Coastal Boating, Sailing, Cruising, Yachting, Racing, Coastal, Sailboat, Yacht, Fleet, Club, Regatta, Commodore, One design, Social, Long Island Sound, Narragansett Bay, Buzzards Bay, Chesapeake Bay, Island, Seamanship, NE waters, NOAA, NWS


There and back again
The First Rule of Boating
Navigation displays for the rest of us
Patchy Fog

Tools and more info

The Importance of Practicing MOB Drills and Crew Training
Tools for coastal navigation
What is the meaning of Zulu time?
Lots more Navigation Resources


Top 10 Online Boating Resources
Top 10 Challenging Cruising Waters in the Northeast

What is the meaning of Zulu time?

In a recent editorial, we talked about the ethereal concept of Fiji time.  This month, in contrast, never to be complacent, we’re tackling the very real concept of Zulu time. You can tell it's mid-winter and we're starting to get a little stir crazy. Maybe this will help or maybe it will put you to sleep. Here goes...

Our natural concept of time on earth is based in days which are measured by the amount of time it takes the earth to complete one rotation on its axis – or 24 hours.  As ways to measure time became more accurate and the concept of longitude was defined, it became critical to designate some point on earth the Prime Meridian – the place where measurement of time would begin each day and the place where longitude could be measured from.  Since Britain was the world’s greatest power at the time longitude was defined, that’s who got to have the starting point.  The Prime Meridian is at zero degrees and runs through the Royal Greenwich Observatory southeast of London

When the concept of time zones was introduced, GMT or Greenwich Mean Time became the standard.  Zulu time is, simply put, that which you know as GMT.  When it is noon at the observatory is it five hours earlier in Washington DC under standard time. 

Unfortunately, the Earth does not rotate at an exactly constant rate and so a new timescale was developed for greater accuracy.  Coordinated Universal Time (UTC) has been adopted by the Navy and civil aviation and replaces the term GMT.  They use the letter “Z” (Zulu) to refer to the time at the Prime Meridian.   The US Naval Observatory in Washington DC serves as the nation’s official timekeeper.   This is the time used in GPS systems, computer/internet clocks, and so on. 

Time zones in the United States include:

  • Eastern (“R” Romeo)
  • Central (“S” Sierra)
  • Mountain (“T” Tango)
  • Pacific (“U” Uniform)
  • Alaska (“V” Victor)
  • Hawaii (“W” Whiskey)

Why do you need to know this? Because lots of things happen in Zulu time, one of which is the weather. More accurately the weather prediction reports from NOAA based on satellite imagery which is referenced to zulu time. You have to know what time it is relative to Zulu time to know when a storm might come your way. That's why they have a calculator for converting Zulu time to local time.

What's even more fascinating (okay, at least to me) is the concept of leap seconds. Periodically, the time has been adjusted to match the earth's rotation and there will be a UTC TIME STEP on the 1st of January 2006. Leap seconds can be introduced in UTC at the end of the months of December or June, depending on the need. A bulletin is mailed every six months, either to announce a time step in UTC or to confirm that there will be no time step at the next possible date. A positive leap second will be introduced at the end of December 2005. The sequence of dates of the UTC second markers will be:

  • 2005 December 31, 23h 59m 59s
  • 2005 December 31, 23h 59m 60s
  • 2006 January 1, 0h 0m 0s

This adjustment results in a difference between UTC and the International Atomic Time (TAI), which has been - 32s from January 1, 1999 to January 1, 2006. As of 2006 January 1, 0h UTC, until further notice the difference between UTC and TAI will be - 33s. Pretty cool, eh? They're just going to subtract a second from the clocks. Hmm, does that mean it will disappear from all time and will everything that happened then be reassigned to another time? Oh well, let's ponder that another time.

As an aside, you may have heard of taking noon sites with a sextant to fix your position on earth.  It is calculated as the difference in time between noon wherever you are and noon at the Prime Meridian with a corresponding difference in longitude.   Now, which clock do they use there?

Joy of sailingCoastal Boating (Reg. in Ireland No. 443222) is a division of Knowledge Clinic Ltd.
Port Aleria, Rosnakilly, Kilmeena, Westport, Co. Mayo, Ireland - USA: PO Box 726, Mahwah, NJ 07430
All content on this site is subject to Copyright© - All rights reserved.
Contact us - Advertising - Privacy - Terms & Conditions - Copyright & Trademark - Webmaster