Page 93 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

2.2
MARCH OF THE SEASONS
Looking down on the solar system from above Earth’s North Pole, you can follow the changing seasons. As Earth orbits the Sun, the 23.5° tilt of its axis produces continuous changes in day length and sun angle.
June Solstice
In the Northern Hemisphere, this is the summer solstice, marking the beginning of summer. The circle of illumination includes
the North Polar region, so everything north of the
Arctic Circle receives 24 hours of
March Equinox
In the Northern Hemisphere, this is the vernal equinox, marking the beginning of spring. The circle of illumination passes through both poles, so that all locations on Earth experience 12 hours of day and night. At the North Pole, the Sun rises for the first time since the previous September.
daylight—the Midnight Sun. Over the next six months, daylength shortens
and sun angle
North Pole
View from above the North Pole
declines.
North Pole
North Pole
Sun
Circle of illumination
Describe: For the South Pole, describe day length and position with respect to the circle of illumination at the December solstice and the March equinox.
September Equinox
2.3
Changes in Sun Angle
March 21 Sept 22 Noon
June 21 Noon
OBSERVING SUN DIRECTION AND ANGLE
The Sun’s altitude at local
noon at 40° N latitude
increases from a 26° angle
above the horizon at the
winter (December) solstice to
a 73° angle above the horizon
at the summer (June) 26° Noon solstice—a range of 47°. Sun angle
South
Explain: Why does the Sun's angle at zenith change by 47 degrees between the June and December solstices?
73° Noon Sun angle
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.
Zenith
Changes in Sunrise and Sunset
In the midlatitudes of the Northern Hemisphere, the position of sunrise on the horizon migrates
In the Northern Hemisphere, this is the autumnal equinox, marking the beginning of autumn. As with the March equinox, days and nights are of equal length.
SW Sunset
West Sunset
NW
Sunset
What an Observer Sees
At 40° N an observer sees a 73° noon Sun angle at the June solstice, a 50° angle at the equinoxes, and a 26° angle
at the December solstice.
North Pole
December Solstice
In the Northern Hemisphere, this is the winter solstice, marking the beginning of winter. Notice that the North Pole is dark. It lies outside the circle of illumination. Over the next six months, daylength and sun angle increase.
As the seasons change, the Sun’s altitude, or angle above the horizon, also changes, as does its position at sunrise and sunset along the horizon. The diagram below illustrates these effects from the viewpoint of an observer.
Dec 21 Noon
50° Noon Sun angle
Observer 73° 50° 26°
North
SE Sunrise
East Sunrise
NE Sunrise
Antarctic sunset, December 11:30 pm [Bobbé Christopherson.]
GEOquiz
1. Apply Concepts: Ushuaia, Argentina, is located at 55° S
latitude near the southern tip of South America. Describe the
march of the seasons for Ushuaia, explaining changes in day length, sun angle, and the position of sunrise and sunset.
2. Explain: What happens to the amount of insolation an area on Earth’s surface receives as you move from the subsolar point away from the equator? What role does this play in producing the seasons?
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geosystems in action 2EARTh – SUN RELATIoNS
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