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April 9: The Twelve Years' Truce is signed by the Netherlands and Spain August 25: Galileo demonstrates his first telescope.
1609 (MDCIX) was a common year starting on Thursday of the Gregorian calendar and a common year starting on Sunday of the Julian calendar, the 1609th year of the Common Era (CE) and Anno Domini (AD) designations, the 609th year of the 2nd millennium, the 9th year of the 17th century, and the 10th and last year of the 1600s decade. As of the start of 1609, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.