Tonga sits at 22°S 170°W — Page 408, Lesson 361

Let’s pick this up with the Tonga Islands, because it’s the perfect way to see why the International Date Line isn’t a straight line down the 180° meridian. Tonga sits at 22°S 170°W. Fiji is at 17°S 178°E. They’re part of the same Pacific island group, and they want to keep the same calendar day, even though they’re on opposite sides of the 180° meridian. If we used the strict 180°E/W dateline, Tonga would be one day behind Fiji. So the International Date Line is deliberately moved further east, away from 180°, so that Fiji and Tonga share the same Standard Time day. Now here’s the worked example, and I want you to follow the logic column by column. The question: if Local Mean Time in Tonga is 0800 LMT on local mean date 10 May, what is the Standard Time and Standard Date in Tonga? We work through UTC as the bridge. Start with Local Mean Time: day 10, hour 08, minute 00. Then apply the arc/time correction for 170°W. That’s plus 11 hours 20 minutes, because 170° west of Greenwich means LMT is behind UTC by that amount. Adding it gives UTC as day 10, 19 hours 20 minutes. Now we convert UTC to Standard Time. Tonga’s Standard Time correction, from the list, is plus 13 hours. Add that to 19:20 UTC on the 10th, and you get day 11 May, 08 hours 20 minutes Standard Time. Notice the key point: Tonga’s Standard Date is one day ahead of its Local Mean date. The local mean date was 10 May, but the Standard Date is 11 May. That’s the whole reason the dateline was bent — to keep Tonga on the same day as Fiji. And this isn’t unique to Tonga. The same situation exists with Chatham Island, which I mentioned earlier, and with two other anomalies: the extreme far east of Russia and Wrangell Island. In all those cases, the International Date Line has been adjusted. There’s also a mirror-image case: the far western islands of the Aleutian chain, which lie in the Eastern hemisphere. There, the dateline is adjusted further west, so the whole Aleutian chain stays on the same day as the USA. Now the second example, and this one reverses the direction. Find the LMT in Tokyo, Japan, at longitude 140°E, if the Standard Time is 1425 hours on the 16th January. Set up the columns: Standard Time Tokyo, day 16th, hour 14, minute 25. Then apply the Standard Time Difference, which is minus 09 hours. Japan’s Standard Time is UTC plus 9, so to get back to LMT you subtract 9 hours. That gives you the Local Mean Time in Tokyo. So the pattern to hold onto: LMT is tied to the sun and your longitude; Standard Time is the legal clock time of your zone. The International Date Line is a human adjustment, bent east or west, to keep politically related islands on the same calendar day.