Mapping Toolbox

Time Zones

Early in this manual, we gave an example of a time zone calculation. The timezone function returns a navigational time zone, that is, one based solely on longitude with no regard for statutory divisions. So, for example, Chicago, Illinois, lies in the statutory U.S. Central time zone, which has irregular boundaries devised for political or convenience reasons. However, from a navigational standpoint, Chicago's longitude places it in the S (Sierra) time zone. The zone's description is +6, which indicates that 6 hours must be added to local time to get Greenwich, or Z (Zulu) time. So, if it is noon, standard time in Chicago, it is 12+6, or 6 p.m. at Greenwich.

Each navigational time zone is 15° of longitude in width and has a distinct description and designating letter. The exceptions to this are the two zones on either side of the Date Line, M and Y (Mike and Yankee). These zones are only 7-1/2° wide, since on one side of the Date Line, the description is +12, and on the other, it is -12. Navigational time zones are very important for celestial navigation calculations. Although the Mapping Toolbox does not contain any functions designed specifically for celestial navigation, a simple example can be devised.

It is possible with a sextant to determine local apparent noon. This is the moment when the Sun is at its zenith from your point of view. At the exact center longitude of a time zone, the phenomenon occurs exactly at noon, local time. Since the Sun traverses a 15° time zone in 1 hour, it crosses one degree every 4 minutes. So, if you observe local apparent noon at 11:54, you must be 1.5° east of your center longitude.

You must know what time zone you are in before you can even attempt a fix. This concept has been understood since the spherical nature of the Earth was first accepted, but early sailors had no ability to keep accurate time on ship, and so were unable to determine their longitude. The engineering exploitation of this scientific truism had to wait until the invention of the chronometer.

The timezone function is quite simple. It returns the description, zd, an integer for use in calculations, a string zltr of the zone designator, and a string fully naming the zone. For example, the information for a longitude 123°E is the following:

• [zd,zltr,zone] = timezone(123)
  zd =
      -8
  zltr =
  H
  zone =
      -8 H
  

Returning to our simple celestial navigation example, the center longitude of this zone is:

• -(zd*15)
  ans =
      120
  

This means that at our longitude, 123°E, we should experience local apparent noon at 11:48 a.m., 12 minutes early.

Drift Correction

Time Notation