TOKYO — A celestial visitor, comet Lemmon, is gracing Earth’s night sky. Despite its name, suggesting a yellow lemon floating in the sky, it has no actual connection to the fruit. Discovered in January 2025 by the Mount Lemmon Survey in the United States, this comet will make its closest approach to Earth on Oct. 21, the day of the new moon, coming within about 90 million kilometers, or roughly 60% of the distance between the Earth and the sun. It is currently at its prime visibility.
Comets, often called “dirty snowballs,” release fine dust and vaporized gas from their surfaces as they approach the sun. The Japanese term “hokiboshi,” meaning “broom star,” derives from their distinctive tail.
According to NASA, comet Lemmon is making its return after approximately 1,350 years. It will come closest to the sun on Nov. 8, at a distance of about 80 million kilometers. Typically, comets shine brightest when nearest to the sun. However, since comet Lemmon will approach Earth before that, it is expected to maintain its brightness as it traverses the night sky.
On the early morning of Oct. 17, this reporter was at the base of Mount Fuji and could clearly see its faintly extended tail through binoculars even in the moonlight. The comet is anticipated to be at its brightest from late October to early November.
Comet Lemmon shines with its thin, faint tail stretching toward the upper left, as seen in Narusawa, Yamanashi Prefecture, at 4:27 a.m. on Oct. 17, 2025. (Mainichi/Koichiro Tezuka) (15-second exposure, with a small equatorial mount)
Initially, comet Lemmon was predicted to brighten to around magnitude 4, but it is now expected to reach the magnitude-3 range. While it may be challenging to spot in city lights, it should be visible to the naked eye in dark skies with minimal moonlight interference. Binoculars, even at low magnification, will enhance the viewing experience.
On Oct. 18, the comet moved from the constellations Canes Venatici to Bootes, with prime viewing shifting from dawn to dusk. One hour after sunset, it will be visible in the west-northwest sky at an altitude of approximately 18 degrees. By around Oct. 24, it will pass about 10 degrees above the first-magnitude star Arcturus in Bootes, serving as a helpful reference point.
Toward the end of the month, the comet’s altitude will rise slightly, moving southward. On Oct. 28, it will be at an altitude of 22 degrees in the west. By Nov. 8, when it approaches closest to the sun, it will be at an altitude of about 15 degrees in the west-southwest. From then on, its altitude will decrease daily, making it harder to spot. These altitudes are based on Tokyo’s latitude, but the comet’s position in the night sky will be consistent across Japan.
From Oct. 25, the moonlight will increasingly interfere, with a full moon on Nov. 5. From Nov. 8, the moon will rise later, reducing its impact, but the comet’s altitude will also decrease.
The head of comet Lemmon shines with its tail extending, showing a thin ion tail within the broad dust tail, as seen in Narusawa, Yamanashi Prefecture, at 4:30 a.m. on Oct. 17, 2025. (Mainichi/Koichiro Tezuka) (With an equatorial mount, composite of eight averaged images)
While the media often highlights the closest approach date, comet Lemmon is not only visible on Oct. 21. Considering factors including altitude and moonlight, the best viewing opportunity may be around Oct. 25. I hope it will continue to shine brightly despite the moonlight. It will likely be easier to spot in the suburbs where the night skies are darker.
Around this time last year, astronomical news was dominated by “comet Tsuchinshan-ATLAS,” a great comet visible to the naked eye, with a tail that could be photographed with a smartphone.
Comet Lemmon is not expected to be as bright, but comets visible to the naked eye are not an annual occurrence. The next return of this comet is predicted to be at least 1,100 years from now, due to slight orbital changes, making this a once-in-a-lifetime opportunity.
The comet can be seen during the early evening. On clear days, I encourage readers, while being cautious of bears in some regions, to take the chance to search for the shining comet Lemmon in the western sky.
(Japanese original by Koichiro Tezuka, Photo and Video Department)
