Total solar eclipses are rare events. Although they occur somewhere on Earth every 18 months on average, it is estimated that they recur at any given place only once every 360 to 410 years, on average. The total eclipse lasts for only a maximum of a few minutes at any location, because the Moon's umbra moves eastward at over 1700 km/h. Totality currently can never last more than 7 min 32 s. This value changes over the millennia and is currently decreasing. By the 8th millennium, the longest theoretically possible total eclipse will be less than 7 min 2 s. The last time an eclipse longer than 7 minutes occurred was June 30, 1973 (7 min 3 sec). Observers aboard a Concorde supersonic aircraft were able to stretch totality for this eclipse to about 74 minutes by flying along the path of the Moon's umbra. The next total eclipse exceeding seven minutes in duration will not occur until June 25, 2150. The longest total solar eclipse during the 11,000 year period from 3000 BC to at least 8000 AD will occur on July 16, 2186, when totality will last 7 min 29 s. For comparison, the longest total eclipse of the 20th century at 7 min 8 s occurred on June 20, 1955, and there are no total solar eclipses over 7 min in duration in the 21st century.