Estimating comet magnitudes

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    Dominic Ford

    As some people will be aware, I’ve had a project going for a while to produce ephemerides listing the future brightnesses of comets. Some of the results end up on (and perhaps even in the BAA Observing Calendar), but I tend to hide them away because they’re mostly very bad. A complaint from somebody earlier today about my dodgy predictions encouraged me to have another look into what’s going wrong.

    Looking at the prospects for 2020, Jonathan Shanklin’s Comet Prospects for 2020 says, for example, that 289P/Blanpain will probably not exceed 18th magnitude.

    I tried to reproduce this value from the comet’s absolute magnitude (H) and slope parameter (G). Roger Dymock gives an equation of the magnitude of a comet here, which is equivalent to equation 33.14 in Jean Meeus’s “Astronomical Algorithms”.

    If I go onto the Minor Planet Center website… (file CometEls.txt)

    … I can look up the values of H and G, which are given as H=10, G=4. In December 2019, Blanpain will be about 1 AU from the Sun, and about 0.1 AU from the Earth (see, e.g. <>)

    Crunching the numbers together, I get an estimate for Blanpain’s brightness which is around 5th magnitude — completely wrong. But this seems in keeping with the data on the MPC website: if the absolute magnitude is H=10, then they’re saying it would be 10th magnitude when 1AU away from the Sun and Earth. In practice, it’s much closer to the Earth than that.

    So, what data do people like Jonathan use when they make future predictions of the brightnesses of comets like this? Am I missing something, or is the data on the MPC website complete nonsense?


    Hello Dominic,

    For many years I used to try to do this for the annual Yearbook of Astronomy. It was a bit of a nightmare. The basic problem is that comets are a law unto themselves. Only the magnitudes of the best behaved well-established periodic comets can be predicted with some certainty, and that is rarely as good as +/- 1 mag!  My approach with periodic comets was simply to see how they behaved last time round, see what Guide predicted for that apparition, note the error, and then see what Guide said for the next apparition!

    The MPC website can give reasonable mag predictions for well-established periodic comets, now and again, but for new comets it is often very inaccurate. Brand new comets may have been discovered in outburst or may brighten rapidly in the weeks after discovery.

    As for Blanpain, well, that is a rather extreme example as it was healthy in 1819, then declared ‘dead’, then rose from the dead in 2003 – 2005. So, anyone who can accurately predict its magnitude for the coming apparition would be a genius!

    I really wouldn’t worry if your predictions are ‘dodgy’ Dominic as predicting the magnitudes of comets is like nailing multiple jellies to the wall….


    Dominic Ford

    Hi Martin,

    Thanks for the reply. It’s good to know that I’m not alone.

    My reaction would be to query what the point of publishing these absolute magnitude values is, if the numbers produce such wild predictions. Surely, the only reason to publish absolute magnitudes is for people like me to use them to predict the future brightnesses of comets. Isn’t it inviting naive people like me to start telling the world that Blanpain will be a fifth magnitude comet?

    In the past I’ve heard people express exasperation when comets get over-hyped, only to disappoint. And so, wouldn’t it be better to publish absolute magnitudes at the conservative end, rather than ones that lead to wildly optimistic forecasts? It would help if there was any kind of error bar quoted, to signify which absolute magnitudes are vaguely trust-worthy.

    I should add that the absolute magnitudes on the BAA Comet Section pages are much more reliable than those on the MPC website – I suspect in part because the BAA only publishes values for moderately well-behaved comets. The BAA does not, for example, publish any values for Blanpain.

    Even then, by my calculations, the BAA’s published absolute magnitude for 2017 T2 puts it at magnitude -1 next May, which is about 5-6 magnitudes brighter than most people seem to be expecting.

    Best wishes,



    Hello again Dominic,

    >My reaction would be to query what the point of publishing these absolute magnitude values is, if the numbers produce such wild predictions.<

    I think you might be assuming that the MPC/IAU/CBAT have a team of dedicated comet experts calculating the absolute magnitudes, but they don’t. They are a small group of people who are really, in the case of the MPC, orbit experts, but not light curve experts. When something goes wrong with their computers they take months to fix because they have little spare capacity…this is currently the case with their orbital elements for software packages data….not updated for months!

    Really, where magnitude data is concerned, it’s the amateurs who know the most about likely comet performances, based on the developing light curve over recent months and previous apparitions. The guys at the MPC have to put a value for H into the data and presumably it is generated from the initial discovery mag, taking 5 log delta and 10 log r into account, but with little feedback from observers, apart from the magnitude column in astrometric measurements. While the astrometry will only be accepted if it is of a high standard the magnitude data is often very inaccurate. In contrast, the magnitude data found at COBS ( ) is likely to be far more accurate and would be far better for deriving a comet’s likely performance. If the MPC analysed data from, e.g., the BAA’s Comphot software I’m sure their predictions would be far more accurate, but they just don’t have the manpower to keep up to date with all the brightening comets around at any one time. Even if they did, comets are always going to be a law unto themselves as they approach the inner solar system!


    Nick James

    As Martin noted the magnitude params computed by the MPC are based mostly on estimates made as a byproduct of doing astrometry. They are therefore generally based on nuclear magnitudes which will be underestimates for comets with larger apparent diameters. The COBS data are generally total magnitudes and the magnitude parameters derived from those are generally better than the MPC ones assuming that there are enough observations to work with. Jonathan uses total magnitudes too and adds a human element and much experience.

    The magnitude of a comet depends on its activity level and this can be quite unpredictable. Even well-known periodic comets can behave badly (cf 17P/Holmes). That is what makes them fun to observe! You identified a good example in 289P/Blanpain which was mentioned in the 2019 Handbook with a big warning about its magnitude:

    C/2017 T2 is another example where the mag params are not well known at the moment:

    Again, this is why we are observing it.

    I guess that the key message is that no fully automated comet prediction website is likely to work. There will always need to be a human element checking what it says and adapting it to the latest observations.

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