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How often do solar eclipses occur?


Total solar eclipse

People watch a total eclipse over Australia from Lyndhurst in South Australia in 2002 (Source: David Gray/Reuters)

How often do different types of solar eclipses occur and why? Why don't you see one every time there's a new moon? Has their frequency changed over time?

Solar eclipses, when the moon totally or partially blocks out the Sun, still seem to many people a random and almost magical phenomenon, but they can be very accurately predicted, says Fred Espenak, an American astrophysicist and world authority on eclipses.

"Certainly within 100 to 200 years we can predict when an eclipse will occur to within a second", says Espenak, "but the pattern of occurrence is a complicated one. They don't repeat on a time schedule like the seasons of the year".

There is an average of 2.4 solar eclipses every year, and four types of eclipse, he explains, all occurring during the new moon phase of the moon's cycle, when it passes between the Earth and the Sun.

"With a total eclipse the darkest part of the moon's shadow sweeps across the Earth's surface and if you happen to be in the path of that very small shadow, usually 100 to 200 kilometres diameter, you will see the moon completely block the Sun's disc", he explains.

"Total solar eclipses occur once every one to two years but are only visible from less than half a percent of the Earth's surface".

Whether an eclipse is total, when the Sun is completely blocked with just a halo of light appearing around it, or annular, when the moon appears smaller and is silhouetted against the Sun, depends on which point of its orbit the moon is in at the time.

"Total eclipses occur along the section of the moon's orbit called perigee, where the moon is closest to the Earth. At the other end of the moon's orbit, called apogee the moon is further from the Earth and appears smaller than the Sun".

Anatomy of a total solar eclipse

Similar in frequency to the annular eclipse but, according to Espenak, much less interesting to eclipse fans, is the partial eclipse.

"In that case the moon's central shadow completely misses the Earth and you only get the outer edge of the shadow, the penumbral shadow, and during that type of eclipse you see the moon move along either the top or bottom edge of the Sun but not through the middle of the Sun".

Rarest of all is the hybrid eclipse, says Espenak. "The moon's orbit is elliptical so if you happen to get a solar eclipse just as the moon's distance is changing from a little bit smaller than the Sun to a little bit larger than the Sun, what you can get is an eclipse path that begins as annular and ends up total, or vice versa". The last hybrid eclipse was in 2005 and next will be visible from the Americas, Europe and Africa in 2013.

Espenak says the frequency of eclipses hasn't changed in thousands of years, and only immensely slow geological factors like the very gradual slowing of the Earth may make them difficult to predict many thousands of years from now.

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New moon

So why do we not see an eclipse every time there is a new moon?

"Because the moon's orbit is tipped about 5 degrees to the Earth's orbit around the Sun", he says.

"During most new moons when the moon passes between the Earth and the Sun, the moon's orbit takes it a little bit above the plane of the Earth's orbit or a little bit below the plane of the Earth's orbit and what that means is you see the moon passing a little bit above the Sun or a little bit below the Sun, but at least twice a year we get some type of a solar eclipse. They're not as rare as people think, just only visible from a small section of the Earth's surface".

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Eclipses over Australia

Espenak will shortly travel to Australia to view the next total eclipse, visible from Cairns in Northern Queensland on 14 November. And by coincidence, Northern Queensland is also a potential viewing spot for the next annular eclipse in May 2013.

"It's the luck of the draw — it's quite unusual to have annular and total visible from same geographical area within six months of each other".

Espenak has calculated when and where eclipses will occur around the world up to 2050. Visit the NASA eclipse site to find out when each eclipse will occur. The map below shows eclipses over Australia for the next four decades.

Eclipses over Australia 2012 - 2050

Fred Espenak is a retired astrophysicist from NASA's Goddard Space Flight Center in Greenbelt, Maryland. He is best known for his predictions of eclipses. His two books, Fifty Year Canon of Solar Eclipses: 1986 - 2035 and Fifty Year Canon of Lunar Eclipses: 1986 - 2035 have become standard references on the subject. He was interviewed by Annie Hastwell.

Tags: astronomy-space, the-moon

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Published 06 November 2012

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