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The frozen planet

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# 5.1 The seasons

There is one final point to discuss related to the cycles of energy and that you most likely noted when you looked at Figure 2. When it is warm in the Northern Hemisphere (the black line on Figure 2), it is clearly cold in the Southern Hemisphere (the red line). This is because of the seasons, and anyone living away from the equatorial regions of the Earth has experienced their effect. Figure 10 shows the hypothetical situation in which the Earth’s axis of rotation is at right angles to the plane of its orbit. (If you have difficulty thinking about this you can again use an orange and a felt pen to mark out a location at high latitude and the Equator.)

Figure 10 Hypothetical situation in which the Earth’s axis of rotation is at right angles to the plane of its orbit around the Sun. (Note that at the poles the Sun’s rays graze the Earth’s surface.) The black region on the left is night, and the green region is daytime.
• Imagine the Earth in Figure 10 with the 0° longitude line just at the boundary between the black and the green. If you rotate the Earth by one complete cycle, how long will the day be at the Equator compared with high latitude?

• Night and day would always be the same length (i.e. 12 hours each) everywhere around the globe, except at the poles, which would have perpetual twilight.

But no planet in the Solar System has an axis of rotation that is aligned as shown in Figure 10.

• What would be the implication on the day length if in Figure 10 the axis of rotation was not vertical?

• The length of time each latitude on the Earth was facing the Sun would be different.

The axis of rotation of the Earth is currently at an angle of 23.4° to the vertical (see Figure 11) and, over the course of a year, the direction the axis points to changes relative to the Sun. Following on from the questions you have just answered, you can see that there will be implications for the day length at different latitudes and times of the year. At midday the Sun is overhead at the Equator only twice a year at what are called the equinoxes. These equinoxes fall on 21 March and 21 September. At other times the latitude at which the midday Sun is overhead moves between 23.4° N, which is called the Tropic of Cancer, and 23.4° S, which is called the Tropic of Capricorn, and back again (Figure 11b).

Figure 11 (a) The four seasons of the Northern Hemisphere in relation to the Earth’s orbit around the Sun. The Earth’s axis is tilted at approximately 23.4° to a line at right angles to the plane of its orbit around the Sun. (b) The angle of tilt of the Earth’s axis (at present 23.4°) determines the latitude of the tropics (where the Sun is overhead at one of the solstices) and of the Arctic and Antarctic Circles (66.6°, which is 90° minus 23.4°). (c) The passage of the seasons shown in terms of the position of the midday Sun in relation to the Earth: (1) the midday Sun overhead at the Tropic of Cancer, i.e. the northern summer solstice (cf. b); (3) the midday Sun overhead at the Tropic of Capricorn, i.e. the southern summer solstice; (2) and (4) at the equinoxes, by contrast, the Sun is overhead at the Equator, and the Northern and Southern Hemispheres are illuminated equally; days and nights are the same duration at all latitudes except at the poles, which are grazed by the Sun’s rays for 24 hours.

Figure 11a shows the passage of the seasons for the Northern Hemisphere. Along the Tropic of Cancer, the midday Sun is overhead and maximum solar radiation is received, during what is called the spring equinox. The longest day in the Northern Hemisphere is on 21 June, which is called the summer solstice. After this date the days begin to shorten. At the autumn equinox on 21 September both day and night are of equal length, but day length continues to shorten until the winter solstice on 21 December, which is the shortest day of the year. After this, days lengthen until the spring equinox on 21 March where again the day and night are of equal length.

• In Figure 11 when will the Equator receive maximum solar radiation at the surface? (Hint: when is the Sun overhead at midday?)

• The Equator receives maximum solar radiation at the equinoxes in March (spring) and September (autumn): see Figures 11c (2) and (4).

Poleward of the tropics, the Sun is never overhead, although it is at its highest elevation (the angle between the Sun and the horizon) at the summer solstice (Figure 11b and c (1) and (3)).

• What will be the day length at the North Pole and South Pole at the northern summer solstice?

• At the North Pole, the axis of rotation is facing the Sun and so it will never be dark. At the South Pole, the axis of rotation will be facing away from the Sun and so there will be 24 hours of darkness.

In their respective summers, the high latitudes experience the so-called ‘midnight Sun’ where the Sun never sets, and in their winters there is a ‘polar night’ where the Sun never rises and there is complete darkness. The lowest latitude at which there is polar night and midnight Sun has a special significance and a name which you are certain to have heard of. The current tilt of the Earth’s axis is 23.4° and there are 90° of latitude between the Equator and the poles (Box 2). So 90° – 23.4° = 66.6° and the lowest latitude that you would be able to experience the midnight Sun in the northern summer and the polar night in the northern winter is 66.6° N. This is called the Arctic Circle. The Antarctic Circle is at 66.6° S.

As the Sun dips below the horizon, the atmosphere can also bend the light, which keeps some daylight present – to experience complete darkness of a true polar night one would have to be at higher latitudes than 66.6°.

On the ground the distinction between polar day and polar night is not completely obvious. Just like the winters you experience on successive days, the midday Sun gets lower and lower in the sky. As it gets close to the horizon, if it is not completely overcast, the days become one very long sunset (Figure 12) with intense purple and red colours.

Figure 12Approaching the polar night at high latitudes the days turn into an apparently endless sunset or dawn with beautiful red and purple light colouring the land. This is a view of the ocean as it freezes at 74° S.

To demonstrate your understanding of the sections you have read so far, now answer Question 4 [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] [Tip: hold Ctrl and click the link to open it in a new tab.]

The polar regions receive less solar energy than close to the Equator because of the shape of the planet and because the solar radiation has to pass through more atmosphere. The seasons occur because the axis of Earth’s rotation is at an angle to the Sun. This angle is also responsible for the latitude of the Arctic and Antarctic Circles, and polar day and polar night.