Chapter 3: Rotation And Revolution
On the third of January, a student in Agra pulls a blanket tighter and waits for the fog to lift. It is one of the coldest mornings of the year. That same morning, a penfriend in New Zealand is looking for shade, because it is the middle of a hot summer afternoon there. Same day, same sun, two completely opposite experiences of it. Now add the part that makes no sense at first. On or about that very date, the earth is closer to the sun than at any other time in the whole year — about 147.3 million km away, against 152 million km in early July, when north India is at its hottest. So the cold morning in Agra happens when the sun is nearest, and the hot afternoon in July happens when the sun is farthest. Distance from the sun, it turns out, is not what makes a season. Something else is doing the work: a planet spinning from west to east, tilted permanently to one side, carrying that tilt all the way round its orbit. This chapter is about those two motions and everything they decide for us.
Diagram
Fig. 3.A1 Caption: On 3 January the earth is nearest the sun, yet the northern half is tilted away from it. Brief: Draw the sun at left with parallel rays striking the earth at right; show the earth as a globe with its axis tilted 23½° so the North Pole leans away from the sun; label Equator, Tropic of Cancer, Tropic of Capricorn, Northern Hemisphere and Southern Hemisphere; shade the lit half and the dark half; mark the sun–earth distance as 147.3 million km with an arrow; do not show country boundaries.
3.1 Rotation Of The Earth
The earth, like every other planet in the Solar System, revolves round the sun in an elliptical orbit, and at the same time it spins on its own axis. These two movements are known as revolution, the earth’s yearly journey round the sun, and rotation, its daily spin. Neither one stops for the other; the earth is doing both at every moment.
Almost everything a student of Geography will study later rests on these two motions. Day and night, the seasons, the direction in which winds blow and the paths that ocean currents follow all depend on them. That is why this chapter comes so early in the book.
What Rotation Actually Is
Rotation is the spinning of the earth on its axis from west to east, once in twenty-four hours. The axis is an imaginary line passing through the centre of the earth, exactly like the metal rod running through a globe in a classroom. Nothing is really there; it is a line we draw so that we can describe the spin.
The two end points through which this axis passes are the North Pole and the South Pole. The axis is not upright. It is tilted at an angle of 23½° from the vertical, and this tilt never changes as the earth travels round the sun.
The tilt can be measured against a second line as well. If a line were drawn parallel to the plane of the earth’s orbit, the axis would make an angle of 66½° to that line, as shown in Fig. 3.1. The two figures, 23½° and 66½°, describe the same tilt measured from two different references, and together they add up to 90°.
Diagram
Fig. 3.1 Caption: Inclination of the earth’s axis. Brief: Draw the earth as a circle with the Equator marked and the axis running from North Pole to South Pole; show the vertical dashed line and the dashed line of the plane of the earth’s orbit; mark 23½° between the axis and the vertical and 66½° between the axis and the orbital plane; label North Pole, South Pole, Equator, earth’s axis and plane of the earth’s orbit.
3.1.1 Characteristics Of Rotation
Three things about the spin need to be fixed in your mind, because the rest of the chapter is built on them.
- Direction: the earth completes one rotation on its axis from west to east in 23 hours, 56 minutes and 4.09 seconds, which is taken roughly as 24 hours.
- Inclination of axis: the earth is always tilted in the same direction, towards the Pole Star, as it moves round the sun. The angle at which the earth is tilted is known as the inclination of the earth’s axis.
- Speed: the speed of the earth’s rotation at the Equator is 1670 km per hour, because the circumference of the earth is 40,075 km and that whole distance is covered in 24 hours. The speed decreases towards the poles.
The speed falls away from the Equator for a simple reason. Every point on the earth completes one turn in the same 24 hours, but a point on the Equator has the widest circle to travel, while a point nearer a pole travels a much smaller circle in the same time. Less distance in equal time means a slower speed.
3.1.2 Effects Of The Rotation Of The Earth
Rotation is not a fact to be memorised on its own. It is a cause, and it has a long line of consequences on the earth’s surface.
- Day and night occur because the parts of the earth facing the sun have day, while the parts turned away from it have night.
- Sunrise and sunset are caused by the rotation of the earth from west to east, which brings every part of the surface into the sun’s rays at some time and takes it out again. The part moving from darkness into the sun’s rays experiences sunrise, and the part moving away from them experiences sunset.
- The Coriolis Effect is the force experienced by any object moving over the surface of a rotating body such as the earth. It plays an important part in the movement of air, which we call winds, and of water, which we see as ocean currents.
- The sun, the moon and the stars seem to move from east to west, because the earth itself spins from west to east.
