La "luna blu" tornerà a farsi vedere oggi, venerdì, 31 luglio. In realtà il satellite della Terra brillerà del suo solito colore argentato, ma la particolarità è che per la seconda volta del mese sarà piena: un fenomeno astronomico piuttosto raro, che si verifica all'incirca ogni tre anni. L'ultima volta era nell'agosto 2012, la prossima sarà nel gennaio 2018.
Nuvole permettendo, la seconda luna piena del mese, dopo quella del 2 luglio, dovrebbe essere visibile a partire dalle 20.19. "Attenzione alle bufale che spesso sulla rete spacciano per 'luna blu' immagini truccate al computer", avverte Paolo Volpini, dell'Unione Astrofili Italiani (Uai).
La vera luna blu
La luna è apparsa azzurra solo in rarissime occasioni, nelle quali grandi quantità di polveri presenti nell'atmosfera hanno agito come una sorta di filtro. «È accaduto nel 1883, in occasione dell'eruzione del vulcano Krakatoa, in Indonesia. Ma si tratta di casi che non hanno nulla a che fare con l'evento astronomico della doppia luna piena in un mese», spiega l'astrofisico Gianluca Masi, responsabile del Virtual Telescope.
L’altra
Viene detta luna blu anche la terza delle lune piene di una stagione che, fenomeno piuttosto inabituale, ne ha quattro.
Diem/ANSA
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