The Green Flash

 In the mid 1950s, observatory director Fr. Daniel O’Connell SJ and Br. Karl Treusch SJ (see photo on east wall) carried out research on the “green flash”: when the atmosphere is particularly clear and calm, the last segment of the sun at sunset (or the first at sunrise) appears green. 

For a long time scientists were divided in their opinion as to whether this was real or just an optical illusion. O’Connell could see it very clearly from the window of his office at Castel Gandolfo looking out over the Mediterranean, and so he conceived the idea of obtaining color photographs of the phenomenon by using the Specola’s telescopes. 

The work was not trivial; in addition to the fact that the green flash is not easy to predict, the phenomenon lasts for an extremely short time and it is, therefore, very difficult to photograph. But after some initial tests, Br. Treusch became enthusiastic about the work. They eventually co-authored a 200 page book, The Green Flash and Other Low Sun Phenomena (1958). Their photos were republished in many places, including the cover of Scientific American for January, 1960 [image available]. For many years it was one of the most-referenced results to come from the Specola Vaticana.