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Measurement of the size and shape of the Earth's umbra during a lunar eclipse

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One of the most interesting measurements we can take during a Lunar eclipse is the careful timing of lunar craters as they enter and exit the umbra. This way we can get with precision the shape of Earth shadow, that varies from one eclipse to the next and it is related to the Earth's atmosphere.

More information can be find in this article in pdf: Size and shape of the umbra during a lunar eclipse.

The following Table lists 20 well-defined craters with predicted umbral immersion and emersion times during the lunar eclipse. Note that all predictions presented here use Danjon's rule of shadow enlargement. In particular, the diameter of the umbral shadow has been calculated assuming an enlargement of Earth's radius of 1/85 to account for the opacity of the terrestrial atmosphere. The effects of Earth's oblateness have also been included.

Data from NASA Eclipse Home Page . Fred Espenak, NASA / GSFC

Crater timing is easy! Record the instant when the most abrupt gradient at the umbra's edge crosses the apparent centre of the crater. In the case of large craters like Tycho and Copernicus, record the times when the shadow touches the two opposite edges of the crater. The average of these times is equal to the instant of crater bisection.

In order to obtain useful measurements, a precision of 0.1 minutes (6 seconds) is needed, so a clock synchronized with radio time signals could be very helpful. Moreover, this measurements are best made with low-power telescopes.

You should be thoroughly familiar with these features before viewing an eclipse in order to prevent confusion and misidentification. This map, as well as the one from the document in pdf above, can be of help as a planning guide.

craters.jpg
Names of some of the most important Moon craters.

 
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