We're starting a brand-new chapter now — Chapter 25, "Time (2)" — Page 393, Lesson 346

We're starting a brand-new chapter now — Chapter 25, "Time (2)". This is the second part of your time work in General Navigation, and it builds directly on the hour-angle concepts you just finished in Chapter 24. So before we dive into the new material, I want to make sure we're anchored on what came before, because this chapter assumes you're comfortable with it. In Chapter 24 you learned about hour angles — the angular distance of a celestial body west of a meridian, measured in degrees. You had Local Hour Angle, which is measured from your own meridian, and Greenwich Hour Angle, measured from the Greenwich meridian. That's the foundation. Now, in Chapter 25, we're going to convert that angular measurement into time. That's the whole point of this chapter: arc to time, and then the time systems that aviation actually uses. Let me walk you through the structure of what's coming, because it's a logical progression. The chapter opens with "Conversion of Arc (Angle) to Time" — that's the fundamental skill. The Earth rotates 360 degrees in 24 hours, so 15 degrees of longitude equals one hour of time, 1 degree equals 4 minutes, and 1 minute of arc equals 4 seconds of time. You'll need that conversion cold, because everything else in the chapter hangs off it. Then we move to "Local Mean Time" — that's the time at your particular meridian, based on the mean sun, not the apparent sun. The mean sun is a fictitious sun that moves at a constant rate along the celestial equator, so Local Mean Time runs at a perfectly steady rate. That's why we use it for navigation rather than apparent solar time, which speeds up and slows down through the year. From there we go to "Co-ordinated Universal Time" — UTC. This is the time standard that aviation operates on worldwide. It's essentially the Local Mean Time at the Greenwich meridian, and it's what your flight plans, your ATC clearances, and your navigation logs all reference. You'll learn how to convert between Local Mean Time and UTC, and the chapter has a dedicated section on "Local Mean Time/UTC Problems" to drill that conversion. Then we get to "Zone Time" — ZT. The Earth is divided into 24 time zones, each 15 degrees of longitude wide, and each zone uses the Local Mean Time of its central meridian. That's why a flight crossing several zones needs you to change your clock. And finally, "Standard Time" — that's the legal time adopted by a country or region, which may not align perfectly with its zone boundaries. Some places shift their standard time by half-hours or even quarter-hours for political or economic reasons. The chapter ends with a summary, then three sets of questions, then the answers to those questions, and finally an extract from the Air Almanac — that's the reference publication you'll use to look up GHA values for celestial navigation. Now, one thing I want to flag right away. The excerpt I have in front of me is mostly the answer key for Chapter 24's questions, plus this table of contents for Chapter 25. So I can see the answers to the Chapter 24 questions — 1 is a, 2 is c, 3 is d, and so on — but I'm not going to read those out to you as a batch, because those are practice questions you should work through yourself. What I can tell you is that the answers are there on page 390 for you to check your work against. What I really want to do is set you up for Chapter 25 properly, because this is where time becomes a working tool for navigation. Let me give you the core idea before we even open the first section. The Earth rotates once on its axis in 24 hours — that's 360 degrees of rotation. So if you divide 360 by 24, you get 15 degrees per hour. That single relationship is the bridge between the angular world of hour angles and the clock world of time. When you know the Greenwich Hour Angle of a body, you can convert that angle into the time at Greenwich. When you know your longitude, you can convert that into the time difference between your meridian and Greenwich. Here's the practical picture. Imagine you're flying east. Your Local Mean Time is ahead of UTC, because the sun rises earlier at your longitude. Every 15 degrees of longitude east of Greenwich, your LMT is one hour ahead of UTC. Every 15 degrees west, it's one hour behind. That's the conversion you'll be doing over and over — longitude to time, time to longitude. And there's a figure in the book that shows this relationship — it's Figure 24.9, "Local hour angle/Greenwich hour angle," which you saw in the previous chapter. That's the visual anchor for everything we're about to do. So here's where we stand. You've finished the hour-angle material in Chapter 24, and you have the answer key for that chapter's questions. Now we're moving into Chapter 25, where we take those angles and turn them into the time systems that govern every flight you'll ever plan or fly. The first thing we'll tackle is the conversion of arc to time — that 15 degrees per hour relationship — and then we'll build Local Mean Time, UTC, Zone Time, and Standard Time on top of it. Let's start there. The conversion of arc to time is your foundation stone. The Earth rotates 360 degrees in 24 hours, so 15 degrees of longitude equals one hour. From that, 1 degree equals 4 minutes, and 1 minute of arc equals 4 seconds of time. You'll use these three equivalences constantly — converting a longitude like 75 degrees west into 5 hours behind Greenwich, or converting a GHA of 240 degrees into 16 hours of time. Get comfortable with that arithmetic now, because every time problem in this chapter — and every time problem on your exam — starts with it. When you're ready, we'll move into Local Mean Time proper, and I'll show you how the mean sun gives us a steady clock where the apparent sun doesn't.