Reply To: Mercury’s socium tail

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Chris Hooker

Thanks for the link, Mike.
Strictly speaking the +/-16 days from perihelion are the times when Mercury’s radial velocity is greatest, and therefore the excitation of the tail is also greatest. The best time to image it may differ significantly from that depending on where Mercury is with respect to elongation. In the recent observing window I was able to image in the evenings 2, 3, 6 and 7 days after perihelion, and by 6 days the tail was already relatively bright and extended. If I had been able to image on the previous evening I’m fairly sure I would have captured something then. On the 17th the radial velocity was at its maximum, but Mercury was very faint and sinking rapidly lower in the twilight, so that was far from the optimal time to capture the tail. The best times are when Mercury is 16 +/-10 days from perihelion and simultaneously within about 10 days of greatest elongation (eastern in spring, western in autumn). Those two conditions do not necessarily coincide!