It is flattened at the poles and bulges at the Equator, so an oblate spheroid more closely represents its shape. A better model for the shape of Earth’s shadows is needed.Įarth’s figure is not a perfect sphere.
Both methods assume the cross sections of Earth’s shadows are exact circles.Īlthough a circular approximation for Earth’s shadows works well in most cases, it is less successful for an eclipse like May 26 where the Moon is barely immersed inside the northern edge of the umbra. It should be noted that the Chauvenet method is flawed in that it does not correctly scale the enlargement factor of Earth’s shadow as the Moon’s distance changes.Īlthough some authorities still use Chauvenet’s method, most use Danjon’s method (e.g., NASA Eclipse, La Connaissance des temps, TimeAndDate, EclipseWise). More details on how these two methods differ can be found at Shadow Enlargement. Both methods assumed a circular cross section for the umbral shadow. William Chauvenet (1891) formulated one method to account for the shadow enlargement while André-Louis Danjon (1951) devised another. He found the predicted radius of Earth’s umbral shadow needed to be enlarged by about 1/41 in order to fit shadow timings made during recent lunar eclipses. This results in a small enlargement of Earth’s two shadows, giving them soft or “fuzzy” edges.Īstronomer Philippe de La Hire noticed this in 1707. The transparency drops significantly in the lower atmosphere where the air is thicker and dustier. 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. 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. This same grazing shadow geometry also makes for a sensitive test on the accuracy of lunar eclipse predictions. 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 time lapse sequence shows the partial and total phases of the total lunar eclipse of 2014 April 15 (©2014 Fred Espenak)Īnticipation is growing for the upcoming total eclipse of the Moon on May 26.
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