Page 94 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

58 part I The energy–Atmosphere system
Sun’s path above the horizon determines the thickness of the atmosphere through which the Sun’s rays must pass. The illumination may be enhanced by the presence of pollution aerosols and suspended particles from volcanic eruptions or forest and grassland fires.
At the equator, where the Sun’s rays are almost di- rectly above the horizon throughout the year, dawn and twilight are limited to 30–45 minutes each. These times increase to 1–2 hours each at 40° latitude, and at 60° latitude they each range upward from 2.5 hours, with little true night in summer. The poles experience about 7 weeks of dawn and 7 weeks of twilight, leaving only 2.5 months of “night” during the 6 months when the Sun is completely below the horizon.
Seasonal observations In the midlatitudes of the North- ern Hemisphere, the position of sunrise on the horizon migrates from day to day, from the southeast in December to the northeast in June. Over the same period, the point of sunset migrates from the southwest to the northwest. The Sun’s altitude at local noon at 40° N latitude in- creases from a 26° angle above the horizon at the winter (December) solstice to a 73° angle above the horizon at the summer (June) solstice—a range of 47° (Figure GIA 2.3).
Seasonal change is quite noticeable across land- scapes away from the equator. Think back over the past year. What seasonal changes have you ob- served in vegetation, temperatures, and weather? Recently, the timing of seasonal patterns in the bio- sphere is shifting with global climate change. In the middle and high latitudes, spring and leafing out are occurring as much as 3 weeks earlier than in previous human experience. Likewise, fall is hap- pening later. Ecosystems are changing in response. In Chapter 1, we emphasized the impacts of humans and their activities on Earth systems and processes. Humans affect, and are affected by, the systems com- prising Earth’s four spheres. The Human Denominater feature (Figure HD 2) illustrates important examples of human–Earth interactions, with summary text high- lighting the direction of influence. For example, sea- sons affect humans by determining the rhythm of life for many societies. Humans are affecting seasonal pat- terns through the activities that cause climate change, and the related impacts on biological cycles. Through- out Geosystems, you will find similar illustrations reviewing human interactions with the systems and processes presented in each chapter.
CRITICAlthinking 2.3
Use the Analemma to Find the Subsolar Point
If you marked the location of the sun in the sky at noon
takes a figure­8 shaped path called an analemma. on the analemma chart in Figure CT 2.3.1, you can locate any date,
then trace horizontally to the y­axis and find the sun’s declination, which is the latitude of the subsolar point.
Along the Tropic of Capricorn, the subsolar point occurs
on December 21–22, at the lower end of the analemma.
Following the chart, you see that by march 20–21, the sun’s declination reaches the equator, and then moves on to the 6° Tropic of Cancer in June. As an example, use the chart to
calculate the subsolar point location on your birthday.
The shape of the analemma as the sun’s declination moves between the tropics is a result of earth’s axial tilt and elliptical orbit. As earth revolves in its elliptical orbit around the sun, it moves faster during December and January, and slower in June and July. This is reflected in the equation of
+ Sun fast Equation of time (in minutes) Sun slow –
N 16M14M12M 10M8M 6M 4M 2M 0M 2M 4M 6M 8M 10M12M14M 24°
22° each day throughout the year, you would find that the sun 20°
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18° 16°
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14° 12°0 10°0
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4° 2° 0° 2° 4° 6°
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time at the top of the chart. 8° An average day of 24 hours (86 400 seconds) is the basis for 10°
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mean solar time, time measured by a clock (and introduced
earlier in the chapter). however, observed solar time is the
observed movement of the sun crossing your meridian each
day at noon. This sets the apparent solar day. you see on the
chart that in october and november, fast-Sun times occur
and the sun arrives ahead of local noon (12:00), as noted on
the x-axis along the top of the chart. In February and march 22° the sun arrives later than local noon, causing slow-Sun times. 24° What was the equation of time on your birthday? S
12° 14° 16° 18° 20°
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search online for more information about the analemma (begin at www.analemma.com). Can you explain the figure­8 shape? •
▲Figure CT 2.3.1 The analemma chart.
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Declination of Sun
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