Saros Cycle

The saros is a cycle of 6,585.32 days, about 18 years, 11 days and 8 hours, after which the Sun, Moon and Earth return to almost the same arrangement. Three of the Moon's rhythms line up over that time: 223 synodic months, the cycle of lunar phases; 242 draconic months, the time the Moon takes to return to the same node, where its orbit crosses the plane of Earth's; and 239 anomalistic months, from one perigee to the next. Because all three match so closely, an eclipse is followed one saros later by another of almost the same type, length and geometry. The extra third of a day matters: Earth turns about 120° in eight hours, so each repeat falls about a third of the way round the world to the west. Eclipses linked in this way form a numbered saros series, which begins with small partial eclipses near one pole, runs through central eclipses for centuries and ends with partial eclipses near the other pole, over 1,226 to 1,550 years and 69 to 87 eclipses. About 40 solar series are running at any one time. The total eclipse of 2 August 2027 belongs to Saros 136. Lunar eclipses follow the saros too, in series of their own. Saros 136 has produced some of the longest totalities of the past century, including 7 minutes 8 seconds on 20 June 1955.