Page 90 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

54 part I The energy–Atmosphere system
12 hours of night. All other latitudes experience un- even daylength through the seasons, except for 2 days a year, on the equinoxes.
The length of a true day varies slightly from 24 hours throughout the year. However, by international agree- ment a day is defined as exactly 24 hours, or 86400 sec- onds, an average called mean solar time. Since Earth’s rotation is gradually slowing, partially owing to the drag of lunar tidal forces, a “day” on Earth today is many hours longer than it was 4 billion years ago.
Tilt of Earth’s Axis To understand Earth’s axial tilt, imag- ine a plane (a flat surface) that intersects Earth’s elliptical orbit about the Sun, with half of the Sun and Earth above the plane and half below. Such a plane, touching all points of Earth’s orbit, is the plane of the ecliptic. Earth’s tilted axis remains fixed relative to this plane as Earth revolves around the Sun. The plane of the ecliptic is important to our discussion of Earth’s seasons. Now, imagine a perpen- dicular (at a 90° angle) line passing through the plane. From this perpendicular, Earth’s axis is tilted about 23.5°. It forms a 66.5° angle from the plane itself (Figure 2.13). The axis through Earth’s two poles points just slightly off Polaris, which is named, appropriately, the North Star.
The tilt angle was described above as “about” 23.5° because Earth’s axial tilt changes over a complex 41000- year cycle (see Figure 11.16). The axial tilt ranges roughly
Perpendicular to plane of ecliptic
North Pole
▲Figure 2.13 The plane of Earth’s orbit—the
ecliptic—and Earth’s axial tilt. note on the illustration
that the plane of the equator is inclined to the plane Animation of the ecliptic at about 23.5°. Earth–Sun
Rotations, Seasons
between 22° and 24.5° from a perpendicular to the plane of the ecliptic. The present tilt is 23.45°. For convenience, this is rounded off to a 23.5° tilt (or 66.5° from the plane) in most usage. Scientific evidence shows that the angle of tilt is currently lessening in its 41000-year cycle.
Axial Parallelism Throughout our annual journey around the Sun, Earth’s axis maintains the same alignment rela- tive to the plane of the ecliptic and to Polaris and other stars. You can see this consistent alignment in Geosystems in Action, Figure GIA 2.2. If we compared the axis in dif- ferent months, it would always appear parallel to itself, a condition known as axial parallelism.
Sphericity Even though Earth is not a perfect sphere, as discussed in Chapter 1, we can still refer to Earth’s sphericity as contributing to seasonality. Earth’s approxi- mately spherical shape causes the parallel rays of the Sun to fall at uneven angles on Earth’s surface. As we saw in Figures 2.8, 2.9, and 2.10, Earth’s curvature means that insolation angles and net radiation received vary be- tween the equator and the poles.
All five reasons for seasons are summarized in Table 2.1: revolution, rotation, tilt, axial parallelism, and sphericity. Now, considering all these factors operating together, we explore the march of the seasons.
CRITICAlthinking 2.2 Astronomical Factors Vary over Long Time Frames
The variability of earth’s axial tilt and orbit about the sun, as well as a wobble to the axis, are described in Figure 11.16. please refer to this figure and compare these changing conditions to the information in Tables 2.1 and 2.2 and the related figures in this chapter.
What do you think the effect on earth’s seasons would be if the tilt of the axis was decreased? or if the tilt was increased a little? or if earth was lying on its side? you can take a ball or piece of round fruit, mark the poles, and then move it around a light bulb as if revolving it around the sun. note where the light falls relative to the poles with no tilt and then with a 90° tilt to help complete your analysis. now, what if earth’s orbit was more circular as opposed to its present elliptical shape? earth’s ellipti­ cal orbit actually does vary throughout a 100000­year cycle. (Check the answer at the end of the key Learning Concept review.) •
Georeport 2.4 Measuring Earth’s Rotation
A slight “wobble” along earth’s rotating axis causes it to migrate irregularly along a circular path with a radius up to about 9 m. The accuracy of modern navigation systems such as gps relies on measuring this “wobble.” scientists at the
International earth rotation service track earth’s rotation indirectly by monitoring fixed objects in space using radio telescopes (see www.iers.org/). In 2011, a research group made the first accurate, direct measurements of earth’s annual rotation using two counter­ rotating lasers stored deep underground. The next goal is to precisely measure changes in earth’s rotation over a single day.
E
q
u
a
t
o
r
Plane of ecliptic
23.5°
66.5°
Orbit