- Centrifugal force is an apparent outward force that acts on a rotating object, always directed outward, away from the axis of rotation. The speed of rotation has created a centrifugal force that has produced a bulge in the middle portion of the earth and a flattened top at the poles.
- Rotation causes a difference in time over various places on the earth, and that difference works out at 4 minutes for each degree of longitude.
- The side of the earth turned towards the sun constantly gains heat, while the side away from it constantly loses heat by radiating it into outer space. Rotating in a tilted position gives the earth two advantages here.
- The short rotational period keeps the variation of temperature between day and night moderate.
- Rotation in combination with revolution, the earth’s movement round the sun, causes seasons and variations in the lengths of day and night over different places on earth.
A Day, Read Hour By Hour
Day and night do not switch on and off. They arrive with precision and progression, because the earth turns steadily and a place is carried into the light a little at a time. The time when the sun begins to cast its light in the sky is known as dawn. At noon the sun is overhead, at dusk it is twilight and the sun is seen disappearing from the sky, and at night it is completely dark.
The duration of day and night is not equal at all places on the earth, and the reason is the inclined axis. The length of days at a place varies with the seasons and with its latitude, which is the whole subject of the sections that follow.
Which Way A Moving Thing Bends
The Coriolis Effect does not push winds or currents along; it turns them aside once they are already moving. Because of it, winds and ocean currents move clockwise and are deflected to the right in the Northern Hemisphere.
In the Southern Hemisphere the deflection is the mirror image. There, winds and ocean currents move in the anticlockwise direction and are deflected to the left. The hemisphere a moving mass of air or water finds itself in therefore decides which way it will curve.
One Turn, Many Clocks
The time difference between places follows straight from the spin, and it can be worked out exactly.
What this figure gives you is a rule you can apply to any two places. Two places 15° of longitude apart are carried into the sun’s rays one full hour apart, because \(15 × 4 = 60\) minutes. The sun does not rise everywhere at once, and this arithmetic is exactly why.
The Order Of A Single Day
The four stages of a day follow one another in a fixed order and then begin again, because the earth’s spin never stops. Fig. 3.A2 sets out that order.
Flowchart
Fig. 3.A2 Caption: One rotation carries a place through the whole sequence of a day and returns it to the start. Nodes: place turns towards the sun → dawn, the sun begins to cast its light → noon, the sun is overhead → place turns away from the sun → dusk, twilight as the sun disappears → night, complete darkness → back to dawn
3.2 Inclination Of Earth’s Axis And Its Significance
We have said that rotation is the spinning of the earth on its axis from west to east, and that the axis is the imaginary line passing through the centre of the earth on which it rotates. What has not yet been explained is why that line matters so much.
The earth’s axis is not vertical. It makes an angle of 23½° with the vertical, or 66½° with the plane of the Earth’s orbit. That single lean, held steadily year after year, is responsible for almost every difference between one place and another in this chapter.
An Earth With An Upright Axis
The easiest way to see what the tilt does is to imagine it gone. Let us suppose the earth had a vertical axis. Then at all places on the earth the length of day and night would have been equal, as shown in Fig. 3.2.
Take a place L on the Equator. As the earth turns, L would be carried to position L’ after 12 hours.
Distance LO = Distance OL’
∴ Length of day = Length of night.
The two distances are equal because an upright axis cuts the lit half and the dark half of the earth into equal parts along every parallel. What is true of L is true everywhere. At all places on the earth, the length of day and night would be equal if the axis of the earth was not inclined.
Diagram
Fig. 3.2 Caption: Hypothetical Vertical Axis Brief: Draw the earth as a circle with an upright axis from NP to SP and the Equator horizontal across it; shade the half away from the sun as night and the half facing the sun as day; show the sun’s rays as horizontal arrows striking from the right; mark point L on the lit side of the Equator, O at the centre of the Equator line and L’ on the dark side; label NP, SP, Equator, Day, Night and Sun’s Rays.
3.2.1 Effect Of The Inclined Axis On The Duration Of Day And Night
The real earth is tilted, so the neat halves of that imagined globe do not appear. Four consequences follow from the inclination.
- The duration of day and night is not equal at all places.
- The time of the day at any place is determined by the height of the sun in the sky.
- If the axis of the earth was vertical instead of inclined, places on earth would have the same temperature throughout the year.
- If the earth did not revolve around the sun, then even with its inclined axis there would be no seasons.
The last of these is worth pausing on. The tilt by itself changes nothing; it is the tilt carried round the sun through a whole year that produces a changing pattern. Tilt and revolution have to work together.
