There are two main applications of time knowledge in your flying career — Page 377, Lesson 333

We're starting a brand-new chapter now — Chapter 24, "Time." And I want to begin by telling you exactly why this matters to you as a pilot, because it's not just academic. There are two main applications of time knowledge in your flying career. First, you need to know why different countries keep different times, and how to find what the standard time is in any country you may land in. Now, most laymen — and I have to say, some pilots when they're using the Public Address to passengers — call this "local time." But the correct professional term is Standard Time. That's the term you need to use and understand. Second, you need to be able to work 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 becomes especially critical if the airport of destination does not have airfield lighting. You need to know whether you'll be arriving into darkness or daylight. Now, before we go any further, let's look at the fundamentals. Our measures of time are based on three things: the Earth's rotation about its own axis, the Earth's rotation around the Sun, and the movement of the solar system in our galaxy, and the movement of our galaxy in the Universe. So you see, time is not just a clock on the wall — it's tied to astronomy at every level. That's why we need to know something about elementary astronomy. Let's start with the solar system. The solar system consists of the Sun and the major planets, of which the Earth is one. Now, the planetary orbits — and therefore the Earth's orbit — are governed by Kepler's laws of planetary motion. There are three of them, and I want to walk you through each one carefully. The first law: The orbit of each planet is an ellipse with the Sun at one of the foci. So the Earth doesn't travel in a perfect circle around the Sun — it travels in an ellipse, and the Sun sits at one of the two focal points of that ellipse, not at the centre. The second law: The line joining the planet to the Sun, known as the radius vector, sweeps out equal areas in equal time. Let me explain what that means. Imagine drawing a line from the Sun to the planet — that's the radius vector. As the planet moves along its orbit, that line sweeps out an area. Kepler's second law says that in any equal interval of time, the radius vector sweeps out an equal area. The practical consequence is that the planet moves faster when it's closer to the Sun and slower when it's farther away — because it has to sweep equal areas in equal times. The third law: The square of the sidereal period of a planet is proportional to the cube of its mean distance from the Sun. Now, the sidereal period is the time it takes a planet to complete one full orbit around the Sun relative to the fixed stars. So this law relates how long a planet takes to go around the Sun to how far away it is from the Sun. Now, I want to be clear about which laws are the important ones for our purposes. The first two laws are the important ones, and they're illustrated in Figure 24.1, which shows Kepler's laws. The first law gives you the shape of the orbit — the ellipse with the Sun at one focus. The second law gives you the varying speed of the planet along that orbit. So let me pull this together. We've established why time matters to a pilot — for Standard Time in different countries, and for predicting daylight and darkness. We've established that time is based on Earth's rotations and movements through the Universe. And we've established the three Kepler laws that govern planetary orbits, with the first two being the critical ones for our study. That's the foundation we're building on as we move into the rest of this chapter.