In order to measure the longitude difference between two distant sites, Inoh Tadataka and his technical team observed three lunar eclipses when surveying the Japanese Islands at the beginning of the 19th century, and left some observation records. The local times when the partial eclipses began and ended were measured with a pendulum clock. By introducing the equation of time to the calendar date of the observations, the time difference can be obtained between the local time observed by Inoh and the lunar eclipse timetable of NASA shown in Japanese Standard Time at 135E degrees of longitude. Consequently, the accuracy of the longitude survey performed by Inoh Tadataka is evaluated in comparison to the precise map of the Geographical Survey Institute.
