Can we predict the (non-)visibility of C/2024 G3 around peak brightness in strong twilight from the recent experience with C/2023 A3's around its solar conjunction? The following assumes a) that G3 follows the brightness slope used by JPL Horizons right now all the way to perihelion and back, b) that the offset of actual observations in COBS from what Horizons predicts remains constant, i.e. that the comet is ~1.7 mag. brighter (no change of slope and no significant scattering effects), and c) that the degree of condensation and thus strength to make it through a severely brightened sky is similar. Here is what I get, for my place in Germany (Bochum at 51.5¡ã North) and Johannesburg at 26¡ã South:
?
Jan. 07 | +2.0 mag. | Elongation 14¡ã | vertical Delta for GER 4¡ã | vertical Delta for RSA 12¡ã
Jan. 08 | +1.2 mag. | Elongation 13¡ã | vertical Delta for GER 4¡ã | vertical Delta for RSA 10¡ã
Jan. 09 | +0.5 mag. | Elongation 12¡ã | vertical Delta for GER 5¡ã | vertical Delta for RSA 10¡ã
Jan. 10 | -0.5 mag. | Elongation 10¡ã | vertical Delta for GER 5¡ã | vertical Delta for RSA 7¡ã
Jan. 11 | -1.7 mag. | Elongation 09¡ã | vertical Delta for GER 5¡ã | vertical Delta for RSA 5¡ã
Jan. 12 | -2.9 mag. | Elongation 07¡ã | vertical Delta for GER 5¡ã | vertical Delta for RSA 1¡ã
Jan. 13 | -3.8 mag. | Elongation 05¡ã | vertical Delta for GER 3¡ã | invisible in the RSA
Jan. 14 | -2.9 mag. | Elongation 06¡ã | vertical Delta for GER 4¡ã | vertical Delta for RSA 2¡ã
Jan. 15 | -1.6 mag. | Elongation 09¡ã | vertical Delta for GER 4¡ã | vertical Delta for RSA 6¡ã
Jan. 16 | -0.5 mag. | Elongation 11¡ã | vertical Delta for GER 4¡ã | vertical Delta for RSA 9¡ã
Jan. 17 | +0.5 mag. | Elongation 13¡ã | vertical Delta for GER 3¡ã | vertical Delta for RSA 11¡ã
Jan. 18 | +1.2 mag. | Elongation 15¡ã | vertical Delta for GER 2¡ã | vertical Delta for RSA 13¡ã
Jan. 19 | +2.0 mag. | Elongation 16¡ã | vertical Delta for GER 2¡ã | vertical Delta for RSA 16¡ã
?
Note that this applies to the morning only until Jan. 12, for the morning and evening about equally on Jan. 13 (perihelion is near noon in Europe and Africa) and to the evening only from Jan. 14. "Vertical Delta" means the actual difference in altitude of the comet and the Sun at any given time rising or setting, respectively, and it is always smaller than or (when the comet sits straight above the Sun) equal to the elongation. E.g. with a Delta of 12¡ã you can have the comet 6¡ã above and the Sun 6¡ã below the horizon or the comet 3¡ã above and the Sun 9¡ã below the horizon etc.
?
This Delta had the greatest effect on the visibility of comet Tsuchinshan-ATLAS in the days before and after solar conjunction, more so even than the brightness: from what I gather from the (few!) reports in https://cobs.si/obs_list?id=2410 (page 4 by now) as well as my own observations (quoting a Delta for Europe here) from Oct. 11 to 14 this comet was
- invisible at twilight on Oct. 10 with a Delta of 6¡ã while at roughly -3 mag.,
- visible with great difficulty on Oct. 11 with a Delta of 10¡ã while at roughly -1 mag.,
- easy, esp. with binoculars, on Oct. 12 with a Delta of 14¡ã at about +0.3 mag.,
- pretty easy with long tail on Oct. 13 with a Delta of 18¡ã at about +1.3 mag.,
- spectacular with a long tail on Oct. 14 with a Delta of 21¡ã at about +1.7 mag.
So in a nutshell for Germany the Delta never exceeds 5¡ã when the comet is brighter than +2 mag., which - applying the Tsuchinshan-ATLAS experience - means that we are completely screwed ... unless something really wild happens in the last few days before perihelion and the brightness rises several magnitudes above the already optimistic brightness model here. (Or the tail's length and esp. surface brightness comes to the rescue, but that would probably require West-/Lovejoy-style nucleus damage at precisely the right instant ...)
?
75¡ã further south the situatiion is worse on perihelion day +/- one day but better before and especially a few days after perihelion: around Jan. 18-20 ATLAS, performing to plan or better, may actually briefly approach the interesting visibility of Tsuchinshan-ATLAS on Oct. 12-13. But a view approaching the latter's best performance - on Oct. 14 - is mathematically impossible even in the Southern hemisphere as the elongation doesn't rise fast enough while the coma brightness fades. Again, though, a fine tail performance may help to some degree.
?
Daniel
