Marine Glossary

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Universal Time

In the Starpath celestial and weather courses we have often used the abbreviation GMT, as opposed to UT, for the universal time system used in navigation. Either name or abbreviation is OK. As far as most practical applications of the Nautical Almanac and celestial navigation are concerned universal time is the same as Greenwich mean time. As of the second edition of our cel nav text (2015), we have removed all reference to GMT and changed to UTC... a belated milestone of sorts.

For the most part, our use of GMT simply means that some of our materials predate the switch over in government publications to the use of UT. But there is no drawback to calling this GMT. In some government applications you see UT, others UTC, others UT1 as a substitute for GMT. These are not strictly the same times, but the use of GMT (= UT1) will not cause problems. The latest Nautical Almanac still states GMT as an alternative name.)

For more information on the details see http://aa.usno.navy.mil/faq/docs/UT.php.

Briefly: The heading in the almanac called UT is actually UT1 (GMT), ie it is effectively telling us where the celestial bodies are on earth relative to where the sun is.

But the location of the sun is affected by the rotation rate of the earth, which though remarkably regular does have very slight variations as the moment of inertia of the earth might change with melting ice caps or other effects.

The time we learn from the official NIST radio broadcasts is UTC, which is an absolute atomic-clock system that is not affected by earth rotation. These two times can differ by as much as 0.7 sec or so, and never more than 0.9 sec, because the government (NIST and USNO) changes UT1 by adding a "leap second" as needed in June and December of each year.

To navigate most accurately you can find the correction to UTC needed for precise UT1 from the WWV time ticks. Listen carefully and several of the first 8 tics or the second 8 tics at each minute will be doubled. Each doubled tick means a correction of 0.1s to the UTC to get UT1.

NIST Instructions: "If you need UT1 with an uncertainty of 0.1 s, you can apply a correction to UTC. UT1 corrections are encoded into the broadcasts by using doubled ticks during the first 16 s of each minute. You can determine the amount of the correction (in units of 0.1 s) by counting the number of doubled ticks. The sign of the correction depends on whether the doubled ticks occur in the first 8 s of the minute or in the second 8 s. If the doubled ticks are in the first 8 s (1-8) the sign is positive. If the doubled ticks are in the second 8 s (9-16) the sign is negative. For example, if ticks 1, 2, and 3 are doubled, the correction is +0.3 s. This means that UT1 equals UTC plus 0.3 s. If UTC is 8:45:17, then UT1 is 8:45:17.3. If ticks 9, 10, 11, and 12 are doubled, the correction is -0.4 s. If UTC is 8:45:17, then UT1 is 8:45:16.6. If none of the ticks are doubled, then the current correction is 0."

You can listen to the time tics on the phone at (303) 499-7111 (not toll free).

See this page for more info: http://www.nist.gov/pml/div688/grp40/wwv_format.cfm

See: sextant sight, standard time, time zone, universal time, watch error, watch rate, watch time, zone description, and zone time.

See also: Time Keeping in Navigation and Weather.

Abbreviation: UT