Equator is commonly modeled as a sphere flattened 0.336% along its axis. This makes the Equator 0.16% longer than a meridian (a great circle passing through the two poles). The IUGG standard meridian is, to the nearest millimetre, , one arc-minute of which is , explaining the SI standardization of the nautical mile as , more than less than the geographical mile. The sea-level surface of the Earth (the geoid) is irregular, so the actual length of the Equator is not so easy to determine. "Aviation Week and Space Technology" on 9 October 1961 reported that measurements using the Transit IV-A
Nautical time maritime usage, GMT retains its historical meaning of UT1, the mean "solar" time at Greenwich. UTC, "atomic" time at Greenwich, is too inaccurate, differing by as much as 0.9 seconds from UT1, creating an error of of a minute of longitude at all latitudes and which is at the equator but less at higher latitudes, varying roughly by the cosine of the latitude. However, DUT can be added to UTC to correct it to within 50 milliseconds of UT1, reducing the error to only . In practice, nautical times are used only for radio communication, etc. Aboard the ship, e.g.