Naked eye astronomical phenomena often make for great opportunities to write about science, and while lunar eclipses are rather common, the one coming tonight/early tomorrow morning is interesting because it’s the first of a “tetrad” of four total eclipses spaced six months apart.
Most news stories noted that the moon will turn the color of blood, or sunsets, but some of us who have watched lunar eclipses have occasionally been disappointed that even total ones can look grey. Sky and Telescope can be counted on to explain this color variability. Here’s Alan McRobert:
Two factors affect an eclipse's color and brightness. The first is simply how deeply the Moon goes into the umbra — because the umbra's center is much darker than its outer edge. The second is the state of Earth's atmosphere all around the terminator. If the air is very clear, the eclipse is bright; if it's mostly cloudy (or polluted with volcanic ash from a major eruption), the eclipse will be dark red, ashen gray, or almost black.
CNN.com's story focused on the clustering of lunar eclipses into tetrads and the clustering of the clusters. The story links to a NASA website saying there were no tetrads between 1600 and 1900, while there will be eight in the 21st century alone. CNN says the clustering is random:
Lunar eclipses — penumbral, partial or umbral — occur in random order, NASA says. Getting four umbral eclipses in a row is like drawing a rare lunar poker hand of four of a kind.
And yet, the website associated with NASA-Goddard Space Flight Center says there is a mechanism for producing total lunar eclipses in foursomes:
The lunar eclipses of 2014 are the first of four consecutive total lunar eclipses – a series known as a tetrad. During the 5000-year period fhttp://www.latimes.com/science/sciencenow/la-sci-sn-lunar-eclipse-blood-moon-20140411,0,2273021.storyrom -1999 to +3000, there are 4378 penumbral eclipses (36.3%), 4207 partial lunar eclipses (34.9%) and 3479 total lunar eclipses (28.8%). Approximately 16.3% (568) of all total eclipses belong to one of the 142 tetrads occurring over this period (Espenak and Meeus, 2009). The mechanism causing tetrads involves the eccentricity of Earth's orbit in conjunction with the timing of eclipse seasons (Meeus, 2004). During the present millennium, the first eclipse of every tetrad occurs sometime from February to July. In later millennia, the first eclipse date gradually falls later in the year because of precession.
Italian astronomer Giovanni Schiaparelli first pointed out that the frequency of tetrads is variable over time. He noticed that tetrads were relatively plentiful during one 300-year interval, while none occurred during the next 300 years. For example, there are no tetrads from 1582 to 1908, but 17 tetrads occur during the following 2 and 1/2 centuries from 1909 to 2156. The ~565-year period of the tetrad "seasons" is tied to the slowly decreasing eccentricity of Earth's orbit. Consequently, the tetrad period is gradually decreasing (Meeus, 2004). In the distant future when Earth's eccentricity is 0, tetrads will no longer be possible.
A few outlets, such as USA Today, focused on a silly end-of-the-world forecast associated with the tetrad, as if we haven’t had enough apocalypse stories for one millennium.
And here at Philly.com, the emphasis was on the coincidence of the eclipse and first night of Passover.
Unfortunately, this story failed to note that it’s not much of a coincidence at all. In fact, lunar eclipses are much more likely to fall on Passover than on Christmas, New Years, 4th of July, Labor Day, Halloween, Thanksgiving, or Valentine’s Day. That’s because the start of Passover is calculated according to a lunar calendar. Both the springtime Jewish holiday and lunar eclipses only happen on full moons.
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