cometa c2023 a3 Tsuchinshan-ATLAS

The “comet of the century” C/2023 A3 becomes visible at sunset: where, how and what time to see it from Italy

Comet C/2023 A3 observed on October 2, 2024. Credit: cafuego, CC BY–SA 2.0, via Wikimedia Commons

The best days for observing the world are beginning “comet of the century” C/2023 A3 Tsuchinshan-ATLAS in our hemisphere, which today 9 October 2024 is located at peak of its brightness. This is the first “great comet” in the Northern Hemisphere since the Hale-Bopp in 1997 and it is the brightest comet of the last 13 years. On September 27, the comet passed the perihelion (the point of its orbit closest to the Sun) unscathed and these days it is visible at our latitudes for about 15 minutes after sunset. The close passage of the comet discovered in 2023 by our star caused a large quantity of volatile materials that make up the nucleus of the comet to sublimate, thus producing a coma approximately 0.5° in the sky and a long tail that extends for approx 20°. Result: the comet has become very brightso much so that the latest measurements have revealed a magnitude –3.3: Translated into simple words, C/2023 A3 is currently a little dimmer than Venus, which is the brightest celestial body in the sky after the Sun and the Moon.

brightness of comet C2023 a3
The orange line represents the predicted brightness curve for comet C/2023 A3, while the white and green crosses are the measured magnitudes. On October 8, 2024, the comet reached magnitude –3.3. Source: COBS

Will comet C/2023 A3 be visible to the naked eye?

Does this mean that between now and the next few days we will be able to see it with the naked eye? Not necessarily: it’s not that simple (with comets it’s not never simple). It is true that the magnitude of C/2023 A3 is comparable to that of Venus, but in the case of the planet this brightness is concentrated in a small dot in the sky, while the brightness of the comet is “spread” over a large area when the Moon is full. This means that the “density” of brightness (i.e. how many magnitudes there are in a given area of ​​sky) is much lower. Furthermore, the comet appears very close to the Sun at less than 5° angular distance, therefore its brightness must compete with that of twilight.

Image
Comet Tsuchinshan–ATLAS at twilight observed on September 30, 2024. Credit: KPNO/NOIRLab/NSF/AURA/R. Sparks, CC BY 4.0, via Wikimedia Commons

On the other hand, however, today is the day in which the phenomenon of forward scattering (literally “forward scattering”), in which the dust and ice crystals around the comet’s nucleus scatter sunlight towards us which backlights the comet. This should substantially increase the brightness of the celestial body, although it is difficult to say by how much. In general, when talking about comets forecasts of brightness and actual visibility are always complexand the best thing to do is raise your eyes to the sky – perhaps armed with a good pair of binoculars or a small telescope – and enjoy the show.

Where and what time to observe the “comet of the century” from Italy (in the sky and in live streaming)

One thing is certain: the absolute best days for Tsuchinshan-ATLAS observation are as they go from today 9 October to Saturday October 12when the comet will reach the maximum proximity to the Earth (about 70 million km away) and will be a little further from the Sun.

visibility of comet atlas
Path of comet C/2023 A3 in the sky from 9 to 16 October 2024. Credit: Dominic Ford via Sky Live

This evening the comet can be observed around 7pm for a few minutes after sunset, very low on the horizon in a westward direction at a short angular distance from the Sun in the constellation of Virgo. Alternatively, you can follow the observation of the comet in the project’s live online broadcast Virtual Telescope from Manciano, in the province of Grosseto, starting from 7.00 pm.