Tomorrow October 12, 2024 “comet of the century” C/2023 A3 (Tsuchinshan-ATLAS) will reach the minimum distance from Earth, at approximately 70 million km from our planet, with the possibility of also being able to observe it to the naked eye. The comet is currently visible in an initial window of one hour from sunset and is as bright as Jupiteralthough seeing it with the naked eye is difficult since its brightness is “spread” over the entire dimension of the coma (i.e. the “head” of the comet) and is immersed in the light of the sunset. The comet is however clearly visible with binoculars, and indeed these are the best days to observe and photograph the “comet of the century”. Alternatively, the Virtual Telescope project has plans for October 12th from 7pm a live web observation of the comet.
How to pinpoint the location of the “comet of the century”
To be able to locate the comet you first need a completely clear western horizon from any obstacle. Follow the Sun during sunset, identify the exact point where it disappears behind the horizon and look a little higher to the left. Using outstretched fist at arm’s length as a unit of measurement, the comet will be at approx a fist to the left of the setting point of the Sun. It is possible to attempt observation with the naked eye, but to avoid the risk of being disappointed we recommend theuse at least a small pair of binoculars.
In the diagram above we show the position of the comet in the days close to maximum proximity to the Earth. Each position refers to coordinates of Rome. THE’October 11th evening at 7:00 pm, the comet will be located at approx 5th from the setting point of the Sun, setting at approximately 7.30pm. Each night, the comet’s height above the horizon will increase by about 3°, setting about 20 minutes later. The October 12 at 7:00 pm the comet will be at approx 10° above the setting point of the Sun and will set at 7.50pm.
By the evening of October 14th the comet will be well positioned in the sky a halfway between Venus (top left) and the bright orange star Arthur (bottom right). The comet will be located approximately 21st (i.e. about “two fists”) from both the planet and the star. On the evening of October 19, the comet will be about 30° (about “three fists”) above the west-southwest horizon, setting three and a half hours after the Sun. By October 26, the height above the horizon will have increased to at 38° west-southwest (almost “four fists”), setting a full four and a half hours after the Sun, but by then the comet will be too faint to be easily spotted, even with binoculars.
How to photograph Tsuchinshan-ATLAS: advice
The most important requirement is to look for a area with little light pollution and a completely clear western horizon. Secondly, we recommend that you equip yourself with one reflex or mirrorless cameraor simply a smartphone equipped with advanced settings that allow you to take photos in night mode. It is also useful to have a tripod since the exposure time can vary from under a second (shortly after sunset) to about ten seconds. Also have a lens equipped with a large field of view it helps, especially if you are unsure of the exact location of the comet.
We also provide some advice on the shooting settings to adopt, although the latter vary greatly depending on the observation site and the moment of shooting. The ISO should be set between 400 and 800with one aperture f/2.8-f/4 to store as much light as possible. Given the small size of the object, it is necessary to use the manual focusfocusing on a bright, distant object, such as the Moon or a star. The exposure time varies from under a second very shortly after sunset to tens of seconds 30-45 minutes after sunset. The exposure time will likely increase as the comet becomes fainter in the days following its closest approach to Earth.
One last piece of advice on tail of the cometthe most spectacular part to photograph. Currently the hair measures approx 209,000 km in diameter and is accompanied by one tail about 29 million km long. On October 12, the comet’s tail will be pointing almost directly toward Earth, so it may appear a bit shortened. In the following days, however, in addition to being visible for longer after sunset, the tail will appear to rotate rapidly towards the east (left) and lengthen considerably night after night. Consequently, to photograph the spectacular tail it is advisable to wait 2-3 days after the closest proximity to the Earth on October 12th. This is not a problem because, although the comet will tend to become dimmer, the photographic equipment will allow you to take long exposure photos that compensate for the decrease in brightness.
Updated forecasts on the visibility of the comet
The comet has reached its maximum brightness on October 9th, when the phenomenon of forward scattering (“forward scattering”), due to the ice grains surrounding the comet and its position between the Earth and the Sun, increased the brightness of the comet until it was similar to that of Venus.
Unfortunately on October 9 the comet was still too close angularly to the Sun, so the sudden increase in brightness could not be fully appreciated. From October 9, the comet began gradually become weaker and at the time of writing the article, the magnitude is comparable to that of Jupiterequal to approximately –2.5. The latest projections predict that the comet will have its closest proximity to the Earth magnitude equal to 0.1similar to that of Saturn.
Although the latter is easily visible to the naked eye, the same is not necessarily the case for the comet. Having a magnitude (a term used by astronomers to describe brightness) comparable to that of Saturn has a different meaning if it is associated with a planet or a comet, since in the latter case the brightness is “spread” in a large area when the Moon is full.
The comet could therefore be visible to the naked eye from lightly polluted skies, free of obstacles to the west, but we still recommend using binoculars or a small telescope to avoid being disappointed by the celestial event. The binoculars in fact there it will help locate the comet more easilyand then attempt observation with the naked eye once its position is known.
By October 16, its brightness may have dimmed to second magnitude, though still as bright as North Star. By October 19, forecasts suggest that C/2023 A3 Tsuchinshan–ATLAS will shine like an ordinary third-magnitude star, while by October 26, the comet will likely have dipped to fifth magnitude, the equivalent of a very faint star visible to the eye naked only from a dark place.