As you probably know, some of us must be able to observe (weather permitting) the next transit of Mercury on November 11, 2019.
In Europe, we are able to see the first contacts and from America, just the last ones.
One of my friends: Jean Bourgeois ask to try to determine the depth of the solar chromosphere. I do not know if you know about it, but we do not yet know exactly how big that chromosphere is.
The next transit of Mercury allowed observers to time the beginning and / or end of this event in two different wavelengths: in green light, corresponding to the photosphere, and in Halpha, corresponding to the chromosphere. The difference between these two times would give an estimate of the thickness of the chromosphere at the solar latitudes concerned. We do not take into account chromospheric spikes, but only the chromospheric layer, which in fact corresponds to the difference of the sun's radius in these two wavelengths.
This type of observation is rarely achieved. The literature mentions only three observations, one by Russ Sampson in 1994 (Sky & Telescope, November 1994), the other by Jean Bourgeois himself in 1996 (IOTA / ES, symposium of 1997) and the last of April 2003 via IOTA and the association of solar observers also by Jean Bourgeois. Unfortunately, almost all the efforts to time the events in Halpha failed, due to clouds, low-precision images or technical problems.
We have now a good program to reduce the observations, this was done by another of our friends, Jean Meeus.
So, if you want to be part of this new challenge, do not hesitate to contact me via this CN post or directly via Private Message.
I attach here under the paper that Jean Bourgeois wrote in 2003, with more explanations on this phenomenon and on the way to organize the observation.
Mercury-Transit.pdf 368.33KB 88 downloads
Clear sky to you all