Seen from Maintenon / Chartres France
TS130mm telescope on MK2 mount, ASI178 cam approx.30 / 300 images, despite terrible weather in the Beauce during this night.
FireCapture, AS3, Registax
It is the grazing lighting on the terminator, during certain first quarters of the Blanchinus, La Caille and Purbach craters by the Sun illuminating the summits which draws the letter X.
This phenomenon only happens a few times a year and only lasts a maximum of 4 hours and a little less for the 4 branches of the X
We can also see, a V on the Ukert crater
His next 2021 appearances will be July 16, September 13, and November 11.