Rare double comet sighting as Lemmon and SWAN brighten Oct 20–21

On Thursday evening, Oct. 16, stargazers in the Northern Hemisphere will have a rare double treat as two recently discovered comets become readily observable: green-hued Comet C/2025 A6 Lemmon will appear near the bright double star Cor Caroli close to the Big Dipper, while bluish-green Comet C/2025 R2 SWAN will be visible low in the southwest after sunset. Both are expected to brighten further later in the week, with peak visibility around Oct. 20 and Oct. 21 as they make their nearest approaches to Earth.

Comet Lemmon was discovered Jan. 3, 2025, by the Mount Lemmon Survey. Astronomers estimate its orbital period at roughly 1,350 years, with recent calculations suggesting gravitational interactions during its current inward journey may shorten that to about 1,150 years. Lemmon will make its closest approach to Earth on Oct. 21, passing at about 55 to 56 million miles. Comet SWAN was first reported in September and has brightened rapidly as solar heating drives sublimation of surface ices. It is expected to pass considerably closer, at roughly 24 to 25 million miles, around Oct. 20 to Oct. 21.

Skywatchers have two primary viewing windows on Oct. 16. About 90 minutes after local...