fractional second is the time difference between utc and gmt.
what is the time difference between utc and gmt?
fractional second
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the purpose of a chronometer is to measure accurately the time of a known fixed location, for example greenwich mean time (gmt). this is particularly important for navigation. knowing gmt at local noon allows a navigator to use the time difference between the ship's position and the greenwich meridian to determine the ship's longitude. as the earth rotates at a regular rate, the time difference between the chronometer and the ship's local time can be used to calculate the longitude of the ship relative to the greenwich meridian (defined as 0°) using spherical trigonometry. in modern practice, a nautical almanac and trigonometric sight-reduction tables permit navigators to measure the sun, moon, visible planets, or any of 57 selected stars for navigation at any time that the horizon is visible.
marine chronometer