The effect of the inclined axis on the duration of day and night is that their duration differs, as shown in Fig. 3.3. Reading five places off that figure makes the pattern clear.
Diagram
Fig. 3.3 Caption: Inclined Axis Brief: Draw the earth as a circle with the axis tilted, the Equator, the Tropic of Cancer, the Tropic of Capricorn, the Arctic Circle and the Antarctic Circle drawn across it; shade the lit half and the dark half with sun’s rays as arrows from the right; mark A and A’ on the Arctic Circle, B and B’ on the Tropic of Cancer with X on the circle of light, C and C’ on the Equator with O at the centre, D and D’ on the Tropic of Capricorn with Y, and E on the Antarctic Circle; label NP, SP and Sun’s Rays.
Here is what each of the five places tells us.
- At the Equator, place C would be at position C’ after 12 hours, and distance CO = distance OC’, so the length of day equals the length of night.
- On the Tropic of Cancer, the parallel at 23½° N, place B will be at B’ after 12 hours. But distance BX is greater than B’X, so days are longer than nights.
- On the Arctic Circle, at 66½° N, place A will be at position A’ after 12 hours. AA’ remains in continuous light in summer for six months, so there is a day of 24 hours near the North Pole in summer.
- Place D on the Tropic of Capricorn, at 23½° S in the Southern Hemisphere, will be at position D’ after 12 hours. Distance DY, in light, is less than distance D’Y, in darkness, so days are shorter than nights.
- Place E on the Antarctic Circle, at 66½° S, remains in continuous darkness in winter, so there is night for 24 hours near the South Pole in winter for six months.
Read the five together and a single pattern appears. The Equator is the only line where the two halves stay equal; away from it, the further you go towards a pole, the more unequal day and night become, until at the polar circles the shorter one disappears altogether. Notice also that the two hemispheres are always in opposite conditions on the same date.
3.2.2 Effect Of The Inclined Axis On Seasons
The path of the earth around the sun is known as the elliptic. It is a flat plane that passes through the centre of both the earth and the sun. The earth’s axis is not perpendicular to the plane of the elliptic, but is tilted at an angle of 66½° to it, and is therefore inclined at an angle of 23½° from the perpendicular to this plane.
As a result of this, in the course of the earth’s movement around the sun, the North Pole remains inclined towards the sun for a part of the year, while the South Pole is tilted away from it. For the remaining part of the year the situation is reversed, the North Pole being tilted away from the sun and the South Pole being inclined towards it.
This is what gives rise on earth to the changing seasons and to the varying lengths of day and night. The tilt does not change; what changes is where the earth stands in its orbit, and therefore which hemisphere is leaning into the sun’s rays.
If the earth’s axis were perpendicular to the plane of orbit, there would be equal nights and days at all places on the earth, at all times of the year, and there would also be no seasons at all.
Tilted axis and revolution of the earth also create heat zones, which is explained later in this chapter.
3.3 Revolution Of The Earth
The earth revolves round the sun in an elliptical orbit. This motion, taken together with rotation, is what makes the earth’s environment complex: one motion gives us the day, the other gives us the year, and the two run at the same time without interfering with each other.
Fig. 3.4 shows the whole journey in one picture — the four positions the earth passes through as it travels round the sun, with the tilt of the axis held in the same direction at every one of them.
Diagram
Fig. 3.4 Caption: Revolution of the Earth Brief: Draw the sun at the centre with the earth’s elliptical orbit around it; show the earth at four positions — 21 June (Summer Solstice), 23 September (Autumnal Equinox), 22 December (Winter Solstice) and 21 March (Spring Equinox); at each position draw the globe with the axis tilted 23½° and always leaning in the same direction; label NP, SP, Equator, Tropic of Cancer, Tropic of Capricorn, 66½° N, 23½° N, 0°, 23½° S and 90° S, shade the lit and dark halves, and mark the direction of the earth’s motion with arrows along the orbit.
3.3.1 Characteristics Of Revolution
Four features of this yearly journey need to be held in mind.
- The earth moves round the sun in an elliptical orbit at an average speed of 29.8 kilometres per second, or about 100,000 kilometres per hour.
- When the earth is closer to the sun, the gravitational pull makes the earth move faster than when it is away from the sun, so the speed of revolution is not uniform.
- The period taken by the earth to make one complete round of the sun is 365 days, 5 hours, 48 minutes and 45.51 seconds, which is taken as 365 days and 6 hours.
- The earth’s axis is always inclined to its orbital plane at an angle of 66½°, and the motion of the earth is from west to east, in the same direction as the rotation.
The third of these creates a small problem for the calendar, and the way we solve it is worth working through.
