Hi Nick,

in my opinion, it is somewhat difficult to use those 9" measurement for an
overall analysis of lightcurves if the distance of the comet from earth is
changing a lot within the time frame considered.
E.g. in case of C/2023 A3 this distance decreased by more than a factor of 3
between 2023 April and 2024 June. The early mesurements are therefore tracing
a much larger volume of the coma and appear brighter by large amount (1-2 mag).
The lightcurve will be skewed a lot and deriving model parameters similar to
total coma magnitude lightcurves should be considered with caution.

The peak in April is mostly affected by the phase effect as you mentioned, but
also by the projection of the dust tail within the mesurement aperture during
that time.

Best regards
Thomas

> Am Fri, 31 Oct 2025 18:38:49 +0000
> schrieb Nick James <ndj@nickdjames.com>:
>
> With all the current excitement about C/2025 A6 (Lemmon) we shouldn't
> forget the much brighter comet that we had this time last year. C/2023
> A3 was a really impressive naked-eye object in mid-October 2024 but it
> has now faded to around 18th magnitude although it is still available in
> the evening sky.
>
> In response to a query from Jonathan Shanklin I have updated the 9
> arcsec lightcurve. The lightcurve of this comet is very unusual. The
> blue curve shows a simple fit to the data points from 2024 July 1
> onwards. It is a passable, although not very good, fit. The period up to
> 2024 April has a rather poor fit to a different curve. The peak around
> 2024 April 15 is supposed to be mainly due to the phase angle effect on
> the very dusty tail which, at that time, was projected behind the comet,
> but that doesn't really explain why the comet was a lot brighter
> throughout the period up to 2024 April.
>
> Jonathan specifically requests if there are any images that can provide
> data in 2025 January. The comet was at a small elongation then and was
> moving from the evening to the morning but if you do have any images
> which you haven't measured and submitted then please contact me.
>
> It is worth keeping this comet under observation and submitting your
> photometry results. The recent large scatter in data is probably due to
> the very crowded field and the high probability that faint stars are in
> the 9 arcsec radius aperture.
>
> Nick.
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