An eclipse like a few dozen others

¡SkyCaramba! Weekly astronomy blog for the week ending November 17, 2012 

Eclipses have always captivated those who see them. They don’t happen often. Weather could ruin your view. You could be in the wrong place to see it. Most people will be in the wrong place to see the total solar eclipse that crosses part of Australia and the south Pacific Ocean on November 13th.

But a few thousand years ago, people who kept track of things that happened in the sky noticed certain patterns in eclipse timing. Combing through their old records of such events, they noticed the same eclipse seemed to happen every 54 years. They didn’t fully understand how the earth, sun, and moon move. But they recognized this regular recurrence well enough to predict future eclipses.

In more recent times, astronomers have come to understand that the same eclipse isn’t replayed every 54 years—but much sooner. The eclipse that happens on November 13, 2012 is almost exactly like one that happened on November 3, 1994. And it’ll be almost exactly like one that happens on November 25, 2030.

The sun, earth, and moon line up in much the same, but not exactly the same, way every 29.5 days. That’s why there’s a full moon every month but many months go by between lunar eclipses. Every now and then, the lineup is very close to another previous lineup. That happens with a regularity of 6,585.32 days or slightly more than 18 years. This has become known to astronomers as a saros cycle.

Pay attention to the numbers after the decimal point in the number of days in a saros cycle. That’s about one third of a day. So when the eclipse recurs, the earth will have turned a third of the way from where it was the last time. This month’s total eclipse crossed South America and the south Atlantic Ocean when it happened in 1994. When it happens again in 2030, it’ll cross South Africa, the Indian Ocean, and Australia.

People thousands of years ago couldn’t travel to or communicate with other parts of the world as readily as we can today. So they didn’t know the eclipse they were watching repeat after 54 years had already been replayed twice since they last saw it. That didn’t detract from the wonder.

The lineup from one eclipse to the next in a saros cycle is nearly identical. But it’s not exactly the same. Over time, subtleties in the way the earth and moon tug at each other as they go around the sun together cause a drift. The total eclipse you may see from the south seas this month will repeat 20 more times. The time of totality will get shorter each time. The width of the path of totality will narrow. When this eclipse happens in 2391, it will only be a partial eclipse. It will happen for the last time in 2499.

Don’t worry. That doesn’t mean all eclipses will end. The same changes that bring a saros series to an end also bring new ones into existence. So the splendor will keep happening for thousands of years to come.