When February has 29 days, that year is known as a leap year, having 366 days. The adjustment is not a decoration on the calendar; without it, our dates would slowly slide away from the earth’s actual position in its orbit, and in time the calendar would put the solstices and equinoxes on the wrong dates altogether.
3.3.2 Effects Of Revolution
Seasonal Changes
The amount of heat that a place receives on earth depends on the angle at which the sun’s rays reach it. When a place receives vertical sun rays, the place gets hot.
The reason lies in how the heat is spread. The heat carried in direct vertical rays is concentrated on a small area, whereas the same amount of heat is spread over a larger area in the case of oblique rays. The rays are not stronger or weaker; they are simply packed more tightly or more loosely onto the ground, and a square metre of ground under vertical rays therefore receives far more heat than a square metre under slanting ones.
We have already noted that the tilt of the earth’s axis causes the seasons. Revolution then causes those different seasons to arrive in turn, that is, the changing weather conditions that follow from differences in the heating of the earth. Revolution is an annual cycle, so the seasons repeat year after year.
Inclination Of Axis And Seasonal Changes
Since the earth is tilted, the seasonal changes are not abrupt. A hemisphere does not swing from full summer to full winter; it leans towards the sun and then away from it by degrees, and the weather follows that gradual change.
Perihelion And Aphelion Positions
The earth’s orbit is the path along which the earth travels around the sun. This path is elliptical in shape, which means it is a stretched circle rather than a true one, and the sun does not sit at its centre.
Because of this elliptical orbit, the distance between the earth and the sun varies through the year. When the earth is closest to the sun the position is called perihelion, and when it is farthest the position is called aphelion.
The two distances are given in Fig. 3.5. At aphelion the distance of the sun is 152 million km, while at its closest, at perihelion, the earth is 147.3 million km away from the sun.
Diagram
Fig. 3.5 Caption: Perihelion and Aphelion Positions Brief: Draw the sun off-centre inside the earth’s elliptical orbit; mark the earth’s position on 3 January labelled Perihelion with the distance 147 million km to the sun, and on 3 July labelled Aphelion with 152 million km; mark the positions for 21 March, 21 June, 23 September and 21 December on the orbit; draw the Line of Solstice through the orbit and label it.
Owing to the elliptical orbit, the minimum distance of the earth from the sun is reached on or about January 3rd every year. On January 3, the earth in its orbit is closest to the sun, and this position of the earth is said to be Perihelion. On about July 4, the earth is farthest from the sun, and at this time the earth is said to be Aphelion.
It is winter in the Northern Hemisphere at perihelion, because that is when the axis is tilted away from the sun. While the tilt makes the Northern Hemisphere cool in winter, its comparative nearness to the sun helps to make the Southern Hemisphere warm in summer at the same time.
Creation Of Heat Zones
The spherical shape of the earth, along with its movements around the sun, causes a difference in the angles at which the sun’s rays fall on the earth’s surface. That in turn causes a difference in the distribution of heat over the surface. As a result, the world has been divided into three distinct heat or temperature zones, bounded by the parallels shown in Fig. 3.6.
Diagram
Fig. 3.6 Caption: Important Parallels of Latitudes Brief: Draw the globe with the Arctic Circle at 66.5° N, Tropic of Cancer at 23.5° N, Equator at 0°, Tropic of Capricorn at 23.5° S and Antarctic Circle at 66.5° S marked as horizontal lines, with 90° N at NP and 90° S at SP; bracket and label the bands to the right as Frigid zone, Temperate zone, Torrid zone, Temperate zone and Frigid zone from north to south.
The three zones are defined by latitude, and each one takes its climate from the angle at which the rays arrive there.
- The Torrid Zone stretches between 23½° N and 23½° S latitudes and more or less receives the vertical rays of the sun, so this region experiences a moderately warm climate throughout the year.
- The Temperate Zone lies in between the Torrid and the Frigid zones, that is, between 23½° and 66½° North and South. In this zone the sun’s rays fall obliquely, and hence the region experiences a moderate climate.
- The Frigid Zone extends from 66½° N to 90° N, the North Pole, and from 66½° S to 90° S, the South Pole. Since the sun’s rays fall extremely obliquely in this region, it experiences an extremely cold climate.
Fig. 3.A3 sets the same classification out as a branching tree, so that the rule behind each zone stays visible.
