Bright comets will be scarce this year, but you can see one of the better ones this month as it races from Aquila to Canes Venatici. In other news, a new, bright nova has flared in Sagittarius.

Comet ATLAS (C/2020 R4) / Jose Chambo / March 27, 2020
Comet ATLAS (C/2020 R4) displays a green coma about 6′ across slightly elongated in the southwest direction in this photo made on March 27, 2021.
José Chambo

Comets are always prowling about, but most are 10th magnitude and fainter. I have nothing against faint comets. If it's 13th magnitude or brighter I'll pursue a chunk of moving fuzz any hour of the night no matter the declination. Brighter is better, of course. But as I gaze into my crystal ball in search of easy fare, only four comets will crack 10th magnitude in 2021, barring a bright, new discovery.

As always, we'll make the best of what there is. And that would be Comet ATLAS (C/2020 R4), which is just now making a fine appearance in the predawn sky in Aquila. Its name comes from the Asteroid Terrestrial-Impact Last Alert System (ATLAS) search program (ATLAS). Two telescopes, one based on Mauna Loa on the Big Island of Hawai'i and a second on Haleakalā in Maui, scan the entire sky several times a night looking for solar system objects on the move.

Comet ATLAS finder chart April 8 to 20
The path of Comet ATLAS is shown at three-day intervals between April 8–20 as it travels from Aquila into Hercules. Because the comet is best seen in the predawn sky positions are for 4 a.m. Central Daylight Time instead of the standard UT (CDT = UT – 5h). Stars are shown to magnitude 8.
Stellarium with additions by the author

Comet ATLAS triggered this telescopic snare on September 12, 2020, when it was just a 20th-magnitude speck of light. According to the website, ATLAS has the capability of providing "a one-day warning for objects in the 30-kiloton 'town killer' range; a week for a 5-megaton 'city killer' and three weeks for a 100-megaton 'county killer.'"

What's It Look Like Right Now?

I observed the comet in last-quarter moonlight on April 3.4 UT at magnitude 9.5 in my 15-inch (38 cm) Dob. It appeared large and diffuse, about 4′ across with a degree of condensation — or DC — of 3. DC is rated from 0 to 9 from completely diffuse (0) to stellar (9). ATLAS passed perihelion on March 1st and could become as bright as magnitude 8 around the time it passes closest to the Earth on April 23rd at a distance of 69 million km (43 million miles).

While comet tails appear infrequently, the coma, a nebulous envelope of sublimating ices, gases, and dust that forms a temporary "atmosphere" around the icy nucleus, is a distinctive feature of nearly every comet. Diatomic carbon (C2) fluorescing in sunlight causes the coma to glow green, a color enhanced with the use of a Swan Band filter. When I used the filter on ATLAS I noticed a modest improvement in its visibility.

Comet ATLAS finder chart April 20 to May 1
This map covers the comet's path from April 20th to May 1st as it zooms from Hercules to Canes Venatici. Its location is shown for 4 a.m. CDT every 3 days. Stars are plotted to magnitude 7.5.
Stellarium with additions by the author

In early April, ATLAS travels about 2° per day but keeps its foot on the accelerator, topping out around 4.5° a day (11¼′ per hour) while dashing from western Hercules into Corona Borealis around the time of minimum distance from Earth on April 23rd. Throughout its apparition the comet is likely to remain a large, weakly condensed but overall bright object. It should be visible in a 6-inch telescope from dark skies.

Comet 67P/Churyumov-Gerasimenko
Pictured here on November 12, 2015, during its last apparition, Comet 67P/Churyumov-Gerasimenko returns later this year when it could brighten to 9th magnitude.
Damian Peach

During its run, the comet will make several close approaches to brighter deep-sky objects and pass through numerous faint galaxy fields. Several examples are given below. Distances are approximate because the comet is moving rapidly, with the actual separation depending on the hour you see it:

April 8 — 0.3° northeast of the open cluster NGC 6773
April 12 — 1° southwest of the the 7th-magnitude open cluster NGC 6709
April 29 — 1° southwest of the 12th-magnitude galaxy NGC 5623

Brightest in late April, Comet ATLAS is the only comet predicted to reach 9th magnitude or brighter in either hemisphere until November. Comet 7P/Pons-Winnecke may push past 10th magnitude in May and June, but the comet engine won't really start humming again until the icicles return. That's when Comet Leonard (C/2021 A1) climbs to magnitude 8 before becoming potentially as bright as 4th magnitude in December. Returning comets 67P/Churyumov-Gerasimenko and 19P/Borrelly are also expected to reach 9th magnitude in November.

Comet Leonard (C/2021 A1) is presently 3.6 a.u. (539 million km) from Earth and glows dimly at magnitude 17.5, yet Dan Bartlett managed to image its coma and a short tail by combining 60 300-second exposures (5 hours of data) using a Celestron 8-inch f/2 RASA astrograph. Photos were taken the night of April 1-2, 2021. Dan Bartlett

Fainter comets worth a look

Those with larger instruments can track down four additional cosmic critters during April and May: another ATLAS comet (C/2019 L3) at magnitude 12.5–13; Comet SWAN (C/2021 D1) at magnitude 12; Comet Palomar (C/2020 T2), currently at magnitude 11.5 but brightening to 11 in the coming weeks; and 10P/Tempel at magnitdue 10.5 and visible before dawn from the Southern Hemisphere.

