ppl_03_e2

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CHAPTER 1: FORM OF THE EARTH

The early seafarers also realised that a method needed to be devised to produce, on a flat surface, a chart on which directions were reliably represented.

Modern 1:500 000 and 1:250 000 aeronautical charts used by general aviation pilots are charts in which angles, bearings and direction are indicated accurately, at the price of some distortion of distance. Over the small distances represented by these charts, however, the inaccuries of distance are negligible.

THE EARTH’S HEMISPHERES.

The Earth has four identifiable hemispheres. The most well known ones are the Northern and Southern Hemispheres which lie to the North and South of the Equator. Less well known are the Eastern and Western Hemispheres which lie to the East and West of the Prime Meridian. (See Figure 1.6.)

Figure 1.6 Hemispheres on the Earth.

BASIC DIRECTION ON THE EARTH.

Navigators in ancient times found their way on the sea, by reference to the position and apparent motion of the Sun and the stars. Meteorological indications of direction were also used, most notably the directions from which regular and steady winds blew. Sailors distinguished between the cold winds from the North and warm winds from the South. The ancients gave names to 8 principal winds, which were represented as 8 equally spaced points of a wind rose (the rosa ventorum).

North, South, East and West.

The four cardinal points of North, South, East and West were also established several thousand years ago, the great age of these concepts being recognised by the equally great age of the words used to describe them.

East was the name given to the point on the horizon where the Sun rises at the vernal (spring) and winter equinoxes. (The Romans called the East, or morning, ‘oriens’, hence our word orient). West (Latin ‘occidens’), was the name for the point on the horizon where the sun sets at the equinoxes.

Modern

aeronautical charts

represent

angles, bearings and direction accurately.

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CHAPTER 1: FORM OF THE EARTH

Directions measured

with respect to the

North Geographical Pole are “true” directions.

“Magnetic” directions,

indicated by a

compass, are not the same as “true” directions.

In air navigation,

direction is expressed

in degrees with respect to either True or Magnetic North.

North and South were determined as directions lying at 90° to East and West, measured by the shadows cast by the noon-day Sun. This first designation of North and South gave man the concept of the Earth’s geographical North and South Poles, which mark the two ends of the axis about which the Earth spins.

The Sun, Moon and stars all rise in the East and set in the West. But the stars in the

Northern Hemisphere appear to rotate about an apparently fixed star which always lies in the direction of North as viewed from the Earth’s surface. This star became known as the Pole Star, and the Earth’s North and South Poles were seen to be the markers of the Earth’s spin axis.

True North and Magnetic North.

The direction in which the geographical North and South Poles lie later became known as True North, and True South, because, in the 15th Century, it became apparent that North as indicated by a magnetic compass needle, did not point to the “true” North Pole from all locations on the Earth’s surface. “Magnetic” North differed from “True” North by a varying number of degrees depending on the observer’s location, and the difference became known as “magnetic variation”. (See Chapter 3.) It is of the greatest importance in navigation that the difference between true indications and magnetic indications of direction should always be allowed for.

We still use the astronomical concepts of North, South, East and West to indicate direction, though the subdivision of the original, four cardinal points has been much refined. For instance, the midway directions between North (N), East (E), South

(S) and West (W) are designated North-East (NE), South-East (SE), South-West (SW), and North-West (NW), known as the ordinal points. There are also further sub divisions such as West North West (WNW) and North North West (NNW), which need not concern us too much here. (See Figure 1.7.)

However, in air navigation, instead of referring to direction by using the names of the cardinal points and their sub divisions, bearings, headings or tracks are indicated in degrees with respect to either True or Magnetic North. Compass indication cards are graduated clockwise from 0° (or 360°), which marks North, to 359°.

Figure 1.7 The Points of the Compass.

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CHAPTER 1: FORM OF THE EARTH

Dividing by Sixty.

The system of dividing the compass into 360° is known as the sexagesimal system, because it is based on 60. This system was chosen by the navigators and scientists of old, because 60 is a number which makes division easy, being divisible by 2, 3, 4,

5, 6, and 10. If North is defined as 000°, East becomes 090°, South becomes 180°, and West becomes 270°. Continuing the rotation back to North makes North 360° (see Figure 1.8). In practice, the use of 000° or 360° for North is often a personal choice. However, an air traffic controller is likely to refer to North as 360°.

When the datum point for direction, measured in degrees, is the North Geographic

Pole, the direction is referred to as true direction. When the datum is the Earth’s

Magnetic North Pole, (see Chapter 3) direction is referred to as magnetic direction.

Note that a 3-figure group is always used to indicate direction (e.g. 000°, not 0° and 090°, not 90°). The use of 3-figure groups for directions has been adopted to avoid ambiguity, particularly in transmitting messages by Radiotelephony.

Figure 1.8 Measuring direction by degrees: 0° to 360°.

Figure 1.9 A compass rose showing cardinal points, ordinal points and degrees.

(Image by courtesy of Dave Gittins.)

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CHAPTER 1: FORM OF THE EARTH QUESTIONS

Representative PPL - type questions to test your theoretical knowledge of the Form of the Earth.