Flowchart
Fig. 3.A3 Caption: The angle at which the sun’s rays arrive is what divides the world into three heat zones. Nodes: earth’s spherical shape and its movements round the sun → sun’s rays fall at different angles on different latitudes → rays more or less vertical? yes → Torrid Zone, 23½° N to 23½° S, moderately warm all year; no → rays oblique? yes → Temperate Zone, 23½° to 66½° N and S, moderate climate; rays extremely oblique? yes → Frigid Zone, 66½° to 90° N and S, extremely cold
3.4 Solstices And Equinoxes
The Circle Of Illumination
The lighted part of the earth is called the circle of illumination. It is the boundary between the half of the earth that the sun’s light reaches and the half that lies in darkness.
This circle moves with the apparent movement of the sun, so that only half of the earth faces the sun at any given point of time. The other half is always in night. What changes through the year is not how much of the earth is lit, but how the lit half is divided between the two hemispheres.
Diagram
Fig. 3.A4 Caption: The circle of illumination always cuts the earth in half, but the tilt decides how that half is shared between the hemispheres. Brief: Draw the globe with the axis tilted 23½° and the Equator marked; draw the circle of illumination as a bold line separating the lit half from the shaded half, with the sun’s rays as parallel arrows from one side; show that the lit portion north of the Equator is larger than the lit portion south of it, and label circle of illumination, day, night, NP and SP.
The Two Solstices
On June 21, the earth is in the position where the North Pole is tilted towards the sun. On this day the sun is overhead at the Tropic of Cancer, and the day is known as the Summer Solstice. The Southern Hemisphere, however, is tilted away from the sun on the same date and experiences winter.
On December 22, the position is the other way round. The Southern Hemisphere is tilted towards the sun, while the Northern Hemisphere is tilted away from it. This day is known as the Winter Solstice. It is summer in the Southern Hemisphere and winter in the Northern Hemisphere.
Notice that the names belong to the Northern Hemisphere’s experience. The same date is a solstice for the whole earth, but the season it brings depends on which hemisphere you are standing in.
The Two Equinoxes
On two days in a year, the sun shines vertically over the Equator, that is, at an angle of 90°. This angle decreases as one moves away from the Equator.
These two days are 21st March, the Spring or Vernal Equinox, and 23rd September, the Autumnal Equinox. At the equinoxes the days and nights are of equal length throughout the world, because the circle of illumination then cuts every parallel into two equal parts.
On March 21 it is spring in the Northern Hemisphere, and on September 23 it is autumn there. The position is reversed in the Southern Hemisphere, where March brings autumn and September brings spring.
At the two equinoxes the sun is at 90° at the Equator, and as one moves away from the Equator the altitude of the sun decreases. After March 21 the apparent northward movement of the sun continues up to June 21st, the summer solstice, and through those weeks there are variations in the duration of daylight from place to place.
Telling The Two Apart
These two words are easy to confuse, so it is worth setting them side by side. A solstice refers to the time when the sun reaches its highest or lowest point in the sky at noon, resulting in the shortest and the longest days of the year. An equinox refers to the time when the sun crosses the plane of the earth’s Equator, making night and day of equal length.
Their names carry their meanings. The word Solstice is derived from the Latin words ‘Sol’ meaning ‘sun’ and ‘sistere’ meaning ‘to stand still’. The word Equinox comes from the Latin words ‘aequus’ meaning ‘equal’ and ‘nox’ meaning ‘night’.
Both happen twice a year, but on different dates and in different parts of the seasonal round. The solstices happen on June 21, the summer solstice, and on December 22, the winter solstice, and they fall during summer and winter. The equinoxes happen around March 21, the Spring or Vernal Equinox, and September 23, the Autumnal Equinox, and they fall at the start of spring and autumn.
So the four dates mark the two extremes and the two balance points of the year: June 21 and December 22 are the turning points of day length, while March 21 and September 23 are the days on which the whole world shares day and night equally.
3.5 Angle Of Incidence And Duration Of Daylight
Everything said so far about tilt, seasons and day length can be checked against numbers. The angle of incidence of the sun’s rays and the duration of daylight at different latitudes are given in Table 3.1, for the equinoxes and for both solstices.
The double dots in the second column are ditto marks: they repeat the entry above, so the duration of daylight on September 23 and March 21 is 12 hours at every latitude in the table. A duration entered as 0 means no daylight at all on that date.
Working Out The Angle Of Incidence
The altitude, or the angle of incidence, is also measured in degrees. It is calculated by subtracting the value of the latitude in degrees from 90°, which is the maximum reached at the Equator.
\[\text{Angle of incidence} = 90° - \text{latitude of the place, in degrees}\]
Here 90° is the angle of the sun’s rays at the Equator on an equinox, and the latitude is the angular distance of the place north or south of the Equator. Degrees are divided into minutes, with 60 minutes making one degree, which is why a latitude is written in a form such as 66°30’.
