Newspaper article The Christian Science Monitor

Missed Monday Night's Lunar Eclipse? Here's How to Catch the Next One

Newspaper article The Christian Science Monitor

Missed Monday Night's Lunar Eclipse? Here's How to Catch the Next One

Article excerpt

Last night's "blood moon" was the first of four total lunar eclipses that will occur in a regular pattern over the next two years. This series of four is known as a "tetrad," and it's caused by the shape of Earth's orbit.

The first eclipse of the current tetrad came in the pre-dawn hours of April 15th, when the full Moon passed through Earth's shadow. The next total lunar eclipse comes Oct. 8, 2014, then April 4, 2015, and the last on Sept. 28, 2015. Eclipses can be seen from only part of the Earth, but coincidentally, all four of this lunar tetrad will be at least partially visible from North America.

"During the 21st century, there are 8 sets of tetrads, so I would describe tetrads as a frequent occurrence in the current pattern of lunar eclipses," said NASA eclipse expert Fred Espenak in a press release. "But this has not always been the case. During the three hundred year interval from 1600 to 1900, for instance, there were no tetrads at all."

Astronomer Giovanni Schiaparelli noticed in the 19th century that there are periods when tetrads occur comparatively frequently, interrupted by eras when they are rare. More recently, astronomers have calculated that tetrads peak every 565 years. For example, there were no tetrads from 1582 to 1908, but 17 tetrads between 1909 and 2156. The most recent tetrad fell in 2003-2004.

During the 5000-year period from 1999 BC to 3000 AD, there 142 tetrads, accounting for about 16 percent of the 3479 total lunar eclipses during that period. During that same 5000-year window, there were 4378 penumbral eclipses and 4207 partial lunar eclipses.

The tetrad "seasons" are tied to the slowly decreasing eccentricity of Earth's orbit, which is still slightly oval-shaped. Once Earth's orbit becomes a perfect circle, in the distant future, tetrads will no longer be possible.

In general, solar and lunar eclipses occur about 3 to 5 times every year. …

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