Here’s the problem we’re solving — Page 402, Lesson 356

We’ve just been looking at Local Mean Time and Zone Time, so now I want to take you into the next step: Standard Time, which is what we actually use on land. Here’s the problem we’re solving. Zone Time, or ZT, is a great convenience for ships at sea, because it keeps the hours of the day roughly in step with daylight and darkness. But that system would not be appropriate on land. Think of Nigeria. The 7.5°E longitude line runs right through the middle of the country. If we used strict zone time, western Nigeria would keep a time one hour in advance of eastern Nigeria. That would be absurd for a single country. Local Mean Time, or LMT, is equally inappropriate as a national standard. In the UK, if we used LMT, then at 1200 LMT in Greenwich, the time at Oxford, which is at 001°20’W, would be 1155 LMT. And the time in Penzance, at 005°W, would be 1140 LMT. So you’d have a twenty-minute difference across the country. That situation actually did exist in the UK up until the mid-1800s, when lines of communication were slow. Then came the railways. With the advent of the railway system, the need for a standardized system was recognized. In those very early days, train guards leaving London carried a watch set to Greenwich time. At each station out of London, the station clock would be reset to the guard’s watch — that is, to GMT. And subsequently, church wardens would set the church clocks to the station clock, so effectively to London time. In that way, each town in the UK set its clocks to a standard time for the whole country. That was the foundation of the Standard Time system, abbreviated ST. So for the UK, Standard Time is UTC — which is the same as GMT — at all times of the year. In summer, we adjust our clocks forward one hour and call that British Summer Time, or BST. BST is simply one hour added on to our Standard Time, UTC. Summer Time and Daylight Saving Time corrections are discussed later in the notes. Now, the key point: each country nominates its own Standard Time, and it’s normally notified in the Air Almanac as a correction to UTC. Each country aims to fix its Standard Time so that 1200 hours ST occurs about the middle of the period of light, and 0000 hours ST occurs about the middle of the period of darkness. That means Standard Time will be close to both Local Mean Time and Zone Time, but it will not be exactly the same as either. For your navigation work, the Standard Time corrections for most countries are listed in the Air Almanac on pages 1 to 4 inclusive. The countries are listed in three general areas. List 1 is on pages 1 and 2. And that’s where we’ll pick up next — the structure of those lists and how you actually read the correction for a given country.