Time Zones

Time zones used for navigation are uniform 15° extents of longitude. 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 15° navigational time zone 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 there are no Mapping Toolbox™ 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 invention of accurate chronometers in the 18th century solved this problem.

The timezone function is quite simple. It returns zd, an integer for use in calculations, zltr, a character vector of the zone designator, and zone, a character vector 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 the 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.

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