This is a view of comet C/2012 S1 ISON, a comet that will be in the news at the end of 2013. Currently beyond the orbit of Jupiter, comet ISON is unusually bright at this distance. When it arrives at perihelion in December, grazing a few million miles from the Sun’s surface, some predictions have the brightness of this comet reaching that of the full moon, but in daylight (i.e. will have the appearance of a small bright white cloud against the blue sky).
The extreme predictions of ISON being a brilliant comet should be taken with significant caution. Comets are notoriously fickle objects. What to expect from this comet depends on your background. The general public pays attention to comets brighter than magnitude 1, and are in the evening sky. Meanwhile amateur astronomers are excited at any comet that exceeds the naked eye limit of magnitude +6 (that’s a hundred times fainter than magnitude +1). The perceived brightness of a comet also depends on the background sky.
A magnitude +1 comet in a dark sky is stunning, but in a twilight sky can be barely visible. SO when you read about comet ISON being near the brightness of the Full Moon, remind yourself that this will occur when the comet is a few degrees away from the Sun in a bright sky. However, if the comet turns out a few hundred times fainter than predictions, it will appear in the evening sky in December as a stunning object, but only assuming it does not break up as it rounds the Sun. If this breakup occurs, the brightness will be short-lived, but has the chance of providing a nice dust tail. For sure, amateur astronomers who are photographing the comet and observing in binoculars and wide-field telescopes will have a great time. If it will be a great public comet remains to be seen, but everyone’s hope are high. Only nature will reveal the reality of Comet ISON.