It was early January 2025, when the Mt. Observatory A pale spot of featherlight was observed in Lemmon, Arizona. Further observations revealed that the object was a comet coming from the outer edge of the Solar System and was named Comet Lemmon (C/2025 A6). Its “period” – the time it takes to complete one orbit of the Sun – is approximately 1,350 years.
Initially, the comet was so pale that it could only be seen with specialized equipment. However, as the months passed, it quickly brightened and by the time fall came, it was vivid enough to be seen through binoculars. It will now become even brighter as it approaches both Earth and the Sun, and may even become noticeable to the naked eye.
According to NASA, Comet Lemmon will be closest to Earth around October 21, 2025passing at a distance of about 0.60 astronomical units, or AU (1 AU is the distance between the Earth and the Sun). The comet is expected to reach perihelion – the point at which it is closest to the Sun – on November 8. Around this time, the comet will reach perihelion, will likely reach maximum brightness, and may be noticeable to the naked eye if conditions are favorable in the shadowy sky.
The comet’s tail will also be at its longest point near its perihelion point. Tails form on comets when solar radiation heats frozen ice and gases in the comet’s nucleus, causing them to sublimate – or change from a solid to a gas – and the radiation then causes some of this material to be emitted from the comet in a characteristic streak. The stronger the radiation, the greater the size of this effect, so comet tails grow as they approach stars and shrink again as they move away.
Comet Lemmon stands out with a greenish tint. This is caused by a cloud of gas surrounding the comet’s nucleus, called its “coma”, whose color is created by diatomic carbon (molecules of two carbon atoms) interacting with sunlight.
How to see a comet
From early October, Comet Lemmon can be found just below the handle of the Massive Dipper (or Plow, as it is called in the UK), which forms part of the constellation Ursa Major. If you look at the northern sky, you will be able to trace its path. In the northern hemisphere, yes is predicted appear in the northwestern and western skies after sunset from mid-October. It will be at a low altitude above the horizon and is expected to emit a pale glow at dusk just after sunset.
The best time to observe the comet is about a week before and after its closest approach. In November, the comet will cross the celestial equator and become noticeable from the southern hemisphere.
Binoculars or diminutive telescopes are best for observing, but if you choose a location with little featherlight pollution, you may be able to capture them with the naked eye. If you look up at the sky right after sunset, when it’s still a little vivid, you might be able to see them. If you’re having trouble finding the comet, employ a star map app on your smartphone or planetarium software to lend a hand you pinpoint its location.