Читать книгу Exploring the Solar System - Peter Bond - Страница 68

Planets have seasons because their rotational axes are tilted, rather than perpendicular (upright) in relation to the planes of their orbits. Venus and Jupiter have negligible axial inclinations, so there is no difference between the amount of solar radiation arriving at their equator or poles throughout their year.² This is not the case with planets such as Earth, Mars, and Saturn, which have noticeable tilts. Even more unusual conditions occur on objects which are more or less spinning on their sides, such as Uranus and Pluto.

The northern hemisphere experiences summer when the North Pole is tilted toward the Sun. Six months later, when Earth has traveled halfway around its orbit, the northern hemisphere is tilted away from the Sun and experiences winter. (The seasons are reversed in the southern hemisphere.)

Figure 3.3 Since Earth's axis is inclined to its orbital plane, the amount of radiation any one place receives varies throughout the year. In June, the North Pole is tilted towards the Sun, resulting in longer days and warmer temperatures (i.e. summer). In December, the North Pole is tilted away from the Sun, causing longer nights and colder temperatures (winter). The seasons are reversed in the southern hemisphere.

(NOAA)

Figure 3.4 The Sun's motion across the sky, looking south. The maximum height of the Sun in the sky, and its rising and setting points on the horizon, change with the seasons. In the summer, the Sun rises in the north east, reaches its highest maximum height at noon, and stays up longest. The Sun rises in the south east and remains low in the winter when the days are shortest. The length of day and night are equal on the vernal, or spring, equinox (March 20) and on the autumnal equinox (September 23) when the Sun rises exactly east and sets exactly west.

(NASA)

The dates on which Earth's axis is most directly tilted toward or away from the Sun are known as the solstices. They occur on or around June 21, when the Sun is overhead at midday at the Tropic of Cancer (23.44°N), and December 22, when the midday Sun is overhead at the Tropic of Capricorn (23.44°S).

Not only are days longer in summer, but the Sun moves noticeably higher above the horizon at that time of year, providing more heat per square meter of surface area. The opposite is true in the winter. These two factors combined account for much of the difference in seasonal temperature.

When one of the poles is tilted towards the Sun, the surrounding regions at high latitude are bathed in permanent sunlight (hence the term, “the land of the midnight Sun”). The opposite polar region endures 24‐hour darkness and extreme cold (Figure 3.3).

However, even in December or June, the noonday Sun is never far from the zenith at the equator, so the amount of insolation received shows little variation throughout the year (Figure 3.4). Hence, the equatorial regions are always hot.

Midway between the solstices, on or around March 21 and September 23, the Sun is directly overhead at noon at the equator. On those dates, Earth's axis is inclined away from the Sun. Day and night are equal in length across the globe – so they are known as the spring (or vernal) and autumnal equinoxes.

Earth's orbit is slightly elliptical. The planet reaches perihelion in early January, only about two weeks after the December solstice.³ This means that the northern hemisphere winter and the southern hemisphere summer begin about the time that Earth is nearest the Sun. (Similarly, the southern hemisphere's winter and northern hemisphere's summer coincide with aphelion.)

However, the difference between the aphelion and perihelion distances is only about 5 million km or 0.3%. This difference results in a 6% increase in incoming solar radiation (insolation) from July to January – too small to cause any significant seasonal effects.

The dates of perihelion and the December solstice will not always be so close, since the date of perihelion is not fixed, but slowly regresses (becomes later in the year). The rate of orbital precession (see Orbit and Rotation) is about one full day every 58 years. There is some evidence that this long‐term change in the date of perihelion influences Earth's climate.

Figure 3.5 Earth's temperature is determined by its radiation budget – the amount of solar radiation it receives compared with the amount of heat it loses to space. This diagram shows what happens to incoming solar radiation and longer wavelength radiation emitted by the surface. Here, the overall amounts of incoming and outgoing radiation are balanced, so the temperature is in equilibrium. In reality, the global temperature has generally been increasing gradually since the 1970s.

(NASA)