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.
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.
For more information the eclipses and the Saros cycle, visit my web page on the NASA Eclipse web site
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.
Members of the EarthSky community - including scientists, as well as science and nature writers from across the globe - weigh in on what's important to them. Photo by Robert Spurlock.