Marine Glossary

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Equation of Time

The time difference between the actual GMT of local apparent noon at Greenwich and 12:00:00. It varies throughout the year in a smooth but irregular way from about +14 minutes to about -16 minutes. EqT is listed twice daily in the Nautical Almanac. This variation is caused by the tilt of the earth's rotation axis and the varying orbital speed of the earth along its slightly elliptical path around the sun.

Note that there is no actual call for the use of the equation of time in routine celestial navigation. One could in principle use it to make a more accurate prediction of the time of LAN, but in practice this prediction need not be accurate to the minute and it is usually made from a DR postion that is not precise enough to call for this correction anyway.

In emergency navigation, there can be an application in that one could use knowledge of the equation of time to figure Lon at LAN if one did not have an almanac. With an almanac at hand, however, there is again no need for equation of time in any application. See discussion in the Emergency Navigation book, which also includes a prescription for how to memorize how it varies throughout the year.

In the Nautical almanac they give the EqT so we can improve on the mer pass time if needed, as such:

EqT = 15m 52s, at 12 UTC, so the mer pass time that they approximate as 11:44 = 11h 44m, should really be (if you want the highest precision) = 12h 00m 00s - 00h 15m 52s = 11h 44n 08s, which they have rounded to 1144.

Abbreviation: EqT