The result is a rule you can apply to any place on an equinox: the further a station lies from the Equator, the lower the noon sun stands in its sky. That is the whole reason the same sunlight heats the tropics strongly and the high latitudes weakly, since a low sun spreads its rays across a wider patch of ground.
What The Numbers Show
The table also shows that hours of daylight are always 12 hours at the Equator, and that they go on increasing or decreasing towards the Poles. Seasonal contrasts are at their maximum at mid latitudes, rather than at the Equator or the Poles.
Fig. 3.A5 plots the June 21 column, so that the pattern can be seen as a shape rather than read as a list.
Graph
Fig. 3.A5 Caption: Duration of daylight at different latitudes on June 21. Brief: Bar graph; x-axis lists latitude from 90°N to 90°S in the order given in Table 3.1, y-axis shows duration of daylight in hours from 0 to 24; plot the June 21 duration for each latitude, converting the minutes shown in the table into decimal hours; source of data is Table 3.1 of this chapter.
The shape tells the story at a glance. At the Equator the bars never change through the year, at the poles they swing between everything and nothing, and in between lies the belt where the difference between a summer day and a winter day is largest.
3.6 Cycle Of Seasons
The equinoxes and the solstices are the positions of the earth with respect to the sun on the given fixed dates. They help us to understand how seasons occur, and how they are reversed in the Northern and Southern Hemispheres. When it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere.
From Table 3.1, which sets out the variations in the altitude of the sun during the equinoxes and solstices at different latitudes, it is observed that the duration of day and night varies according to the seasons. As a result of these variations, seasons are not the same in all parts of the earth.
In general, there are four seasons, that is, spring, summer, autumn and winter, corresponding to the dates of solstice and equinox shown in Fig. 3.4. The four dates that follow are not separate events. They are four points on one journey, and the earth passes through all of them every year in the same order.
Flowchart
Fig. 3.A6 Caption: The four positions of the earth follow one another every year and return to the start. Nodes: 21 June, North Pole tilted towards the sun → summer in the Northern Hemisphere, winter in the Southern → 23 September, sun overhead at the Equator → autumn in the Northern Hemisphere, spring in the Southern → 22 December, South Pole tilted towards the sun → summer in the Southern Hemisphere, winter in the Northern → 21 March, sun overhead at the Equator again → spring in the Northern Hemisphere, autumn in the Southern → back to 21 June
3.6.1 Position On 21st June
It is summer solstice in the Northern Hemisphere. The rays of the sun fall vertically on the Tropic of Cancer.
The duration of sunlight, or day time, increases from 12 hours at the Equator to 13 hours and 27 minutes at 23½° N, and further to 24 hours at 66½° N, as Table 3.1 shows. Beyond the Arctic Circle, the regions experience 24 hours of daylight.
This phenomenon is termed the midnight sun. Norway, in northern Europe, part of which lies beyond the Arctic Circle, is called the land of the midnight sun, and many tourists travel there to witness it. The sun is visible only at a very low height, just above the horizon, and this continues for almost three months up to the autumn equinox. In the Southern Hemisphere, meanwhile, it is winter.
After June 21st, the hours of sunlight in the Northern Hemisphere begin to decrease, from 12 hours at the Equator to all nights and no days beyond the Arctic Circle. This position can also be observed in Table 3.1. The maximum position is reached on December 21st.
3.6.2 Position On 23rd September
On 23rd September, the sun is vertically overhead at the Equator, and the days and nights are equal. It is autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
On September 23rd, the duration of sunlight is 12 hours at all the places on earth. No latitude is favoured on this date, because the circle of illumination passes through both poles and divides every parallel into two equal halves.
3.6.3 Position On 22nd December
On 22nd December, the Southern Hemisphere is tilted towards the sun and experiences summer. Areas beyond the Antarctic Circle experience 24 hours of daylight.
The midnight sun therefore appears in the far south as well, but few people see it. Being a water hemisphere, the spectacle of the Midnight Sun there, though more interesting, is confined to navigators only. The South Polar region has 24 hours of sunlight continuously for many days.
It is summer in the Southern Hemisphere and winter in the Northern Hemisphere. This date is the exact mirror of 21st June, which is why the two solstices are best learnt as a pair rather than one at a time.
3.6.4 Position On 21st March
After 22nd December, the sun again begins to move towards the Equator, and on March 21st the sun’s rays fall vertically over the Equator. On this day, all places on the earth have equal days and nights.
