Jon's Comet Blog

There is a lot of dross available on the internet, much of it contributed by "experts".  This blog will attempt to redress the balance and give the novice some guidance on points to look for to assess whether a site is authoritative or not.  The initial posts were actually written at the same time, but given different dates to spread them out a bit.  To come - more on light curves.

2013 December 11.  This morning I was interviewed on the local radio about the Rosetta spacecraft.  Nothing had actually happened to it, and nothing will until 2014 January 20 at 10:00 UT when it comes out of hibernation.  So why the interest?  Because of a press release put out at the big AGU meeting in America.  The press release does not say anything new, but recycling of old news is becoming commonplace.  The same meeting announced yet again the discovery of water on Mars, and also the measurement of a record low temperature in Antarctica.  This won't stand as a record because it was measured from space using infra-red and is a measure of the ground temperature.  Under clear winter skies the ground temperature is lower than the air temperature, and in Antarctica strong inversions often make it much lower.  It isn't even the lowest temperature measured over Antarctica - winter temperatures in the ozone layer can get below -100C.   Such press releases are misleading and certainly not the best practice in scientific ethics.
2013 September 2.  It is curious how many experts are convinced that they are right and others are wrong.  As soon as a magnitude estimate either matches their opinion or disagrees with it, out come the comments on comets-ml.  These experts seem to forget that all observers see things differently.  George Alcock's observations of comet tails were thought to be a figment of his imagination, but once CCDs started to image comets some of the types of structure that he drew were found in the images.  Electronic devices are more objective, but how many have been compared one against another for standardisation?  Because observers are different they will inevitably report different magnitudes, and the differences may vary from comet to comet depending on its peak emission wavelength.  Far too many predictions take no account of this variation, nor of the random errors of observation.  It is still far too soon to make any confident prediction of how bright 2012 S1 will get at perihelion, but on the basis of observations received so far, it is more likely than not to be brighter than Venus, but I wouldn't bet on it.
2013 March 14.
  Many observers these days take quite good images of comets, and for a bright comet images come flooding in.  We give very clear guidance on how the images should be named, but sadly many keen and enthusiastic observers don't seem to read them.  There is also a lot more that observers need to do before the data contained in their images can be of use.  You can measure the position of the comet and submit this to the MPC.  You can process the photometry of the comet and submit it to me in ICQ format.  Roger Dymock has developed excellent guidance on how this should be done.  Of the rest who are for one reason or another unable to do this, some of the images might be used to illustrate reports on comets, some may be of use to professionals studying tail development, but sadly most will just gather dust in the archives.  It is the responsibility of the observer to turn their observations into a useful format.  As an analogy much of what is being received is comparable to a variable star observer reporting that they can see Betelgeuse.  
2013 March 10. 
There is often confusion between precision and accuracy.  It is instructive to compare comet orbital elements given in the MPECs and on the JPL Horizons website.  The MPECs almost invariably give elements with a precision of 5 decimal places, with no indication of error, so one might assume they are that accurate.  However, they are not accurate to that precision as simple observation of successive orbits of recently discovered objects shows, often changing by whole degrees never mind decimals.  JPL give even higher precision, but also give an estimate of the uncertainty.  Looking today at their elements for 2013 E1, it is clear that the orbit for that comet is still quite uncertain, with a possible range of perihelion dates of around a year.
2013 March 9.  2012 S1 (ISON).  Many authors and the media are going into hyperbole about this comet, on the basis of very little evidence.  Some say "We remember comet Kohoutek and aren't going to make the same mistakes" then promptly do.  It was clear fairly soon after discovery that the orbit was nearly parabolic, and there was no possibility that it was a returning comet.  This doesn't stop some experts still saying that with a few more astrometric observations there is bound to be a link with comet 1680 V1.  Wrong, there is no link.  The same experts are also stating unequivocally that the comet will reach -12.  What is this based on ?  A few rough astrometric nuclear or quasi-total magnitude estimates and a range in solar distance of some 2 au, when still 5 au from the Sun.  This is only enough to determine an approximate absolute magnitude if you assume the rate of brightening.  Taking a value of 10 log r and extrapolating from 5 au to 0.006 au gives you -12.  The error bars remain huge and anything between -12 and 6 is not unreasonable.  Indeed it could be worse than 6 if the comet fragments prior to perihelion as many have.
2013 March 8.  Comet Names.  The IAU has a clear resolution on the naming of comets, yet few professionals are prepared to follow it, and some actively encourage the media to get it wrong.  I gave an analogy at a recent meeting at the Royal Astronomical Society at Burlington House in London, which has as a near neighbour the Linnaean Society.  If a speaker at an expert meeting of that Society described a plant as a Blackberry it would be clear that either the speaker was not in fact an expert or that the audience were not.  If another speaker referred to Rubus, then the subject would clearly be botanical.  If a speaker discussed the difference in leaf morphology between Rubus cantabrigiensis and Rubus babingtonianus then it would be for expert batologists.  Many of the speakers at the RAS meeting used the astronomical equivalent of Blackberries, with mention of comets Pan-STARRS and Garradd.  Sooner or later Pan-STARRS will discover two comets that become bright at the same time and then confusion will be complete.  Just use 2011 L4 if you actually want to be clear - after all most fans (and Google) can distinguish between a DB6 and a DB9!
2013 March 7.  There are many sites giving light curves for comets and using these to predict future brightness.  Few give any indication of the errors associated with the magnitude parameters that have been derived.  For a comet some way from perihelion these can be large.  Prior to the recovery of 2011 L4 after its 2012 November solar conjunction, the errors associated with the perihelion brightness prediction were considerable, with a range of around 10 magnitudes being possible.  As more observations accumulated the error bounds reduced, but it is important to realise that a simple light curve ignores much of the physics that may be happening on the nucleus - changing illumination of topography, other seasonal effects, changing activity etc.  Surprises are therefore likely and 2011 L4 duly obliged, becoming around 2 magnitudes brighter than the error bounded envelope of what the equation suggested.  It could be an observational error - the comet is at low altitude and extinction corrections will be large, but wait for a week or two before condemning the observers.
2013 March 6.  "The tail of 2011 L4 MUST be longer than shown in the images."  Why?  A good scientist makes predictions based on a theory, but may have to modify the theory if the observations don't support it.  This was the case with Newton's theory of gravity, which couldn't explain the observed motion of Mercury.  Einstein came up with a more complete theory, which explained the motion of Mercury better, but his is not necessarily the final word.  Anyone who simply says the observations don't fit my theory, so the observations must be wrong is probably not a scientist.  Equally anyone who says the observations don't fit the theory so the theory must be wrong could equally not be a scientist, unless they are prepared to suggest an alternative theory that explains them better.  In both cases the errors need to be assessed - it could simply be that the actual observational errors were greater than the observer thought.