Thursday, November 14, 2013

Comet C/2012 S1 ISON fires up! 15-22 November 2013

Location of comet C/2012 S1 ISON in the solar system on 15 November. Click to embiggenMorning sky from Friday November 15 to November 22 looking north-east as seen from Adelaide at 5:00 am ACST in South Australia. Comet C/2012 S1 ISON is close to the bright star Spica. Similar views will be seen elsewhere at the equivalent local time (click to embiggen).

Comet C/2012 S1 ISON has turned on. Gianluca Masi has seeing it naked eye using averting vision from a good dark site. The TRAPPIST team have reported a doubling of gas production in the last 24 hours, and creditable reports put the comet between magnitudes 6.5 and 6.1. This represents around a full magnitude jump in 24 hours.

What this means for the comets future is uncertain, this may be a prelude to a more steady brightening, as the comet is well inside Venus's orbit, or it may be the prelude to disintegration. Only time will tell but now is a good time for observation.

Still, unless there is significant brightening in the next few days it will still be difficult to see from Australian sites.  Visible in 10x50 binoculars it has been described as having a star like appearance. However,  its fuzzy, diffuse nature, its closeness to the horizon (between 2 -8 degrees depending on where you are, except Darwin and Far North Queensland, where it is more like 16 degrees above the horizon see here for charts illustrating the difference at different sites)  means that it can be difficult to spot visually in binoculars and smaller scopes.

The comet is to the right of Mars, and heading for the bright star Spica.  Making finding it relatively easy on a line between Spica and Mars. The best time to look for it is when it is highest in the sky and the sky is darkest.  However, especially in the Southern States, because it never gets very high, the best time to look is astronomical twilight. Astronomical twilight is an hour and a half before local sunrise. 
On November 15 the comet is still in the constellation of Virgo, and is between Earth's and Venus's orbits (see image above). At this time, at astronomical twilight, comet ISON is just two fingers width above the horizon as seen from the latitude of Melbourne, just under a hand span as seen from the latitude of Sydney, just under two hand spans as seen from the latitude of Brisbane and almost 3 hand spans as seen from the latitude of Darwin. A hand-span is the distance across your knuckles when your hand is outstretched at the end of your arm as if making a "stop" sign.

On the 15th comet ISON is a handspan above and to the right of the bright star Porrima, and should be (just) visible to the unaided eye and easily visible in binoculars. However, the comet is also sinking towards the horizon, so you will have to wait deeper in the twilight for the comet to get reasonably high above the horizon, as the sky brightens the comet will be more difficult to see, offsetting its own rise in brightness.

The comet is also moving very quickly now, as it begins its final dive into the sun. Over the next 4 days the comet quickly approaches the bright star Spica, the obvious and brightest star in the constellation of Virgo. Comet ISON is closest on the 18th and 19th and should be very easy to pick up with this obvious guide post.

After this there are no obvious bright stars to act as guide posts, hopefully the comet should be fairly obvious. Whether it will outpace the rising dawn, and be come easily visible to the unaided eye without binoculars is uncertain at this stage. The comet is sufficiently low that you need to start looking at nautical twilight, and hour before local sunrise.

A printable PDF map is here.

A black and white map suitable for printing for use with binoculars and telescopes. The large circle is the approximate field of view of 10x50 binoculars.Time is astronomical twilight.Click to embiggen. The image is the same orientation as the horizon animation above. Click to embiggenA high power stellarium view of the region around Porrima and Spica with ISON (expanded from the unaided eye view shown above) on November 18th at Nautical twilight. ISON is close to Spica at this time.  Click to embiggen.


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