There are two main applications, and both are practical, operational concerns — Page 377, Lesson 333

We're starting a brand-new chapter now — Chapter 24, "Time (1)" — and I want to begin by answering the question that opens it: why do pilots need to study time at all? There are two main applications, and both are practical, operational concerns. First, you need to know why different countries keep different times, and how to find what the standard time is in any country you might land in. Now, most laymen — and I'll add, some pilots when they're using the Public Address system to passengers — call this "local time." But the correct professional term is Standard Time. I want you to lock that in right now: in aviation, we say Standard Time, not local time. The second application is working out when it gets dark in different parts of the world at different times of the year — or when it gets light in the morning. This matters especially if the airport of destination does not have airfield lighting. So you're not just learning this for trivia; you need to know whether you'll be landing into darkness at an airfield that can't light itself up for you. Now, before we get into the mechanics, let's establish the foundation. Our measures of time are based on four things: the Earth's rotation about its own axis, the Earth's rotation around the Sun, the movement of the solar system in our galaxy, and the movement of our galaxy in the Universe. So we need to know something about elementary astronomy to understand time properly. That brings us to the solar system. The solar system consists of the Sun and the major planets — of which the Earth is one. And the planetary orbits, including the Earth's orbit, are governed by Kepler's laws of planetary motion. There are three of them, and I want to give you each one precisely. The first law: the orbit of each planet is an ellipse with the Sun at one of the foci. So the Sun isn't at the centre of the orbit — it's offset, at one focus of the ellipse. The second law: the line joining the planet to the Sun, known as the radius vector, sweeps out equal areas in equal time. That means a planet moves faster when it's closer to the Sun and slower when it's farther away, so that the area swept in any given time interval stays constant. The third law: the square of the sidereal period of a planet is proportional to the cube of its mean distance from the Sun. The sidereal period is the time a planet takes to complete one orbit relative to the fixed stars — I'll come back to that concept shortly. Now, the important laws for our purposes are the first two, and they're illustrated in Figure 24.1, which shows Kepler's laws. Let me bring that up for you. So that's where we stand: we've established why time matters to a pilot, and we've laid the astronomical groundwork with Kepler's laws. Next we'll move into the seasons and then the measurement of days and years, which is where the sidereal period really comes into play.