We normally look at our watches to see what time it is. But for the celestial navigator time is not so simple. In fact there are several different times for the same place at the same time!
Greenwich Mean Time (GMT) or Universal Coordinated time (UTC) is the time at the Prime Meridian, 0°, the standard for the beginning of the day, the time from which all data in the Nautacal Almanac is calculated. From this point we add one hour for each 15° of Longitude West up to 180°, and subtract one hour for each 15° of Longitude East, +12 Hours West, -12 East up to the International Date Line at 180° where you add one day if you are going West or subtract on day if you are going East.
From the above you can see that for each 15° of longitude we have the same Local Standard Time, although each Time Zone is 900 nm wide at the equattor, and the sun takes an hour to go from the eastern edge to the western edge. Although you watch shows noon everywhere in your time zone actually, if you are on the eastern edge of your time zone the sun passed overhead ½ hour earlier and on the western edge it passes overhead ½ later than you watch shows.
Now add to that that the lines of separation for time zones are drawn to meet geographical and political needs and thus don’t match the specifics of the lines of longitude at 15° interviews. Some locations (in the middle east for example) have less than 1 hour differences and then there is the fun of Daylight Saving Time.
It gets more interesting, The earth has an elliptical orbit which causes what we call the Equation of Time, that strange figure 8 you’ve seen on the globe, called the Analemma. Apparent time can be ahead (fast) by as much as 16 min 33 s (around 3 November), or behind (slow) by as much as 14 min 6 s (around 12 February). The equation of time has zeros near 15 April, 13 June, 1 September and 25 December.
Oh, remember what I said about your time zone being 1 hour or 15° wide? So to get the exact time for your exact location, Local Mean Time (LMT) you need to forget what your watch says and calculate from your actual Longitude (and from that, apply the equation of time.)
For example my Longitude is 122° 45’ W. The center of my time zone is 120° W (8 X 15 = 120). Next, I go to the arc to time conversion table in the Nautical Almanac (The Yellow or Grey pages at the back of the almanac,) and see that it takes the sun 8 hours 8 minutes of time to travel 122° and then get 3 minutes additional for the 45’ more of arc, so I know that my physical position is 8 hours 8 minutes 45 seconds from GMT although my watch is set, in my time zone exactly 8 hours plus GMT. So the sun is over my head 8 minutes and 45 seconds after my watch says noon.
BTW, remember the equation of time? If you want to get it exact you need to go back to the Nautical Almanac and add or subtract the equation of time to get it right on the button, Local Apparent Time (LAT).
And you thought time was simple…
© Mark S. Anderson June 2012