This is a repeat of the position on 23rd September, but only in reference to the duration of day and night, and not in reference to seasons or weather. During this period it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
The cycle then closes. From March the sun’s apparent movement continues northward until 21st June, and the whole sequence of four positions begins again, in the same order, year after year.
3.7 Twilight In Low And High Latitudes
It is clear from everything in this chapter that there are generally great seasonal contrasts as one moves from the Equator to the Poles. The variations in the amount of heat received at different parts of the earth during the year depend on the inclination of the sun’s rays.
The difference is best explained by the vertical and slanting rays of the sun. But we should also remember that no place on earth has an abrupt change from vertical to slanting rays or the other way round. The change is gradual, and the same gradualness shows up at the beginning and the end of every single day.
Light Without A Sun In The Sky
During sunrise and sunset, though the sun is below the horizon, we experience a diffused light. Much of this light is a reflection by clouds and dust particles present in the atmosphere, which pass the sunlight on to us before the sun itself has appeared, and after it has gone.
This period of diffused light between sunset and complete darkness is called dusk, or twilight, and that between sunrise and full daylight is called dawn. This phenomenon of dawn and dusk is experienced more in temperate regions, in the belt between 23½° and 66½° north and south.
Diagram
Fig. 3.7 Caption: Twilight Brief: Draw the sun above the horizon at the upper right with the horizon as a horizontal line across the picture; show the path of the rising sun and the path of the setting sun as arrows slanting up to and down from the sun; shade the band above the horizon as Day and the band below it as Twilight; label sun, horizon, path of the rising sun, path of the setting sun, Day and Twilight.
How Long Twilight Lasts
The duration of twilight has a considerable influence on seasons. The duration of twilight is large beyond 50° latitude, and in polar regions twilight lasts for as much as seven weeks at a stretch.
Twilight continues as long as the sun is 18° below the horizon, that is, until the sun has sunk more than eighteen degrees of angle beneath the line where sky meets ground. Its duration is 1 hour and 12 minutes at the Equator, and it goes on increasing towards the Poles. At the poles the duration is 50 days long, during the three months of the alternating period of day and night.
The reason lies in the path the sun takes as it rises and sets. At the Equator the sun rises and sets in a vertical path, so it crosses those 18° quickly and the period during which the refracted light is received is short. In temperate latitudes the sun rises and sets in an oblique path, so the period of refracted light is longer. At the poles the slanting path is longer still, and that is why the winter darkness there is only twilight.
So the tilt of the axis does not merely decide how long the sun stays up. It also decides how gently the day begins and ends, and that gentleness stretches from a little over an hour at the Equator to fifty days at the poles.
3.8 Summary
- The earth has two motions at the same time: rotation, its spin on its own axis, and revolution, its journey round the sun in an elliptical orbit. Day and night, the seasons, winds and ocean currents all depend on them.
- Rotation is the spinning of the earth on its axis from west to east, once in 23 hours, 56 minutes and 4.09 seconds, taken roughly as 24 hours.
- The axis is an imaginary line through the centre of the earth, ending at the North Pole and the South Pole.
- It is tilted at 23½° from the vertical, which is the same as 66½° to the plane of the earth’s orbit.
- It always points in the same direction, towards the Pole Star, all the way round the orbit.
- The speed of rotation is 1670 km per hour at the Equator, where the circumference is 40,075 km, and it decreases towards the poles.
- Rotation has seven main effects on the earth.
- The parts facing the sun have day and the parts turned away have night.
- A place moving into the sun’s rays has sunrise, and one moving out of them has sunset.
- The Coriolis Effect deflects winds and ocean currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
- The sun, the moon and the stars appear to move from east to west, because the earth itself spins from west to east.
- Centrifugal force, acting outward from the axis, has given the earth a bulge in the middle and a flattened top at the poles.
- Time differs from place to place by 4 minutes for each degree of longitude.
- The short rotational period keeps the temperature difference between day and night moderate.
- The inclination of the axis is what makes conditions unequal across the earth.
- If the axis were vertical, day and night would be equal everywhere and every place would have the same temperature all year.
- If the earth did not revolve round the sun, the tilt alone would still give no seasons.
- On a tilted earth, the length of day and night differs with latitude on any given date.
- At the Equator, day and night stay equal throughout the year.
- In the hemisphere leaning towards the sun, days are longer than nights, and the Arctic Circle has 24 hours of daylight in summer.
- In the hemisphere leaning away, days are shorter than nights, and the Antarctic Circle has 24 hours of darkness in winter.
- Revolution has four chief characteristics.
- The earth moves at an average speed of 29.8 kilometres per second, or about 100,000 kilometres per hour.
- The speed is not uniform, because the gravitational pull makes the earth move faster when it is closer to the sun.