1.What is the angle of inclination of the Earth’s orbit to its orbital plane?

a.23½°

b.66½°

c.90°

d.33½°

2.The meridian passing through Greenwich is known as?

a.Main meridian

b.Equator

c.Prime meridian

d.Great meridian

3.A Rhumb line is?

a.A regularly curved line on the Earth’s surface which represents the shortest distance between two points

b.A line showing True North

c.A line on the surface of the Earth whose centre and radius are those of the Earth

d.A regularly curved line on the Earth’s surface which cuts all meridians at the same angle

4.What type of line will always be the shortest distance between two points on the Earth?

a.A Rhumb line

b.A Great Circle

c.A Small Circle

d.A parallel of latitude

5.What item identifies the difference between True North and Magnetic North?

a.A Rhumb line

b.A meridian of Longitude

c.Degrees magnetic

d.Magnetic variation

Question	1	2	3	4	5
Answer

The answers to these questions can be found at the end of this book.

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CHAPTER 2

TIME

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CHAPTER 2: TIME

INTRODUCTION.

Timing in flying, as in many activities, is of crucial importance. Therefore, pilots need to have a good understanding of both the nature and measurement of time.

When flying cross country, a pilot must regularly check that his aircraft is on heading, on track, on speed, and that its progress along the desired track is on time. If ground features confirm that an aircraft has covered the planned distance, on track, in the planned time, a pilot knows exactly where he is, and is confident that the forecast wind and his own flying are accurate. This knowledge assures him that his Estimated

Time of Arrival (ETA) at destination, or at the next waypoint, will be as predicted, and that he does not need to worry about running short of fuel or running out of daylight.

Even if a pilot arrives early or late at a particular ground fix along his route, if he knows how early or how late he is, he is able to update ETAs, recalculate groundspeed and revise his fuel management, so that the flight can be continued in safety, and future events remain under his control.

In either case, the correct and competent management of time helps the pilot to remain in control of the navigational task.

An understanding of the nature of time also helps the pilot to appreciate the larger scale passage of time and its consequences.

The times of sunrise and sunset change continuously throughout the year, varying with the changing seasons and with different altitudes. Knowledge and understanding of this phenomenon enable the pilot to plan his flying day and not get caught in the dark, whether by staying airborne too late, or by being fooled that the Sun, still visible

at high altitude, might also still be illuminating the Earth’s surface beneath him.

The Nature of Time.

Definingthenatureoftimehasbeenachallengetophilosophersandsagesthroughout the ages. Does time require space in which it must exist and through which it flows, or can time exist without space? Would time exist anyway, even if there were no material universe? Is the “now” ever present, or does it come into existence only when the time flow reaches it? Such questions show that time can be classified as a philosophy.

This philosophy is of little concern to the air navigator. However, the unstoppable flow of time, by which we are all being carried along, is marked by our clear perception that, with the passage of time, things change. The seasons come and go and man is born, grows old and dies. Change accompanies the flow of time, and regular, cyclic change enables man to measure time, and measuring time is a practical matter with which the pilot navigator is concerned.

A complete rotation of the Earth on its axis gives us the solar day; one orbit of the moon around the Earth takes a lunar month, and a complete revolution of the Earth about the Sun gives us the year. These time periods provide us with natural units of time, which man has used over countless centuries.

Conversely, the time divisions which we call the week, the hour, the minute and the second are man-made time divisions. These divisions, though old, are less ancient.

Correct and competent

management

of time helps the pilot to remain in control of the navigational task.

Regular and cyclic change enables us to measure time.

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In the

navigation skills test

for the PPL, a candidate must arrive at the destination,

or at a waypoint, within 3 minutes of the declared ETA.

The progression

of the seasons and the changing duration of the periods of daylight and darkness are caused by the tilt of the Earth’s axis and the rotation of the Earth about the Sun.

For the pilot flying cross-country, it is the hour, the minute and the second which are of importance, more especially the minute.

In the navigation skills test for the PPL, a candidate must arrive at his destination, or at a waypoint, within 3 minutes of the declared ETA.

THE MEASUREMENT OF TIME.

Defining the nature of time is difficult, but measuring time is not. Although there are different systems and zones of time that a pilot must know about, these are not difficult to learn.

The direction of the Earth’s rotation causes the Sun, the Moon and the stars to appear to rise in the East and set in the West. The interval of time between two successive apparent passages of the Sun across any meridian of longitude on which an observer is located is the solar day.

The sidereal day is the time interval between two successive apparent passages of a given star across a given meridian. There is a difference of only minutes between a solar day and a sidereal day.

A sundial will indicate solar time, and, as every one knows, although the length of the solar day has for a long lime been divided by man into twenty-four hours, the length of the periods of daylight and darkness change continually as the seasons progress.

The Length of the Day and the Seasons.

The progression of the seasons and the changing duration of the periods of daylight and darkness, in the 24-hour day, result from the tilt of the Earth’s axis with respect to its orbital plane, and the Earth’s rotation about the Sun. (See Figure 2.1.)

Figure 2.1 The Northern Hemisphere Seasons, and the changing length of daylight and night.

