The earliest datable noctilucent cloud observation (Parma, Italy, AD 1840)
Noctilucent clouds (NLCs) are an uncommon phenomenon that provides information about the conditions and dynamics of the mesosphere. The first observation of NLCs was recorded in 1884/1885, following Krakatoa’s eruption in 1883. The literature speculates that this observation was trigged by the injection of millions of tons of H2O by the Krakatoa into the stratosphere. We have discovered that 43 years before Krakatoa, Antonio Colla observed an NLC in Parma. He was a meticulous astronomer and meteorologist with special interest in astronomical and atmospheric phenomena occurring during twilight. On 18 June 1840, from 21:00 to 22:15 (Local Mean Sideral Time), Antonio Colla observed a ‘phosphoric cloud’. Analysis of the Colla’s description, the local sky and the condition of the observation proves that he was recording an NLC. This finding forces to develop a new hypothesis to explain the early NLC observations and encourages the rescue of NLC observations from documentary sources.