Two Bright Comets Converge on Northern Hemisphere Skies

An exaltation of larks. A dazzle of zebras. A murmuration of starlings. These are a few of the more poetic and evocative collective nouns used to describe natural phenomena. How about a convergence of comets? By early October, both Comet SWAN (C/2025 R2) and Comet Lemmon (C/2025 A6) will be wagging their diaphanous tails across the sky for observers in mid-northern latitudes. While the former has been observed and photographed primarily by Southern Hemisphere observers, it will become widely visible to those of us at mid-northern latitudes starting the second week of October. Watch for it to make an entrance low in the southwestern sky in Libra at dusk, then track northeastward through Scorpius and into Ophiuchus.

A recent outburst on September 26th kicked up Comet SWAN's brightness about a half magnitude, to 5.9. It currently displays a strongly condensed 5′ coma and an approximately 1° tail pointing southeast. We expect the blue-green fuzzball to remain around 6th magnitude through mid-October before fading. Glare from the full Harvest Moon on and around October 6th will compromise views of the tail, but the compact head of the comet should still punch through even in binoculars.

From latitude 40° north, SWAN stands about 5° high in late twilight (an hour after sunset) on October 2nd. But as it vectors toward its closest approach to Earth on October 19th, the comet's elongation from the Sun increases, and it gains altitude. By October 6th the Oort Cloud visitor stands 5° high at the end of astronomical twilight. Three nights later, with the return of a moonless sky, it's in line with the trio of bright stars that outline the head of Scorpius. As Comet SWAN continues to wing eastward and ascend, don't miss an easy observing opportunity on October 13th, when it will pass 1.2° southwest of 2nd-magnitude Sabik, Eta (η) Ophiuchi.

In the meantime, SWAN's counterpart has been luring Northern Hemisphere observers into the morning sky for their first look at a binocular-bright comet since Comet Tsuchinshan-ATLAS paid a visit last fall. Thanks to a recent spate of clear nights, I've seen lots of Lemmon. From a rural sky, it's immediately obvious in 10×50 binoculars as a small, round ball of fuzz with a denser, brighter core and a hint of a tail aimed northwest. On September 26th, through my 15-inch telescope at 64×, the 5′-wide, moderately condensed coma glowed a subtle but tantalizing blue-green hue, with about 1° of tail visible. This is a nice comet. I estimated its brightness that night at magnitude 7.5. Two mornings later, its inner coma was noticeably more condensed, and the comet's overall brightness had inched up closer to magnitude 7.

Recent brightness predictions have been revised for Comet Lemmon, further whetting excitement for what's to come. It's possible it could vault to around 3rd magnitude around the time it makes its closest approach to Earth on October 21st. With no Moon to interfere at that time, this rare planet-trotter should be obvious without optical aid from the countryside from mid-October through month's end. Of course, that's a prediction. As always, comets are under no obligation to perform as anticipated!

As a side note, I have to share how much fun I've been having in photographing Comet Lemmon with my smart telescope. While I'm taking in views of the comet (or Jupiter) with binoculars or my 15-inch, the tiny, automated Seestar S50 camera-scope is busy stacking images, like volunteers piling pancakes on plates at a Lion's Club fundraiser. I typically expose between 5 and 8 minutes, long enough to capture details in the tail and distinct color in the coma. If you own one, go for it. Modest comets like SWAN and Lemmon make perfect targets.

Like Comet SWAN, Lemmon is "picking up speed" and covering more distance in the sky as it approaches Earth. It begins October in eastern Lynx and spends three nights in Leo Minor before crawling across the claws of the Great Bear, Ursa Major. Around October 13th, when the comet crosses into Canes Venatici, it will have transitioned into the evening sky, visible low in the northwest starting in late twilight. The timing is perfect because it escapes the Moon just as the comet's brightness is approaching its peak. The object's relatively low altitude in a dark sky will provide excellent opportunities to frame it with a pretty foreground scene — possibly even with a smartphone.

Maximum brightness occurs during early evening hours toward the end of the second week and during the third week of October, when the Moon will be mostly absent and Lemmon trucks across Boötes some 10° north of Arcturus. What a wonderful time for comet-watchers this will be, with Lemmon highlighting the northern sky and SWAN showing off in the southern. Speaking of which, Southern Hemisphere observers, no doubt chomping at the bit, will finally see Comet Lemmon at the end of October/early November as it heads south across Serpens into Ophiuchus.