The Earth, which is inclined at 66½° to its orbital plane, rotates about its own axis once in 24 hours to give us our solar day. The Earth also revolves around the Sun once every 365. 2422 days to give us our year. Possessing the characteristics of a

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gyroscope, the Earth’s axis always maintains the same orientation (i.e. points in the same direction) in space. On one particular day in its orbit around the Sun, which modern man labels as 21st June, the Earth’s North Geographical Pole is tilting directly towards the Sun. In this position, not only is the Northern Hemisphere in midsummer and the Southern Hemisphere in midwinter, but the Northern Hemisphere days are at their longest and the Southern Hemisphere days at their shortest. Six months later the North Geographical Pole is pointing directly away from the Sun; at this point in the Earth’s orbit, the Northern Hemisphere is experiencing its shortest day in midwinter while the Southern Hemisphere is in midsummer and experiencing its longest day. Through all the Earth’s intermediate orbital positions, seasonal change progresses and the periods of daylight and darkness vary continually in length, with the times of apparent sunrise and sunset changing daily. At the Equator, the length of daylight and darkness is very nearly equal the whole year long, while at 50° North or South the period of daylight varies between 8 hours, in the Winter, and 16 hours in the Summer. At the North and South Poles, the periods of daylight and darkness are each of 6 months’ duration.

The Calendar, Years and Leap Years.

In order to keep the modern Gregorian calendar in phase with the seasons, (basically to keep the vernal equinox, in the Northern Hemisphere, as close as possible to March 21) each normal calendar year has 365 days, with every fourth year (i.e. every year divisible by 4, called a leap year) having 366 days, the “extra” day being 29th February. Centennial years, such as 1800, 1900, etc., are, however, not leap years, unless they are divisible by 400. So, the year 2000 was a leap year, but the years 1800 and 1900 were not.

CHAPTER 2: TIME

At the Equator, the length of daylight and

darkness is nearly equal,

the whole

year long. At the North and South Poles, daylight and darkness last 6 months each.

In an any given season, the relative

lengths of daylight and

darkness

at a particular location on Earth depend on that

THE TIME OF DAY – MEASURING TIME BY THE CLOCK OR CHRONOMETER.

You may be able to deduce from Figure 2.1 that, at any moment in the Earth’s period of rotation, one half of the Earth is illuminated by the Sun and is in daylight, while the other half of the Earth is in the shadow that we call night. Depending on the season, the relative length of the periods of daylight and darkness at any particular location on the Earth’s surface depends on that location’s latitude.

Daylight and darkness are, of course, natural phenomena. But man has divided the day into 24 hours, and has subdivided hours into minutes and seconds, which are arbitrary and artificial units of time. Nevertheless, hours, minutes and seconds, measured by clocks and chronometers, regulate our life, and are of immense importance to navigation. We must, therefore, examine “clock time” in some detail.

There are various systems of clock time in use throughout the world that we must look at. But we can begin by stating that local time at any given locality on Earth will

depend on longitude.

Local Mean Time.

Local Mean Time at any specified meridian of longitude is referenced to the Sun.

If we consider a particular meridian of longitude (it does not matter which one we chose), when the Sun crosses that particular meridian, all places on that meridian have the time 1200: that is, midday or noon, while the local mean time on the meridian displaced by 180° to the meridian that we are considering (its antipodal point) has the time 2400: that is, midnight. Meridians of longitude to the East of our meridian

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Figure 2.2 Time of day - the Earth viewed from the North Pole at one of the equinoxes.

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The Local Mean Time

at any given locality on Earth depends on that locality’s longitude.

Local Mean Time

advances by 4 minutes per degree of

longitude, counting eastwards, and regresses by 4 minutes per degree of longitude, counting westwards.

will already have a local mean time later than noon, and meridians to the West of our meridian will have a local mean time earlier than noon. (See Figure 2.2.)

Local mean time, then, depends on longitude. As there are 24 hours in a day, and the Earth, in one rotation, turns through 360°, local mean time advances by four minutes per degree of longitude, counting eastwards, and regresses by four minutes per degree of longitude counting westwards. (24 x 60 = 1440 minutes ÷ 360 = 4).

Every meridian of longitude, then, has a different local mean time. For example, in the United Kingdom, Birmingham lies

approximately on the 2° West meridian, and Swansea is on the 4° West meridian. When the Sun transits the 2° West meridian, the local mean time at Birmingham is 12 o’clock midday; at the same moment, the local mean time in Swansea, 2° further West, is behind that of Birmingham by 8 minutes (2° x 4 minutes), making Swansea’s

local mean time 11:52, or eight minutes to midday.

Time Zones and Standard Time.

It may seem obvious to us, nowadays, that local mean time is not at all convenient for a nation to work to. Having different times in Swansea and Birmingham would make it extremely difficult for government and commerce to regulate their affairs.

Nevertheless, before the advent of the railways, local time was computed in that way. 12 o’clock noon was the moment when the Sun was at the highest point in the sky that it would reach according to the season (its zenith), wherever a person happened to be.

Figure 2.3 Time Zones

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