Observing circumstances for 2012 S1 (ISON)


2013 November 28.  Update
2013 March 10.  Images added
2013 February 23.  Pages created in draft.  

Observing diagrams

To see the full resolution diagram click on a thumbnail. The diagrams were generated using the GraphDark software written by Richard Fleet.
  50N 30N 10N 10S 30S
2013 March -
2014 May
12s150na.png (36248 bytes) 12s130na.png (34879 bytes) 12s110na.png (30694 bytes) 12s110sa.png (29775 bytes) 12s130sa.png (29493 bytes)
2013 June -
2014 August
12s150nb.png (36831 bytes) 12s130nb.png (33588 bytes) 12s110nb.png (29898 bytes) 12s110sb.png (28664 bytes) 12s130sb.png (28420 bytes)


 Ephemerides for 2012 S1 (ISON) at 50N , 40N , 30N , 10N , 10S , 30S and 50S which give 1950 and J2000 positions and approximate observing windows for the period 2013 February 23 until 2014 August 31. The light curve is a complex function of the comet's composition, shape, active area, rotation rate and season.  I chose to approximate the magnitudes using the equation m = 5.3 + 5 log d + 7.5 log r for these ephemerides, however the comet has actually brightened much slower than this, with a fainter absolute magnitude.  The true light curve can only be determined after observations confirm the comet's behaviour.  

 2012s1.jpg (346498 bytes)Projected light curve [click on the image for the latest version, 2013 November 28].  Note:
a)  The error bars remain the same.  The perihelion brightness could be around the brightness of Venus, though a brighter magnitude is within the range of the error bars.  It does not remain this bright for very long.
b)  The aperture corrected light curve is currently m = 7.30.1 + 5 log d + 6.20.3 log r
c)  The fitted curve generally appears above the observed points as it has been corrected for aperture in order to predict the naked eye brightness.
d)  The comet's future brightness may deviate from the fitted curve.  
e)  The comet is still brightening and the rate of brightening has increased.  The overall curve suggests that the comet is now brightening faster than a reflective body.  The log r parameter has increased with the addition of this week's data.
f)  John Bortle suggests that a comet has to be brighter than H10 = 7.0 + 6q = 7.1 to survive perihelion.  For 2012 S1, H10 is 7.50.1, and the absolute magnitude is 7.30.1 so it appears to be beyond this limit.



  • Faulkes Telescope observing notes.
  • Comparison star charts


A workshop on ground-based support for Rosetta was held at University College, London on 2012 April 17 and 18.  Meeting notes.

There was a joint Asteroids and Remote Planets Section and Comet Section meeting on 2012 October 6.  This was held in Milton Keynes and included discussion of Rosetta.  For the meeting notes see The Comet's Tale for 2012

There was a Comet Section meeting on 2013 May 18 to discuss observing techniques.  The video of the meeting is on the BAA web and a full write up will appear in "The Comet's Tale". 

We hope to obtain EU funding to support a pro-am European meeting prior to the Rosetta encounter.


  • Europlanet.
  • Sign-up to the Matrix [not yet available]

EU National Groups

Published by jds [at] ast.cam.ac.uk