Page 89 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 2 solar energy to earth and the seasons 53
TAblE 2.1
Five reasons for Seasons
Factor
Description
revolution Tilt sphericity
orbit around the sun; requires 365.24 days to complete at 107 280 km·h−1
Alignment of axis at about 23.5° angle from perpendicular to the plane of the ecliptic (the plane of earth’s orbit) oblate spheroidal shape lit by sun’s parallel rays; the geoid
rotation
earth turning on its axis; takes approximately 24 hours to complete
Axial parallelism
Unchanging (fixed) axial alignment, with polaris directly overhead at the north pole throughout the year
▲Figure 2.12 Earth’s revolution and rotation. earth’s revolution about the sun and rotation on its axis, viewed from above earth’s orbit. note that the moon’s rotation on its axis and revolution are counterclockwise, as well.
Revolution Earth’s orbital revolution about the Sun is shown in Figures 2.1d and 2.12. Earth’s speed in orbit av- erages 107280 km·h−1. This speed, together with Earth’s distance from the Sun, determines the time required for one revolution around the Sun and, therefore, the length of the year and duration of the seasons. Earth completes its annual revolution in 365.2422 days. This number is based on a tropical year, measured from equinox to equi- nox, or the elapsed time between two crossings of the equator by the Sun.
The Earth-to-Sun distance from aphelion to peri- helion might seem a seasonal factor, but it is not signifi- cant. It varies about 3% (4.8 million km) during the year, amounting to a 50 W·m−2 difference between summers at the different poles. Remember that the Earth–Sun dis- tance averages 150 million km.
Rotation Earth’s rotation, or turning on its axis, is a complex motion that averages slightly less than 24 hours in duration. Rotation determines daylength, creates the apparent deflection of winds and ocean cur- rents, and produces the twice-daily rise and fall of the ocean tides in relation to the gravitational pull of the Sun and the Moon.
When viewed from above the North Pole, Earth rotates counterclockwise about its axis, an imaginary line extending through the planet from the geographic North Pole to the South Pole. Viewed from above the equator, Earth rotates west to east, or eastward. This eastward rotation creates the Sun’s apparent westward daily journey from sunrise in the east to sunset in the west. Of course, the Sun actually remains in a fixed po- sition in the centre of our Solar System.
Although every point on Earth takes the same 24 hours to complete one rotation, the linear velocity of rotation at any point on Earth’s surface varies dramatically with latitude. The equator is 40 075 km long; therefore, ro- tational velocity at the equator must be approximately 1675 km·h−1 to cover that distance in one day. At 60° lati- tude, a parallel is only half the length of the equator, or 20 038 km long, so the rotational velocity there is 838 km·h−1. At the poles, the velocity is 0. This variation in rotational velocity establishes the effect of the Coriolis force, discussed in Chapter 6. Table 2.2 lists the speed of rotation for several selected latitudes.
Earth’s rotation produces the diurnal (daily) pat- tern of day and night. The dividing line between day and night is the circle of illumination (as illustrated in Geosystems in Action). Because this day–night divid- ing circle of illumination intersects the equator (and because both are great circles, and any two great cir- cles on a sphere bisect one another), daylength at the equator is always evenly divided—12 hours of day and
Earth
Rotation
Moon
Sun
Georeport 2.3 Why Do We Always See the Same Side of the Moon?
note in Figure 2.12 that the moon both revolves around earth and rotates on its axis in a counterclockwise direction when viewed from above earth’s north pole. It does both these motions in the same amount of time. The moon’s speed in orbit
varies slightly during the month, whereas its rotation speed is constant, so we see about 59% of the lunar surface during the month, or exactly 50% at any one moment—always the same side facing earth.
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