Comets in April 2021 / Jose Chambo
Three additional faint comets — ATLAS (C/2019 L3), SWAN (C/2021 D1), and Palomar (C/2020 T2) — ply the evening sky this month.
José Chambo

To find them, download Gideon van Buitenen's latest orbital elements for your software program and create your own customized maps. His elements are also included in the free planetarium program Stellarium. To use them, start the program and click on the Configuration icon in the lower left corner of the screen. Then follow this path: Plugins tab — Solar System editor — Configure button — Solar System tab — Import orbital elements — Comets button — Bookmarks — Gideon van Buitenen: comets — Get orbital elements — checkmark the comets you want to observe — Add objects. When finished, use the search function (magnifying glass icon) and type in the comet's full name to pinpoint its location in the sky.

I hope you're able to make the most of Comet ATLAS. It officially becomes an evening object by mid-month when it rises around midnight. By the third week of April you can catch it around 10:30 p.m. local time although the Moon begins to interfere. When it departs on April 28th, the comet will stand high in the southeastern sky in central Boötes at nightfall, accessible to all at a reasonable hour while still near peak brightness.

New bright nova in Sagittarius

New Nova Sgr No. 2 (now officially V6595 Sagittarii) is located just 1.75° northwest of 3rd-magnitude Gamma (γ) Sagittarii above the Teapot's spout. Click here for a detailed AAVSO finder chart.

While the recently discovered Nova Cassiopeiae (V1405 Cas) remains bright at magnitude 8.0–8.5, another nova just popped into view in Sagittarius. Andrew Pearce of Western Australia nabbed Nova Sagittarii No. 2 on April 4.8 UT at magnitude 8.4 with a Canon digital camera and 100-mm f/2.8 lens. As of April 6th it's risen to magnitude 7.9. Under good conditions observers should be able to spot the nova in instruments as small as 50-mm binoculars in the southeastern sky before the start of dawn.


Image of Dan


April 8, 2021 at 2:11 am

Thanks Bob. Always love reading the visual descriptions through your 15”. Background on the ATLAS program additionally interesting. Hope your next addition brings new bright discoveries!
Dan Bartlett

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Bob King

April 8, 2021 at 11:44 pm

Thank you, Dan. And thank you also for sharing your photo with our readers!

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April 19, 2021 at 6:57 am

Hi Bob. We finally got a clear morning (4/19) here in eastern Kansas! Faint comets are ghostly enough, but in my light-polluted skies C/2020 R4 (ATLAS) is a ghost of a ghost. If I had not known precisely where it was in relation to the stars in the 79X field, I may not have seen it at all. The difficulty made the observation all the more satisfying. Spotted it 5 minutes before the onset of astronomical twilight, using the 12.5-inch.

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Bob King

April 21, 2021 at 9:13 pm

Hi Doug,
Thanks for sharing your observation. It really IS ghostly. I also looked at it recently, and although the coma was a nice, plump 8' across, there's very little concentration towards the center. While bright and easy is always preferred I like the big, vacuous comets, too.

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April 28, 2021 at 5:23 pm

Thanks for the comet info. As always much appreciated! Clear here on the ‘left coast’, so will pull out my 12 1/2” and shoot for Comet Atlas tonight!

Also, for my brother’s birthday, I bought him a copy of your book, ‘Wonders of the Night Sky you must see .. 5 or 6 years ago, my brother inherited a 4” Meade refractor, it came mounted on a ‘hard to use except as a spotting scope photo mount’, so for his birthday last year, I gave him, mounted it on a German Eq. Mount. He has yet to use it! Am hoping your excellent book will inspire him to actually use that scope! Bet it does..

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Bob King

April 28, 2021 at 8:32 pm

Hi Aqua,

Thank you for getting him the book! I do hope that plus the refractor will provide the motivation to explore the night sky.

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April 28, 2021 at 5:39 pm

P.S. I recently completed training as a docent for ‘The Valley of the Moon’ astronomy club up at the Robert Fergusson Observatory, Sonoma CA. Actually I’ve been a guest docent for a couple years but FINALLY got around to making it official! Of course COVID pretty much put the kibosh last year’s public star gazing events - the observatory was closed .. BUMMER! But now .. things are looking UP! The club is doing ‘socially distanced’ viewing, with small groups - reservations are req’d. All docents have gotten their vaccinations so we are ‘chomping at the bit’ to share the glories above with the public!

I SO miss the hundreds of people who have looked thru my telescope! Public star parties are a TON OF FUN!

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April 28, 2021 at 5:44 pm

Oh yeah.. THANK YOU FOR being you!
Dale Jacobs

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May 13, 2021 at 6:17 pm

I will try for this one tonight. I saw Comet Palomar last night and it was fairly easy in a 13.1 inch scope. I failed to find Comet SONEAR and Comet Pons-Winnecke but both were fairly low on the horizon and a bit fainter than ATLAS and Palomar..

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