By now interest in Comet C/2012 S1 (Comet ISON) is building. This could be the most dramatic comet in years. Where should we look for this oncoming interloper from deep space? This was a monthly guide aimed at observers in the UK and Ireland to help you find it. As of December 2013 the comet appears to have broken up and will not be seen in our skies after all.
To find the comet on any given day and date, I really recommend the free software Stellarium (available from our Free Stuff page). You will need to use the update database feature to add ISON to Stellarium, but this is very straightforward to do.
You can learn 10 Things You Need to know About Comet ISON elsewhere in this blog. Please note that this comet will never be brighter than a full Moon (anyone saying this is using very out of date information) and definitely will never appear larger in the sky than a full Moon, and remember that the behaviour of comets is notoriously unpredictable! Veteran comet watcher John Bortle discussed the possible visibility of Comet ISON in an interesting piece at Universe Today, and cautioned against excessive optimism.
From early June through lateAugust, Comet ISON was almost directly behind the Sun as viewed from Earth, and so it could not be seen. In early August, ISON is approaching the Solar System’s so-called “frost line” some 370 to 450 million km (230 to 280 million miles) from the Sun, when it reaches this region the comet’s nucleus will receive enough solar radiation that water will begin to evaporate making comet will appear brighter. From Earth the comet will appear in the constellation Gemini. It will be low in the bright pre-sunrise eastern sky, you will still need a telescope to see it. On 2 August about 5am, we will be treated to a celestial line-up of Comet ISON, Mercury, Mars, Jupiter and the Moon. From the middle of August ISON will be in Cancer and by the end of August it may be visible for observers with larger telescopes. At the end of the month the comet will appear just above M44, the Beehive Cluster. On 12 August the comet was recovered (that is, seen for the first after it emerged from behind the Sun) by Bruce Gary, an amateur astronomer in Arizona.
By now the comet ought to be an easy object for amateur telescopes. In early September the comet will appear close to Mars in the sky before dawn. On 1 September at about 5am,Comet ISON, Mars and the crescent Moon made a line in the sky. By mid September you will find the speeding up comet between Cancer and Leo and by the end of September it will be in Leo. By late September Comet ISON had developed a greenish tinge to its coma indicating more material was escaping from the nucleus.
On 1 October Comet ISON was at its closest to Mars at about 11 million km where various probes scanned the sky for it. The comet was still not bright enough to spot with binoculars from Earth. Early in the month, ISON was close to the bright star Regulus and both Mars and the Moon was close by too. By 15 October Regulus, Mars and ISON were tightly grouped and easy to find with suitable equipment, the comet was about Magnitude 12 or so, which was far too dim to be seen without a telescope. At the end of October, the comet was below Leo, heading down the sky towards Virgo.
So far the comet has been disappointing, being just visible as a feeble smudge through 16 inch telescopes in early November but this will hopefully be the start of ISON’s glory days. At the beginning of November when the comet was between Virgo and Leo, it showed new activity, developing an ion tail and reaching Mag 8 (visible in the morning sky through larger binoculars) reaching Virgo by the middle of the month. On 11 November the comet crossed the orbit of Venus, and suddenly increased in brightness! In just over a day the comet’s apparent magnitude improved by two magnitudes. By 16 November, it was a faint but visible object in the pre-dawn south-eastern sky to the unaided eye. It was an even more fascinating sight for observers with telescopes. By the third week of November it was be seen in the eastern sky before dawn. By the fourth week of November the comet is so close to the Sun that it will be almost sunrise before its head clears the horizon, so viewing it at all at this time was impossible.
If you cannot see Comet ISON why not try to find the other near-naked eye comet currently in the sky, C/2013 R1 Lovejoy? Try looking at any time of the night at the Plough (the famous asterism in Ursa Major), this comet will be be close to the Plough’s handle . Comet Lovejoy is impossible to see with the unaided eye, so you will need to use binoculars. You are looking for a faint fuzzy star.
Comet ISON is at its closest to the Sun (perihelion) on 28 November (Thanksgiving Day in the USA), when it is just a million km or so above our star. It had been thought that if the comet’s brightness increased according to the most optimistic predictions on this day it might be possible to see the comet in the daytime. The comet’s failure to get brighter at the rate originally predicted means it was actually impossible to view it like this.
What happens during this phase of the comet’s orbit determines how visible it will appear to us. The intense radiation of the Sun caused material to explosively evaporate off the comet. This could have meant the comet would rapidly brighten and develop a more impressive tail, delighting observers, and for a short time this seemed to be happening. Then on 26 November, it was looking increasing like a worst case scenario, to spacecraft the comet seemed to be disintegrating as it moved closer to the Sun, apparently turning into a plume of debris that could rapidly disperse. However after perihelion a small, much-diminished nucleus was seen to round the Sun. It was the end of the show for amateur observers yet as ISON is to all intents and purposes dead.
It is now impossible to see this dead comet, but during December 2013 the remains of Comet ISON will be in both the morning and evening sky as it races through the constellations. In early December after 7am it will between between Libra and Ophiuchus, a couple of weeks later it will be between Serpens and Hercules, on 22 December it will be in Hercules near the globular cluster M13. By 25 December it will be close to the Plough, and is circumpolar from UK and Ireland, meaning it will be in the sky all night long. On 26-27 December, ISON will be at its closest to Earth at 64 million km. At the end of December the comet is in Draco and will be in the north west by evening, in the morning sky before before sunrise in east. Sadly we will not be able to see it.
On 6 January 2014 Comet ISON’s debris will be near Polaris.
(UPDATE: Article last updated on 1 December 2013.)
(Article by Colin Johnston, Science Communicator)