- One complete round takes 365 days, 5 hours, 48 minutes and 45.51 seconds, so the extra 6 hours are collected every fourth year into the leap day added to February.
- The axis stays inclined at 66½° to the orbital plane, and the motion is from west to east, like the rotation.
- Revolution, acting on a tilted earth, produces the seasons, because vertical rays concentrate their heat on a small area while oblique rays spread the same heat over a larger one.
- The earth’s distance from the sun varies because the orbit is elliptical.
- At perihelion, on or about 3 January, the earth is 147.3 million km from the sun, and it is winter in the Northern Hemisphere.
- At aphelion, on about 4 July, the earth is 152 million km away.
- Differences in the angle at which the rays fall divide the world into three heat zones: the Torrid Zone between 23½° N and S, the Temperate Zone between 23½° and 66½° N and S, and the Frigid Zone from 66½° to 90° N and S.
- The lighted half of the earth is bounded by the circle of illumination, and the earth’s position with respect to the sun is fixed on four dates.
- Summer Solstice, 21 June, with the sun overhead at the Tropic of Cancer and the North Pole tilted towards the sun.
- Autumnal Equinox, 23 September, with the sun overhead at the Equator and 12 hours of daylight everywhere.
- Winter Solstice, 22 December, with the Southern Hemisphere tilted towards the sun.
- Spring or Vernal Equinox, 21 March, with the sun again overhead at the Equator.
- The angle of incidence at a place is found by subtracting its latitude from 90°, and Table 3.1 shows that daylight is always 12 hours at the Equator while it lengthens or shortens towards the poles, so seasonal contrasts are greatest at mid latitudes.
- Twilight is the diffused light between sunset and complete darkness, and dawn is the same light between sunrise and full daylight; it lasts 1 hour and 12 minutes at the Equator and up to 50 days at the poles, because the sun sets along a slanting path there.
Flowchart
Fig. 3.A7 Caption: Two motions of one tilted planet account for everything in this chapter. Nodes: the earth, tilted at 23½° and moving in two ways → branch one, rotation from west to east in 24 hours → day and night; sunrise and sunset; Coriolis deflection, right in the Northern Hemisphere and left in the Southern; centrifugal bulge at the middle and flattening at the poles; 4 minutes of time difference for each degree of longitude → branch two, revolution round the sun in 365 days and 6 hours with the axis always pointing the same way → the hemisphere leaning towards the sun receives more vertical rays and longer days → summer there, winter in the other hemisphere → 21 June, sun overhead at the Tropic of Cancer → 23 September, sun overhead at the Equator → 22 December, Southern Hemisphere tilted towards the sun → 21 March, sun overhead at the Equator again → back to 21 June
What This Chapter Cannot Tell You
Table 3.1 gives four dates and nine latitudes, so it shows the extremes and the balance points but not the steady change that runs between them, and it says nothing about places lying between the latitudes listed. The diagrams freeze the earth at a single instant, which is why the same place must be read again on another date before its year makes sense.
The chapter explains the geometry of heating — how much sunlight arrives, at what angle, for how long. That is not the same as the weather a place actually gets, which also depends on conditions studied later in this book.
3.9 Check Your Concepts
Look again at Fig. 3.3. Explain why place B on the Tropic of Cancer has a longer day than night, while place D on the Tropic of Capricorn has a shorter day than night on the very same date.
Using the rule given in this chapter for finding the angle of incidence, calculate the angle at 50°N on March 21. Check your answer against Table 3.1, and state what your result tells you about the height of the noon sun there compared with the Equator.
A wind begins to blow across the surface in the Northern Hemisphere. Explain, step by step, why it does not continue in a straight line, and state how the same wind would be turned if it were blowing in the Southern Hemisphere.
Table 3.1 gives values for four dates in the year only. State one thing about the changing length of the day that this table cannot show you, and explain why the table cannot show it.
Mention any two possible consequences if the axis of the earth was vertical instead of inclined.
What will be the duration of daylight on March 21st at 23°30’N and at 66°30’S latitudes? Give a reason for your answer.
Briefly describe the cycle of seasons, with the change in the inclination of the sun’s rays. Which temperature zone receives almost vertical rays of the sun, and which zones receive slanting rays?
Distinguish between an Equinox and a Solstice.
Give a geographical reason for each of the following.
- Kuala Lumpur rotates faster on the earth’s axis than London.
- We always see the sun rising in the east.
- Norway is called the Land of the Midnight Sun.
Draw a well labelled diagram to show the tilt of the earth’s axis, and how revolution causes seasons and variation in the length of day and night.