Comets C/2025 R2 (SWAN) and C/2025 A6 (Lemmon) will reach the point of closest approach to Earth respectively on October 20th and 21st. Comets will be visible for approximately 4-5 hours after sunset in a west/southwest direction, in the case of SWANand for approx 3 hours in a west/northwest direction in the case of Lemmon. Technically, only the latter is bright and shiny enough to be technically visible to the naked eye (about 4.1 at perigee), however, thanks to the low height above the horizon and the fact that its brightness is “distributed” over the entire area of the comet, sighting with the naked eye will be extremely difficult, requiring in any case skies free of light pollution. The comet SWANhowever, is below the threshold of visibility with the naked eye, so it will require at least good binoculars to be observed. For both it will be necessary to go to areas with a western horizon that is as clear as possible. On October 21, Comet Lemmon will rise again around 6 a.m. in a northeasterly directionaccompanied by the planet Venus, creating a new window of visibility about an hour before sunrise.
Where to observe comets SWAN and Lemmon
Al perigee of October 20, 2025the comet C/2025 R2 (SWAN) will be on the day above the horizon for good 5 hours after sunset (around 6.20pm for Rome, 6.30pm for Milan). Initially, the comet will be in the constellation of Scutum, below that of Aquila with its bright star Altair, about 35 degrees above the southern horizon. As the hours passed, the comet did it will move in a south-westerly directionreaching 30 degrees above the horizon at around 8pm, 20 degrees at 9.30pm, setting In the end around 11.30pm in a west/southwest direction.
As for the brightest comet C/2025 A6 (Lemmon)his permanence on the horizon after sunset it is sunny 3 hours al perigee on October 21st. At sunset, the comet will be 30 degrees above the western horizon in the constellation Bootes, just above the bright star Arcturus which will be your best guide in locating the comet about 5 degrees to the upper right of the star. Lemmon will drop to 20 degrees above the horizon at 7:30 p.m. setting in a north-west direction about two hours later. Unlike SWAN, Lemmon will offer a second window of morning visibilitywhen it arises around 6 a.m. in a northeasterly direction in the company of the planet Venus to his right.
Towards end of the monththe comet Lemmon it will decrease its window of visibility after sunsetwhich will drop to about two hours, while it will no longer be observable in the morning. As for the SWANhowever, towards the end of the month it will be a more than 40 degrees on the southern horizon at the time of sunset, remaining visible for six hours. Despite this, for both SWAN and Lemmon the best moment of visibility is near the perigeesince in the following days the Moon will appear in the sky together with the two comets, making their observation even more difficult.
How to best observe them
There magnitude of the two comets at the perigee it will be approximately 6.3 for SWAN and 4.1 for Lemmon. From a technical point of view, therefore, the Lemmon is above the threshold of visibility with the naked eyewhile the SWAN below. Despite this, it will be extremely difficult to spot with the naked eye Lemmon. The magnitude refers to the total brightness of the object, which however is distributed over the entire area covered in the sky by the comet, which makes sighting difficult. Our advice is to equip yourself with good binocularsbetter yet a small telescopeand to go to a sky free of light pollution with clear western horizon. These small tips should help you enjoy the celestial spectacle of the two comets to the fullest. If you then want to try your hand at a session astrophotographyessential will be a easel where to place the camera, as it will be necessary to carry out long exposure photo.
The distance from Earth to perigee
Comets C/2025 R2 (SWAN) and C/2025 A6 (Lemmon) will reach perigee just one day apart, thus offering a great opportunity to observe them almost at their maximum expected brightness. Despite the contemporaneity of the event, this it does not imply that the two comets are close to each other. At the perigee of 2nd Octoberthe comet C/2025 R2 (SWAN) will be found at 39 million km from Earth intersecting its orbit. The comet C/2025 A6 (Lemmon)instead, at the perigee of October 21st it will be good 89 million km from the Earth, far above the Earth-Sun orbital plane and projected between the orbits of Venus and Mercury. The different position in the Solar System also explains why the two comets are observable in two different areas of the sky, in a south-west direction for SWAN and north-west for Lemmon.
If you cannot observe comets “live”, you can still follow one of the many online live broadcasts, such as that of the Italian project Virtual Telescope Project:
