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May 20-21 eclipse of sun is part of a longer cycle

The solar eclipse of May 20-21, 2012 is part of a cycle – called the Saros cycle – that repeats about every 18 years and 10 days.

By Fred Espenak

Every solar eclipse tends to repeat itself in an 18-year-10-day cycle (or 18-year-11-day depending on the number of intervening leap years) called the Saros. I say “tends to repeat” because the cycle isn’t perfect and only lasts 12 or 13 centuries. Although two eclipses separated by one Saros cycle (18 years and 10 or 11 days) are very similar to each other, they are not exact.

Annular eclipse of the sun – China to Texas – on May 20 or May 21

Image copyright Fred Espenak. Used with permission.

Nevertheless, if we look 18 years into the past, we find that there was an annular solar eclipse on May 10, 1994. This eclipse passed centrally through the USA, and I photographed it near Toledo, Ohio. Those photos will give you a preview of what the May 20-21, 2012 eclipse will look like because the moon and sun are nearly at the same positions and distances as they were during the 1994 eclipse.

Find links to eclipse times for you

Fred Espenak

See more images of the 1994 annular eclipse on my gallery page of solar eclipses

For more information the eclipses and the Saros cycle, visit my web page on the NASA Eclipse web site

Fred Espenak on how to photograph a solar eclipse

Fred Espenak is Scientist Emeritus for Goddard Space Flight Center, and a retired NASA astrophysicist. He is known throughout the world for his work on eclipse predictions. His website lists dates and times for future solar eclipses through the year 2020.

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