Let's pick this up with the LMT/UTC problems, because that's where we are — Page 402, Lesson 352

Let's pick this up with the LMT/UTC problems, because that's where we are. I want to walk you through the two worked examples, and then we'll move into Zone Time, which is a whole new concept. First, a key point about Local Mean Time problems. Notice that latitude is completely unimportant. All points on the 75°W meridian have the same LMT, whether that's Ottawa in Canada, Philadelphia in the USA, Kingston in Jamaica, Bogota in Colombia, or Lima in Peru. Longitude is the only thing that matters. Now, the method. I strongly recommend you always work through UTC, even though it can feel like a longer route. Let me show you why with Example 1. We have point A at 40°N 137°50'W, and its LMT is 1812 on 18 August. We want the LMT at point B, which is 30°S 121°12'E. Here's the layout. We set up columns for Day, Hour, and Minute. At A, we have Day 18, Hour 18, Minute 12. Step one: convert A's longitude to time. A is at 137°50' West. We add that arc-to-time conversion, which is +9 hours 11 minutes. Why add? Because A is in the Western hemisphere, and its LMT is behind UTC. So to get from LMT at A up to UTC, we add. That gives us UTC as Day 19, Hour 03, Minute 23. Step two: now we go from UTC down to LMT at B. B is at 121°12' East. The arc-to-time conversion is +8 hours 05 minutes. We add this to UTC because B is in the Eastern hemisphere, so its LMT is ahead of UTC. That gives us LMT at B as Day 19, Hour 11, Minute 28. So the answer is 1128 LMT on the 19th of August. Now, the note in the text is important. This question could have been done without UTC, by finding the change in longitude between A and B, which is 259°02', converting that to 17 hours 16 minutes. But the method through UTC seems longer yet it's much safer. It's too easy to make a mistake if you take short cuts, especially when the date changes. So always use the UTC method. Let me do Example 2 with you. If it's 1012 UTC on 15 December at 50°N 030°W, what's the LMT in Singapore and in Hawaii? First, note that UTC is the same all over the world. The initial position of 50°N 030°W is a red herring — it doesn't matter at all. For Singapore, at 00°30'N 105°00'E: we start with UTC, Day 15, Hour 10, Minute 12. Singapore is East, so we add the arc-to-time conversion for 105°E, which is +7 hours. That gives us LMT Singapore as Day 15, Hour 17, Minute 12. For Hawaii, at 22°00'N 155°00'W: again we start with UTC, Day 15, Hour 10, Minute 12. Hawaii is West, so we subtract the arc-to-time conversion for 155°W, which is -10 hours 20 minutes. That gives us LMT Hawaii as Day 14, Hour 23, Minute 52. Notice the date went back to the 14th. Now, let's move to Zone Time, or ZT. This is a different system, and I want you to understand why it exists. UTC is the worldwide time datum for aviation, but it has little civil use far from the Greenwich meridian. Think about it — humans tend to rise in the morning and go to bed in the late evening. On a cruise ship in the mid-Pacific, it would be disappointing if you went up on deck to sunbathe when your watch read 1000 hours UTC, because the sun might be setting. This problem was recognized centuries ago, and the system of Zone Time was used to keep ships' time in synchronization with the periods of light and dark. Zone Time is based on the fact that the Sun transits 15 degrees in one hour. The datum for the Zone Time system is the Greenwich Meridian, 0°E/W. Here's how the zones work. All ships within longitudes 7.5°W and 7.5°E keep a Zone Time the same as GMT, which is UTC. The correction from Zone Time to GMT is zero. Ships between longitudes 7.5°E and 22.5°E have a Zone Time one hour later than GMT. This zone is numbered -1. The correction to be applied to Zone Time to calculate GMT is minus 1. Similarly, ships between 22.5°E and 37.5°E are in the zone numbered -2. The numerical labels continue as minus values all the way up to 180°E. For the Western hemisphere, a similar system applies, but the signs flip. Ships at longitudes between 7.5°W and 22.5°W are in the zone numbered +1. That means the zone time is earlier than UTC, and 1 hour must be added to Zone Time to calculate UTC. The numerical labels increase as plus values all the way to 180°W. So the key contrast here: East zones are negative numbers, and the zone time is later than UTC. West zones are positive numbers, and the zone time is earlier than UTC. That's the core of Zone Time. Let me show you the figures that illustrate this. shows the Local Mean Time situation when the mean Sun is transiting the meridian. And shows the relationship between LMT and UTC. That's the foundation. We've got the LMT/UTC method with the two examples, and now the Zone Time system with its zone numbering.