Missouri Solar Eclipse

The Saros Cycle

Posted on Thursday, July 20, 2017 at 7:00 pm


             The Saros Cycle


For all earth observers, including the ancients, the Sun is the beginning, the constant, for explaining nature. First, the Sun defines our cardinal directions. Second, the Sun sets up our calendar as the earth revolves around the sun once to make a year. Ancient markers and tablets show reckoning with seasonal markers like summer and winter solstices and equinoxes. To understand and predict eclipses, however, the Moon is a key. There are 2 to 5 solar eclipses somewhere on earth each year.

The periodicity and reoccurrence of solar eclipses is governed by the Saros Cycle. Saros means to repeat. Natural harmony between the three lunar orbital periods and the alignment of the Sun is the key. Ancients found the three lunar orbital periods all were the same for some eclipses.  Those three orbital periods of the Moon are: the synodic or lunar phases at 29.5 days, the draconic movement where the Moon is at the 5° node of alignment at 27.2 days, and the anomalistic movement or having the Moon the same distance to Earth at 27days13hours18.5 minutes.  Babylonians may have been the first, but with many other civilizations were able to see a repetition in the eclipse schedule at 18 years, 11 days, and 8 hours. This is the Saros Cycle.

The Saros is useful in organizing eclipses into families or series. The total solar eclipse of 2017 is a member of the Saros series 145. Each series typically lasts 12 or 13 centuries and contains 70 or more eclipses. Series 145 is a young one and began on January 4, 1639 with a small partial solar eclipse near the North pole. The series continues as the Moon moves southward with the final eclipse of the 145 Saros series taking place April 17, 3009 with a partial solar eclipse over Antarctica.  All eclipses moving southward are odd numbered Saros and where Saros series moving northward are numbered evenly.

Any two eclipses separated by one Saros cycle share very similar geometries. Location, however, is a problem. The additional 8 hours on the repetition means the earth will rotate an additional 120 degrees. Waiting 18 years, 11 days, and 8 hours, the next eclipse of the Saros 145 series occurs on September 02, 2035 across western China and Japan, ending in the Pacific Ocean 1300 miles from the Hawaiian Islands. Waiting 3 Saros cycles will bring it back to our hemisphere, but

it has then moved south. That location of the September 23, 2071, Saros 145 solar eclipse will center on Mexico, a tip of Texas on the Gulf of Mexico, Caribbean, and the northern part of South America.

The total pattern of Saros cycles is very complicated. In general there will be 38-42 solar Saros Cycles and a similar number of lunar Saros Cycles operating at once. Our next total solar eclipse visible in the US will be part of the Saros Cycle 139 on Monday, April 8, 2024.  Part of Southeastern Missouri will be in for totality.

The Antikythera mechanism is thought to be an ancient analogue computer or orrery (which is a mechanical model of the solar system used to represent their relative positions and motions). The device was found in 1900 by sponge divers in a Roman shipwreck off the coast of the island of Antikythera. The mechanical device was heavily corroded with indiscernible inscriptions but ran on a complicated set of dials and more than 30 bronze gears. This ancient Greek machine is regarded by some experts as the world’s first computer which may have predicted solar eclipses based on the Saros formula.

Changing subject matter, one question asked often is why does the eclipse moves from west to east when the both the Sun and the Moon appear to go the other way –east to west. The movement of the Sun and Moon is caused by the Earth’s rotation of 24 hours (counterclockwise from the North Pole). The Moon orbits earth in the same direction that the Earth rotates, but it takes the Moon 27.3 days to make a complete revolution. The Moon’s revolution in 27.3 days is ahead of  the earth’s rotation by a small bit. The Moon’s movement allows it to be in a different position each night among the stars. The Moon moves 13° East or comes up 50 minutes later each night. As the Moon is moving ahead of the Earth’s rotation, counterclockwise, (west to east), the eclipse appears to move from west to east.