Equator is derived from medieval Latin word "aequator", in the phrase "circulus aequator diei et noctis," meaning ‘circle equalizing day and night’, from the Latin word "aequare" meaning ‘make equal’. The latitude of the Earth's equator is, by definition, 0° (zero degrees) of arc. The Equator is one of the five notable circles of latitude on Earth; the other four are both polar circles (the Arctic Circle and the Antarctic Circle) and both tropical circles (the Tropic of Cancer and the Tropic of Capricorn). The Equator is the only line of latitude which is also a great circle — that is,
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.