diagrams - which is more like 11:00 AM UTC. Whatever it is, clearly you are referring to somewhere in between.

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Image of Bob King

Bob King

December 4, 2021 at 6:48 pm

Hi Warren,

Because it's a wide-field map, it's accurate for most U.S. time zones. However, to answer your question, it is specifically for 5:30 a.m. / 6:30 a.m. CST, which by an oversight I didn't include. The caption has been updated to reflect this. Thank you very much for pointing this out.

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Image of OwlEye


November 5, 2021 at 12:19 pm

Hi Bob,

I had my first look at Comet Leonard (C/2021 A1) this morning at 6:15 local time. I've had to wait a long time to try for it, as it has been sitting in the worst of the KC light dome for weeks, and the moon has been in the way. It was only 42 degrees above the horizon at azimuth 75 degrees - right in the top of the light dome - and as I have seen a bit of variance in its reported magnitude on COBS, I had no idea what to expect. Picked it up
with averted vision; it looked about mag 11.0 with a 0.9 arcmin coma and DC of 5.

We are less than a month from the predicted peak - fun mornings ahead!!


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Image of William


December 5, 2021 at 9:01 pm

Thanks for this info. With the S&T finder chart .. I finally saw C/2021 A1 Leonard this morning, at abut 4:30AM... from a less than ideal location, in a municipality here in the Philippines. I used Canon 15x45 IS binoculars.. and detected the non stellar glow (not well defined), after I was fully dark adapted. The comet had a slightly brighter (very tiny) core area, which I estimated at around mag 7.

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