The Battle of Grand Gulf was fought on April 29, 1863, during th: where Earth's average meridional radius is 6,367,449 m. Since the Earth is an oblate spheroid, not spherical, that result can be off by several tenths of a percent; a better approximation of a longitudinal degree at latitude where Earth's equatorial radius equals 6,378,137 m and ; for the GRS 80 and WGS 84 spheroids, is known as the reduced (or parametric) latitude). Aside from rounding, this is the exact distance along a parallel of latitude; getting the distance along the shortest route will be more work, but those two distances are always within 0.6 m of each other if the two points are one degree of longitude apart.