Page 91 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 2 solar energy to earth and the seasons 55
TAblE 2.2 Speed of rotation at Selected Latitudes
Speed
latitude
km·h−1
Representative Geographic locations Near Each latitude
90° 0 50° 1078 30° 1452
north pole
Chibougamau, Québec; kyyîv (kiev), Ukraine new orleans, Louisiana; pôrto Alegre, Brazil
60°
838
seward, Alaska; oslo, norway; saint petersburg, russia
40°
1284
Columbus, ohio; Beijing, China; Valdivia, Chile
0°
1675
pontianak, Indonesia; Quito, ecuador
Annual March of the Seasons
During the march of the seasons on Earth, daylength is the most obvious way of sensing changes in season at latitudes away from the equator. The extremes of daylength occur in December and June. The times around December 21 and June 21 are solstices. Strictly speaking, the solstices are specific points in time at which the Sun’s declination is at its position farthest north at the Tropic of Cancer, or south at the Tropic of Capricorn. “Tropic” is from tropi- cus, meaning a turn or change, so a tropic latitude is where the Sun’s declination appears to stand still briefly (Sun stance, or sol stice) and then “turn” and head toward the other tropic.
During the year, places on Earth outside of the equa- torial region experience a continuous but gradual shift in daylength, a few minutes each day, and the Sun’s al- titude increases or decreases a small amount. You may have noticed that these daily variations become more pronounced in spring and autumn, when the Sun’s decli- nation changes at a faster rate.
The Geosystems in Action illustration on the fol- lowing pages summarizes the annual march of the sea- sons and Earth’s relationship to the Sun during the year, using a side view (Figure GIA 2.1) and a top view (Figure GIA 2.2). On December 21 or 22, at the moment of the December solstice, or Northern Hemisphere winter solstice (“winter sun stance”), the circle of illumination excludes the North Pole region from sunlight but includes the South Pole region. The subsolar point is about 23.5° S latitude, the Tropic of Capricorn parallel. The Northern Hemisphere is tilted away from these more direct rays of sunlight—our northern winter—thereby creating a lower angle for the incoming solar rays and thus a more diffuse pattern of insolation.
For locations between about 66.5° N and 90° N (the North Pole), the Sun remains below the horizon the en- tire day. The parallel at about 66.5° N marks the Arctic Circle; this is the southernmost parallel (in the Northern Hemisphere) that experiences a 24-hour period of dark- ness. During this period, twilight and dawn provide some lighting for more than a month at the beginning and end of the Arctic night.
During the following 3 months, daylength and solar angles gradually increase in the Northern Hemisphere as Earth completes one-fourth of its orbit. The moment of the March equinox, or vernal equinox in the North- ern Hemisphere, occurs on March 20 or 21. At that time, the circle of illumination passes through both poles, so that all locations on Earth experience a 12-hour day and a 12-hour night. People living around 40° N lati- tude (New York, Denver) have gained 3 hours of daylight since the December solstice. At the North Pole, the Sun peeks above the horizon for the first time since the pre- vious September; at the South Pole, the Sun is setting— a dramatic 3-day “moment” for the people working the Amundsen–Scott South Pole Station.
From March, the seasons move on to June 20 or 21, the moment of the June solstice, or summer solstice in the Northern Hemisphere. The subsolar point migrates from the equator to 23.5° N latitude, the Tropic of Cancer. Because the circle of illumination now includes the North Polar region, everything north of the Arctic Circle receives 24 hours of daylight—the Midnight Sun. In con- trast, the region from the Antarctic Circle to the South Pole (66.5°–90° S latitude) is in darkness. Those working in Antarctica call the June solstice Midwinter’s Day.
September 22 or 23 is the time of the September equinox, or autumnal equinox in the Northern Hemi- sphere, when Earth’s orientation is such that the circle of illumination again passes through both poles, so that all parts of the globe experience a 12-hour day and a 12-hour night. The subsolar point returns to the equator, with days growing shorter to the north and longer to the south. Researchers stationed at the South Pole see the disk of the Sun just rising, ending their 6 months of darkness. In the Northern Hemisphere, autumn arrives, a time of many colourful changes in the landscape, whereas in the Southern Hemisphere it is spring.
Dawn and Twilight Dawn is the period of diffused light that occurs before sunrise. The corresponding evening time after sunset is twilight. During both periods, light is scattered by molecules of atmospheric gases and reflected by dust and moisture in the atmosphere. The duration of both is a function of latitude, because the angle of the
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