On the evening of February 10, 2017, I saw the shadow of the Earth extend all the way to the Moon as night fell.

Nightfall happens every evening. The Sun sets, and towards the east a dark shadow appears, darkening the sky as it strengthens. After an hour the “shadow” has spread itself across the whole sky, and it is night. But on February 10, the start of that night was different. Just as Wendee and I saw the first indications of the Earth’s shadow in the east, the full Moon rose.

Only it didn’t look full. There appeared to be a shading on the Moon’s upper left portion. What we were seeing was the Earth shadow actually project all the way to the Moon. It was a lunar eclipse.

There are several kinds of eclipses of the Moon and of the Sun. Lunar eclipses can be penumbral, in which the partial shadow of the Earth falls on a portion of the Moon. They can be partial, where the full dark shadow of the Earth falls on a portion of the Moon. If the full Earth shadow covers the whole Moon, the eclipse is total.

Eclipses of the Sun, which involve the shadow of the Moon reaching a portion of the Earth, are different. If the Moon shadow covers a portion of the Sun, then it is a partial eclipse. The full shadow of the Moon tracks along a narrow band, no larger than about 160 miles, across a portion of Earth, and along that band there can be a total eclipse of the Sun.

That February eclipse was the ninetieth eclipse I have seen. These eclipses range from tiny penumbral lunar eclipses, like the one last February, to the grand spectacles of total eclipses of the Sun, of which I have seen ten so far, and of which I hope to see my 11th this coming August.

But there is more. The night before the lunar eclipse, while I was out in my observatory, I recalled missing one just like this one, decades ago. On January 9, 1963, I was a 14-year-old patient at the Jewish National Home for Asthmatic Children in Denver, Colorado. I watched the Moon rise that night during observing session No. 99E, never knowing that a soft penumbral eclipse was actually underway. That early eclipse was a member of a Saros (Greek for cycle), saros 114.

It turns out that, unbeknownst to me, I saw that same eclipse (saros 114) on January 19, 1981. That eclipse, also a penumbral lunar eclipse, was a repeat of the one I didn’t recognize in 1963. The saros cycle lasts 18 years, 11 days, and 8 hours; and this was the very next repetition of that eclipse. Because of the eight hours (or a third of a day), the eclipse took place at a different time. Eighteen years after that, I missed the next one, because the third of a day meant that the eclipse was visible only in the predawn hours, and I was under a deck of clouds. That brings us to February 10, 2017. We were now pretty much back to the same time of day, and the eclipse was much like the one from 1963. This third repetition is called an exeligmos. It is Greek for a period of 54 years and 33 days. Thus, on February 10, 2017, I saw the 1963 eclipse, but 54 years later. It will be total along a narrow path that extends from Oregon to South Carolina. From our home in Vail, it will be a deep partial eclipse.

The existence of the saros cycle, and the related exeligmos, make these wonderful events even more remarkable. This coming August 21, some of us witnessing the solar eclipse might recall seeing the exeligmos one, under similar conditions, fifty-four years ago.

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