50 years ago today, at 16:09 UTC on 17 July 1975, the Apollo-Soyuz Test Project culminated in the first docking of the spacecraft - but afterwards an interesting experiment took place: after they separated again Apollo (the final one; no number) occulted the Sun from the point of view of Soyuz 19. Images obtained show mostly outgassing from Apollo forward-scattering light from the hidden Sun but allegedly there is also some outer corona in the images of which https://www.esa.int/ESA_Multimedia/Images/2010/04/Artificial_eclipse_produced_by_US_Apollo_spacecraft shows one with a negative caption. The planned artificial #eclipse experiment was described in the mission press kit https://www.nasa.gov/wp-content/uploads/static/history/astp/documents/astp%20press%20kit%20(us).pdf on PDF pages 37-39 and the outcome is discussed in the didactical NASA brochure https://ntrs.nasa.gov/api/citations/19780019206/downloads/19780019206.pdf on PDF pages 20-27; there is apparently also a more technical paper in a mission science report but I couldn't find that one online.