Between February and May 1793, Thompson made 34 observations of the longitude of Cumberland House using lunar distances. The mean of these observations was 102°12′ W, about 2' east of the modern value. Sebert gives 102°16′ as the longitude of Cumberland House, but Old Cumberland House, still in use at that time, was 2km to the east, see: The mean error of the 34 observations was about 15' of longitude. Broughton (2009) notes that the precision of the type of sextant used by Thompson was 15" of arc, corresponding to 7.5' of longitude giving an absolute limit to the precision of an individual observation. The error in Thompson's mean was several times less than this. The time he took on these observations, about 3 hours of calculation each, indicates that he understood the power of averages.
Between February and May 1793, Thompson made 34 observations of the longitude of Cumberland House using lunar distances. The mean ,of these observations was 102°12′ W, about 2' east of the modern value. Sebert gives 102°16′ as the longitude of Cumberland House, but Old Cumberland House, still in use at that time was 2km to the east, see: The mean error of the 34 observations was about 15' of longitude. Broughton (2009) notes that the precision of the type of sextant used by Thompson was 15" of arc, corresponding to 7.5' of longitude giving an absolute limit to the precision of an individual observation. The error in Thompson's mean was several times less than this. The time he took on these observations, about 3 hours of calculation each, indicates that he understood the power of averages.