A E

sPr¡ng s 5 Intermediate opposition

FIGURE 3.23. Direction of axes of Earth and Mars at various oppositions. (The abbreviations S.E. and A.E. stand for spring and autumn equinoxes and S.S. and W.S. for summer and winter solstices.)

It is seen from the figure that at perihelic (favorable) oppositions, when Earth's North Pole is tipped toward the Sun, the north pole of Mars is tipped away from the Sun (and Earth). Thus, at such oppositions, this pole, and the adjacent high northern latitudes, will not be visible from Earth, although the Martian south pole and the surrounding regions can be observed. Conversely, at aphelic (unfavorable) oppositions, the high northern latitudes and north pole of Mars will be seen, but the south-polar areas will be invisible. There are some differences in the parts of Mars that can be seen from different locations, such as Northern and Southern Hemispheres, on Earth. But the general situation is not affected.

When an opposition occurs approximately midway between perihelion and aphelion, at the times mentioned on page 39, it should be possible to see the whole of the Martian surface. To an observer on Earth, the axis of Mars will appear to be almost perpendicular to the orbital (or ecliptic) plane. At the time of opposition, the entire Martian surface, including both poles, should come into view as the planet rotates.

The Seasons on Earth

Just as the inclination of Earth's axis is responsible for seasonal changes in the lengths of daylight and darkness, and hence in the temperature, so the inclination of the axis of Mars is the cause of similar seasons on Mars. The circumstances can be explained most conveniently by considering, first, the situation on Earth.

As Earth revolves in its orbit about the Sun, its axis of rotation always points in the same direction in space, as indicated in figures 3.22 and 3.23. As a result of the constant direction of Earth's axis, there is one day each year, June 22, when the North Pole is tilted toward the Sun by the maximum angle of 23.5 degrees to the vertical. At the same time, the South Pole is tilted to the same extent away from the Sun. Exactly half a year later, on December 22, the situation is reversed, and the North Pole is tilted 23.5 degrees away from the Sun and the South Pole is tilted the same angle toward the Sun. At the intermediate dates of March 21 and September 23, Earth's axis is perpendicular to the line joining Earth to the Sun, and neither the North nor the South Pole is directed either toward or away from the Sun. The situation may perhaps be seen more clearly in figure 3.24, which is a representation of Earth at various points in its orbit as seen from the north ecliptic pole (fig. 3.17). The heavy dots indicate the position of Earth's North Pole.

During the period from March 21 to June 22, as the tilt of the North Pole toward the Sun increases from zero to 23.5 degrees, it is spring in the Northern Hemisphere. Then,

Sept. 23

Dec. 22 sun

March 21

FIGURE 3.24. Earth's axis at the equinoxes and solstices as seen from the north ecliptic pole. (Eccentricity of the orbit is exaggerated tor clarity.)

Dec. 22 sun

FIGURE 3.24. Earth's axis at the equinoxes and solstices as seen from the north ecliptic pole. (Eccentricity of the orbit is exaggerated tor clarity.)

from June 22 to September 21, as the tilt decreases to zero again, the Northern Hemisphere experiences summer. During autumn, between September 21 and December 22, the North Pole tilts increasingly away from the Sun and, finally, between December 22 and March 21, it is winter in the Northern Hemisphere. Because the tilt of the South Pole to or away from the Sun is opposite to that of the North Pole, the seasons are reversed. Thus, spring, summer, autumn, and winter in the Northern Hemisphere correspond to autumn, winter, spring, and summer, respectively, in the Southern Hemisphere. The dates and lengths, to the nearest day, of the seasons in the two hemispheres are given below.

Dates

Northern Hemisphere

Southern Hemisphere

Days

Mar. 21 to June 22. .

Spring. . .

Autumn. .

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