It will be noted that spring and summer in the Northern Hemisphere are longer than autumn and winter. This is because Earth is in the vicinity of its aphelion during the former period (figs. 3.22 and 3.24) and is consequently moving more slowly in its orbit than it does near perihelion. Thus Earth is actually farther from the Sun during the summer in the Northern Hemisphere than it is in winter. In the Southern Hemisphere the situation is, of course, reversed. The summers in the Southern Hemisphere are consequently slightly shorter but hotter and the winters longer and colder than in the Northern Hemisphere.
The dates March 21, when winter turns to spring in the Northern Hemisphere, and September 23, when summer turns to autumn, are referred to as the spring (vernal) and autumnal equinoxes, respectively. The word equinox, from the Latin meaning equal night, refers to the fact that on these dates the day and night are everywhere of equal length (12 hours). The reason is that Earth's axis is then perpendicular to the Earth-Sun line, as seen earlier, and equal parts of the globe are in sunlight and darkness. The intermediate dates, June 22 and December 22, are called the summer and winter solstices, respectively. The times of rising and setting of the Sun on successive days do not change significantly at the solstices, and this has led to the use of the term which means Sun stands still.
The differences of temperature experienced in the four seasons are determined primarily by two related factors; namely, the duration of daylight and the maximum height reached by the Sun in the sky at noon. In the Northern Hemisphere, the period of daylight is longest at the summer solstice (June 22) and the Sun attains its maximum elevation in the sky. The Sun is directly overhead at noon at 23.5° N latitude, but at other latitudes it is not quite so high. Nevertheless, the Sun is highest in the sky on that day. Throughout the summer, the duration of daylight gets less and less until it is a minimum at the winter solstice (December 22) in the Northern Hemisphere. After this date, during the winter and spring, the hours of daylight increase again toward the maximum on June 22. In the Southern Hemisphere, the days are shortest on this date and longest on December 22.
The actual durations of day and night depend on the latitude. At the Equator (latitude 0°) there are always 12 hours of daylight and 12 hours of darkness. At higher and lower latitudes, the difference in length between day and night changes with the season in accordance with the foregoing remarks. At high latitudes—in the polar regions—from latitude 90° minus 23.5° (66.5°) to 90° North or South, it is possible to have 24 hours (or more) of daylight at the time of a solstice. At 66.5° latitude, the maximum period of daylight is just 24 hours, but at still higher latitudes the periods of continuous daylight increase. At the actual poles, there are 6 months of daylight, during the local summer, and 6 months of darkness, during the local winter. As can be seen in figure 3.24, the north polar regions must be in continuous darkness during winter in the Northern Hemisphere and in continuous sunlight in the summer.
Obviously, the more hours of daylight, the greater the amount of heat radiation received from the Sun by a given area of Earth's surface. But this is not the only consideration. During the autumn and winter in the Northern Hemisphere, the Sun is lower in the sky than it is in the spring and summer. The Sun's rays thus fall more obliquely on the surface during the former seasons. As a result, a given quantity of heat radiation from the Sun is spread over a larger area (fig. 3.25), and it is consequently less effective in heating Earth's surface than when the Sun is higher in the sky, in the spring and summer.
If the duration of daylight and the elevation of the Sun were the only factors involved,
June 22 would be the hottest day of the year and December 22 would be the coldest in the Northern Hemisphere. The actual temperatures at Earth's surface are determined by the heat balance, which is the relation between the heat lost to space and the heat gained from the Sun. The Earth tends to retain heat, especially in the oceans and the atmosphere, with the result that the highest temperatures are not reached in the Northern Hemisphere until some time after June 22. Similarly the lowest temperatures are experienced after December 22. The dates are reversed, but the delays are similar in the Southern Hemisphere.
Although the polar regions receive solar radiation continuously for periods up to 6 months, the Sun is always low in the sky. Considerable amounts of heat are absorbed during the daylight months, but much of this is expended in the partial melting of the thick layers of ice and snow. Because these layers never disappear, the surface temperatures do not increase greatly. In any event, they are always below the freezing point (0° C), although the air temperatures may be slightly higher.
The information given above concerning Earth can now be applied to examine the conditions on Mars. The positions of Mars in its orbit at the two Martian equinoxes and two solstices and the way Mars (and Earth for comparison) would look from the north ecliptic pole at these four locations are depicted in figure 3.26. Because of the angle between the directions in which the axis of the two planets point, it happens that the line of the equinoxes, that is, the line passing through the positions of a planet at its equinoxes, for Mars is almost at right angles to that for Earth.
Figure 3.26 must not be taken to imply that the spring equinox, for example, on Mars occurs within a few days of the winter solstice on Earth. What it does mean is that when Mars and Earth are close together, near an opposition, when it is December 22 on Earth, then spring will just be starting in the northern hemisphere of Mars. When a favorable opposition occurs, it is summer in the Northern Hemisphere of Earth; Mars will then be experiencing late autumn and approaching winter in its northern hemisphere. During favorable (perihelic) oppositions, the north pole of Mars is tilted away from the Sun (and Earth).
Similarly, during the unfavorable oppositions, when it is late winter in the Northern
Hemisphere on Earth, it will be spring, approaching the summer solstice, in the northern hemisphere of Mars. At the intermediate oppositions in the middle and late autumn or middle and late spring on Earth, the northern hemisphere of Mars will be approaching the spring and autumn equinoxes, respectively. A rough summary of the situations for oppositions at various times of the year is given in a table on the next page.
Because there is no established Martian calendar and the Martian and terrestrial years are different, it is not possible to give any precise dates for the equinoxes and solstices on Mars. In terms of Earth dates, they vary from year to year, but they can be readily calculated if required. Of particular in-
Type of opposition
Approaching season in Earth's Northern Hemisphere
Approaching season on Mars
Favorable. .. Intermediate
Winter. . Summer
Spring. . Autumn.
terest, of course, are the seasons at the times of oppositions and these can be derived from figure 3.26 or, approximately, from the table.
The lengths of the Martian seasons in terms of Earth days are, however, quite definite. They are given in the table below to the nearest whole number of days. The total is 687 days, equal to the length of the sidereal year on Mars. The durations of the seasons are seen to be roughly twice those of the seasons on Earth, because of the correspondingly greater length of the year. It will be noted, too, that the Martian seasons are of unequal durations, ranging from 146 days of the northern autumn (southern spring) to 199 days of the northern spring (southern autumn). This is a consequence of the significant eccentricity of the Martian orbit. The differences in the duration of the seasons on Earth are much less because the orbital eccentricity is relatively small.
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