An improved oblateness for the earth's thermosphere from lunar eclipse observations

The current ephemerides include an allowance for umbral oblateness based on the flattening of the Earth's geodetic reference spheroid; there are few published estimates for umbral oblateness derived directly from the geometry of a lunar eclipse. This paper reports umbral oblateness values which have been derived from 7,400 crater contact timing observations of twenty-one lunar eclipses by amateur and professional astronomers since 1972. The observed umbral oblateness was found to be typically three times the geodetic reference spheroid flattening.