»Once in a blue moon« is a famous English expression meaning »very rarely« or »once in a while.« The expression derives from the astronomical term Blue Moon, which refers to the rare occurrence of two full moons within the same calendar month. Since the lunar cycle lasts approximately 29 and a half days, this phenomenon rarely occurs. This photo is the second full moon of May (the Blue Moon) and was taken on May 31, 2026, at the Castle of Santa Severa (Rome). The Castle of Santa Severa owes its name to Severa, a young Christian martyr who is said to have died here on June 5, 298 AD, during the reign of Diocletian, along with her brothers. The early Christian church, dating from between the second half of the 5th and the first decades of the 6th century, is dedicated to her. Part of the church is visible in the Piazza della Rocca. Technical data: Fusion of three shots taken with a Fujifilm GFX 100S II and GF 500/5.6 at 1250 ISO, the castle shot at f/7.1, 2-second exposure. The sky at 1/2 second at f/9 and the moon at 1/60 f/9.

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