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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!Chris
Thanks for your comment, James, and I’m glad you enjoyed the talk.
The recent window for viewing the tail has now ended. Unfortunately the April weather continued to be unfavourable so I didn’t obtain any more images.
The next window is the morning elongation in January 2024. It’s not as good as the last one, with Mercury reaching 4 degrees altitude while the Sun is still below 10 degrees from the 4th to the 10th of January. After that there is another opportunity in the evening from the 22nd to the 30th March, during which Mercury will be significantly higher.
Ii’s been suggested that I should write a short article for the Journal based on my talk at Winchester, and I shall probably do that and include information about times when the tail can be detected. For now I’m attaching the slide from my talk that showed the observing windows for the next few years.Chris
Attachments:
For what it’s worth, in most cases I don’t think it matters which way up an image or an observation is presented, provided the directions are indicated (or obvious). The exception, I would say, is the Moon, which we in the northern hemisphere see as north up without optical aid. Presenting lunar images or drawings with south up is just being contrary for no good reason, although observers in the southern hemisphere would no doubt say the opposite! The argument that we should stick to the tradition is very weak, given that the instruments and techniques in use nowadays are far beyond what those who established the tradition could even have dreamed of.
The definition of astronomical twilight is the geometrical condition that the centre of the Sun is between 12 and 18 degrees below the observer’s horizon. In more practical terms it also means that when the Sun is lower than 18 degrees, the atmosphere above you is no longer illuminated directly by sunlight; the Sun is setting for another observer about 2000 km away. However, even if the Sun is more than 18 degrees below your horizon, the sky in your location could still be slightly illuminated by the brighter twilight sky in the direction of the hidden Sun. If locations 2000 km away were also in astronomical twilight, then the indirect illumination would be negligible, and your sky would be as dark as it can get. This would presumably require the Sun to be around 36 degrees below your horizon. Do you have any timings or measurements of how the sky quality changes, and when the effect you’ve seen ends?
