Comet ISON Finder for December 2013

ISON disappeared from view in the last week of November as it races towards the sun. It will reappear again in the eastern pre-dawn sky in December. It might first be visible on Dec 4th with a better chance of seeing on Dec 6th. The best opportunity will probably be Dec 9th and 10th. It will be visible for a few weeks after that, but fades rapidly as it travels away from the sun.

The animation below shows the path that ISON will take as is makes a close approach to sun, swing behind the sun and then reappear on the other side. (The animation does not show the fly-by correctly. The comet really does go behind the sun. The Starry Night Pro animation misses the fast fly-by and just connects the before and after positions with a line crossing in front of the sun.) The animation covers the dates from Nov 15th to Dec 31st and shows the comet’s position at the same time each morning relative to the pre-dawn horizon (green line) for an observer at latitude 45°. The time selected is the start of astronomical twilight (end of official “night”)  so the sun is positioned 18° below the horizon.

ISON (c/2012 S1) is a “grazing comet” meaning it makes a very close approach to the sun. There are three possibilities for the return path. The least desirable scenario is that the comet breaks apart due to the intense heat from being so close to the sun. In which case there isn’t going to be a return path. The second scenario isn’t much better. The sun may burn off the majority of the volatile shell and that means that while the comet will survive, it will be hard to spot and won’t have much of a tail, if any. If the comet survives it’s close encounter, then it will brighten to an apparent magnitude of 4 as it first comes into view in early December.

From the observatory (which is at latitude 45°) the comet might first be visible on Dec 4th a little after it rises at 5:52am est. The estimated magnitude is 3.8, which is bright enough to see unaided although its low altitude might make that difficult. At 6:16am est – the start of nautical twilight – it will be just 3.5° above the horizon. The sun will be 12° below the horizon and the morning sky will be brightening. By 6:52 nautical twilight will be over and the sky will most likely be too bright to see the comet. It will be at altitude 7.5° at this time – still pretty low.

On Dec 6th, the comet will rise at 5:27am – before the end of official night (and the start of astronomical twilight). By 5:42 – start of astronomical twilight – the comet will still be only 2° above the horizon and the sky starting to brighten. The predicted apparent magnitude is 4.7, which might just be bright enough to find unaided. The thick hazy air near the horizon will make that a challenge. Through binoculars it should be findable and hopefully the tail should be obvious. Nautical twilight starts at 6:18 on Dec 6th and civil twilight starts at 6:53. So there is ample time to scan the sky with binoculars and hopefully get a good view.

Over the next few days ISON will continue to rise earlier which means it will be higher in the with more time to view it under darker skies. But as it gains distance from the sun, its magnitude will also decrease. So it’s a bit of a race between the darker skies, better elevation and the fading brightness. December 9th and 10th are probably the best days to see it.

The comet will be visible for a few weeks after it first reappears, but by late December, its magnitude will have faded to 9 and therefore only visible with a medium to large scope under dark skies.

The chart below shows the positions of ISON for the month of December. The time for each date shown is the start of astronomical twilight. The best viewing will be before the start of astronomical twilight when the sky is at its darkest. But the comet also needs to be at least 5° and more like 10° in altitude to be findable. December 9th or 10th might be the best opportunity, but try and see it as soon as possible.

The details of date, time, magnitude and position are in the table below. For each date the table provides the estimated apparent magnitude. Then the time ISON rises with the azimuth position when it rises (0° is north increasing clockwise to the east). The start of astronomical twilight is given and then the azimuth and altitude of the comet at that time.

Date Mag Rise Az Twilight Az Alt
Dec 4 3.8 5:52 99° 5:40
Dec 6 4.7 5:27 94° 5:42 97°
Dec 8 6.6 5:03 89° 5:44 97°
Dec 10 6.7 4:38 83° 5:46 97° 11°
Dec 12 7.1 4:12 77° 5:47 96° 16°
Dec 14 7.3 3:44 70° 5:48 92° 21°
Dec 18 7.6 2:38 54° 5:51 86° 31°
Dec 22 7.9 12:59 32° 5:53 75° 41°
Dec 26 8.1 dnr 5:55 59° 49°
Dec 31 8.6 dnr 5:56 31° 52°