Same clock time, different calendar date — Page 402, Lesson 351

Let’s pick this up right at the 180° meridian, because that’s where the date itself flips. We’ve already established that Local Mean Time, LMT, is tied to the Sun’s position over a specific meridian. Now, at 180° West — that’s mid-Pacific, in the Western hemisphere — the LMT is 2100 on the 15th of May. But here’s the twist: at 180° East, the LMT is also 2100, yet it’s the 16th of May. Same clock time, different calendar date. So the rule is: at the 180° meridian, the LMT date changes by a day, even though the LMT time stays the same. If you travel westward across that meridian, the date increases by one day. If you travel eastward across it, the date decreases by one day. That’s the International Date Line in LMT terms. Figure 25.2 shows this with an LMT at Greenwich of 0300 on the 19th — which is the same as 0300 UTC on UTC date 19th. You can see how the date steps forward or backward as you cross 180°. Now let’s bring in Co-ordinated Universal Time — UTC. UTC changes at a constant rate, and it’s regulated against International Atomic Time, IAT, which comes from atomic clocks. UTC is regularly corrected to match GMT — that’s the LMT at the Greenwich Meridian — but those corrections are very small. For all practical navigation purposes, UTC equals GMT. And UTC is the datum for world time — the reference everything else is measured against. So we often need to convert LMT into UTC, or the other way around. The Sun appears to travel across the Earth from east to west, and that gives us the rule: Longitude east, UTC least — less advanced. Longitude west, UTC best — more advanced. Let me walk you through the examples. Cairo is at longitude 30° East, and the LMT there is 0900. Since we’re going from LMT to UTC, and Cairo is east, UTC is least — so we subtract the arc/time for 30° East, which is 2 hours. 0900 minus 0200 gives us 0700 UTC. Madrid is at 04° West, LMT 0400. West means UTC is best — more advanced — so we add the arc/time for 4° West, which is 16 minutes. 0400 plus 0016 gives us 0416 UTC. Now the reverse: Berlin is at 13° East, and the UTC is 1345. We want LMT. Since Berlin is east, UTC is least — so LMT is more advanced, and we add. The arc/time for 13° East is 52 minutes. 1345 plus 0052 gives us 1437 LMT in Berlin. And San Francisco, at 122° West, with UTC 2115. West means UTC is best — more advanced — so LMT is less advanced, and we subtract the arc/time for 122° West. That’s the one you’re working through now: 122° West at 15° per hour gives you 8 hours and 8 minutes. So 2115 minus 0808 gives you 1307 LMT in San Francisco. The key habit: always ask yourself which is east and which is west, then apply the rule — east, UTC least; west, UTC best.