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Better Times May’s Total Lunar Eclipse

7 May 2021, 21:24 UTC
Better Times May’s Total Lunar Eclipse
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This time lapse sequence shows the partial at total phases of the total lunar eclipse of 2014 April 15 (©2014 Fred Espenak)

Anticipation is growing for the upcoming total eclipse of the Moon on May 26. What makes this particular eclipse unusual is that the Moon’s orbital path just barely brings it inside Earth’s ruddy umbral shadow, thereby resulting in a very short total eclipse phase.

This same grazing shadow geometry also makes for a sensitive test on the accuracy of lunar eclipse predictions.

To calculate detailed predictions for a lunar eclipse, one starts with an accurate time-dependent set of 3-D coordinates of the Sun and the Moon with respect to Earth. But this is where things get a bit “fuzzy.” Earth’s cone shaped penumbral and umbral shadows do not have sharp boundaries due to the atmosphere surrounding our planet. The transparency drops significantly in the lower atmosphere where the air is thicker and dustier. This results in a small enlargement of Earth’s two shadows, giving them soft or “fuzzy” edges.

Astronomer Philippe de La Hire noticed this in 1707. He found the predicted radius of Earth’s umbral shadow needed to be enlarged by about 1/41 in order to fit ...

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