This month’s total solar eclipse on March 20 belongs to a family of eclipses known as Saros 120. A Saros series is composed of a group of eclipses in which each eclipse is separated from the next (or previous) eclipse by 6,585.3 days. This equals 18 years 10 days 8 hours (or 18 years 11 days 8 hours, depending on the number of leap years over this interval).

The Saros period is special because any two eclipses separated by one Saros are very similar to each other. The Moon is nearly at the same position with respect to its node (the point where the Moon’s orbit crosses Earth’s orbit) and is also at almost the same distance from Earth. Not only that, the eclipse occurs at virtually the same time of year.

These coincidences arise because three of the Moon’s orbital periods repeat after one Saros period of 18 years 10.3 days. The three periods are:

Synodic Month (New Moon to New Moon)    = 29.531 days  = 29d 12h 44m
Draconic Month (node to node)           = 27.212 days  = 27d 05h 05m
Anomalistic Month (perigee to perigee)  = 27.555 days  = 27d 13h 18m

If you work out the math, you find that:

  223 Synodic Months        = 6585.322 days   = 6585d 07h 43m
  242 Draconic Months       = 6585.358 days   = 6585d 08h 35m
  239 Anomalistic Months    = 6585.538 days   = 6585d 12h 54m

The biggest drawback of the Saros period is that it’s not equal to a whole number of days. The extra 8 hours means that Earth rotates an additional 1/3 of a day so subsequent eclipses are visible from different parts of the globe. For solar eclipses in a Saros series, this means that each successive eclipse path shifts about 120 degrees westward.

The paths of solar eclipse of Saros 136 show a westward shift of about 120 degrees with each succeeding eclipse. The northward shift is due to the shift in the Moon’s position with respect to its node. Drawing courtesy of Fred Espenak.

Of course, the agreement between the Moon’s three periods isn’t perfect over one Saros period. Consequently, a Saros series of eclipses has a finite lifetime lasting from 12 to 15 centuries. Each series begins with a small partial eclipse near one of the poles. Each partial eclipse grows larger as the Moon passes progressively closer to the node until its umbral shadow finally crosses Earth producing either a total or annular eclipse. After 50 or 60 of these central eclipses, the Saros series ends with a final group of partial eclipses at the opposite pole.

There are currently 40 different Saros series in progress, each one with its own assigned number. Some of them are relatively young like Saros 145 that includes the next American total solar eclipse on Aug. 21, 2017. Others are old like Saros 120.

This month’s total solar eclipse is the 61st eclipse of Saros 120. The family began with a series of 7 partial eclipses starting on May 27, 933. The first central eclipse was annular and took place on Aug. 11, 1059. After 24 more annular and 4 hybrid eclipses, the series changed to total on June 20, 1582. Subsequent members of Saros 120 were all total eclipses with maximum durations hovering around 2 minutes. One eclipse in the series occurred on Jan. 24, 1925 and passed through New Your City. Another eclipse on Feb. 26, 1979 was the most recent total solar eclipse visible from the continental United States.

The paths of the final 7 total solar eclipse of Saros 120 show both the westward and northern shift of the eclipse paths with each succeeding eclipse. The northward shift is due to the shift in the Moon’s position with respect to its node. ©2015 by Fred Espenak.

The Mar. 20, 2015 eclipse is the 25th total eclipse in the series and actually has one of the longest durations (2 minutes 47 seconds). The next member of the series occurs on Mar. 30, 2033 is the last total eclipse of Saros 120. The following 9 eclipses are all partial terminating with the final eclipse of the series on Jul. 07, 2195. Complete details for the 71 eclipses in the series (in the sequence of 7 partial, 25 annular, 4 hybrid, 26 total, and 9 partial) may be found at: Saros 120.

For more införmation on the Saros and eclipses, see:
Periodicity of Solar Eclipses.

The total solar eclipse of Mar. 20, 2015 is visible from within a wide corridor that traverses the North Atlantic. A partial eclipse is visible from Europe, North Africa and western Asia. ©2014 by Fred Espenak.

Finally, for complete details on March’s solar eclipse, see:
Total Solar Eclipse of 2015 Mar 20.

– Fred Espenak

Additional Information
• Eclipses During 2015 – complete details on all eclipses in 2015
• Periodicity of Solar Eclipses – eclipse seasons, Saros and Inex
• Saros Catalog of Solar Eclipses
  – including Saros series -30 through 190
• Solar and Lunar Eclipses (EclipseWise.com)
• Thousand Year Canon of Solar Eclipses 1501 to 2500 – link to the publication
• Thousand Year Canon of Lunar Eclipses 1501 to 2500 – link to the publication