Comet C/2012 S1 (ISON) will make its closest approach to the Earth on 29 December, at a distance of of 0.42 AU.

The comet's perigee – closest approach to Earth – should not be confused with its perihelion, when it passes closest to the Sun. Comets become much brighter when they pass close to the Sun. At perihelion their surfaces are heated and produce clouds of dust which give rise to their tails and extended coma around the nucleus. As a result, most comets are brightest around the time of their perihelion, not their perigee.

From Cambridge on the day of perigee it will be visible in both the dusk and dawn skies. In the evening, it will become visible at around 17:33 (EDT), 23° above your north-western horizon, as dusk fades to darkness. It will then sink towards the horizon, becoming tricky to observe after 17:58 when it dips 20° above your north-western horizon. At 02:40, it will return to an altitude of 20° above your north-eastern horizon, and reach an altitude of 46° before fading from view as dawn breaks at around 05:58.

The events that comprise the 2013–2014 apparition of C/2012 S1 (ISON) are as follows:

The table below lists the times when C/2012 S1 (ISON) will be visible from Cambridge day-by-day through its apparition:

A more detailed table of C/2012 S1 (ISON)'s position on each night is available here. A diagram of the orbit of C/2012 S1 (ISON) is available here.

Finder chart

The chart below shows the path of C/2012 S1 (ISON) over the course of its apparition, as calculated from the orbital elements published by the Minor Planet Center (MPC). It is available for download, either on dark background, in PNG, PDF or SVG formats, or on a light background, in PNG, PDF or SVG formats. It was produced using StarCharter.

Click to expand »

Comet brightnesses

Comets are intrinsically highly unpredictable objects, since their brightness depends on the scattering of sunlight from dust particles in the comet's coma and tail. This dust is continually streaming away from the comet's nucleus, and its density at any particular time is governed by the rate of sublimation of the ice in the comet's nucleus, as it is heated by the Sun's rays. It also depends on the amount of dust that is mixed in with that ice. This is very difficult to predict in advance, and can be highly variable even between successive apparitions of the same comet.

In consequence, while the future positions of comets are usually known with a high degree of confidence, their future brightnesses are not. For most comets, we do not publish any magnitude estimates at all. For the few comets where we do make estimates, we generally prefer the BAA's magnitude parameters to those published by the Minor Planet Center, since they are typically updated more often.

No estimate for the brightness of comet C/2012 S1 (ISON) is currently available.

The comet's position at perigee will be:

The coordinates are given in J2000.0.

The sky on 14 Mar 2025

Source

This event was automatically generated on the basis of orbital elements published by the Minor Planet Center (MPC) , and is updated whenever new elements become available. It was last updated on 23 Feb 2025.

Image credit

© Andy Roberts 1997. Pictured comet is C/1995 O1 Hale-Bopp.

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