Travelling westward across the 180° meridian, the LMT date increases by one… — Page 402, Lesson 351

Let’s pick this up right where the date line logic left off, because now we’re going to turn that into a working rule you’ll use every day in the cockpit. We just saw that at 180°W, LMT is 2100 on the 15th May, but at 180°E, LMT is 2100 on the 16th May. So at the 180° meridian, the LMT date changes by a day, even though the LMT time stays the same. That’s the International Date Line effect. Travelling westward across the 180° meridian, the LMT date increases by one day. Travelling eastward across it, the LMT date decreases by one day. 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. Now let’s bring in Co-ordinated Universal Time — UTC. UTC changes at a constant rate, and it’s regulated against International Atomic Time, or IAT, which comes from atomic clocks. UTC is regularly corrected to match GMT — that’s the LMT at the Greenwich Meridian — but the 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 you’ll often need to convert LMT into UTC, or the other way around. The basis is that the Sun appears to travel across the Earth from east to west. From that, we get the rule: Longitude east, UTC least — less advanced. Longitude west, UTC best — more advanced. Let me walk you through the examples so you see exactly how this works. First: find UTC if LMT in Cairo, longitude 30°E, is 0900 hours. LMT Cairo is 0900. We subtract the arc/time for longitude 30°E, which is 0200. That gives UTC 0700 — and note, UTC least, because we’re east. Second: find UTC if LMT in Madrid, longitude 04°W, is 0400 hours. LMT Madrid is 0400. We add the arc/time for 04°W, which is 0016. That gives UTC 0416 — UTC best, because we’re west. Now the reverse: find LMT in Berlin, longitude 13°E, if UTC is 1345. UTC is 1345, and since Berlin is east, UTC is least, so we add the arc/time, 0052. That gives LMT Berlin 1437 — longitude east. And one more: find LMT in San Francisco, longitude 122°W, if UTC is 2115. UTC is 2115, and since San Francisco is west, UTC is best, so we subtract the arc/time. The excerpt cuts off right there, but the pattern is exactly the same — you’d subtract the arc/time for 122°W to get the LMT. So the key takeaway: east of Greenwich, your local time is ahead of UTC, so you subtract to get UTC. West of Greenwich, your local time is behind UTC, so you add to get UTC. That’s the whole conversion in one line.