Page 88 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

52 part I The energy–Atmosphere system
June Solstice
Equinox
December Solstice
Sunrise Observatory
(a) Solar observatory and towers.
(b) Sunrise at first tower, June solstice.
▲Figure 2.11 Solar observatory at Chankillo, Peru. The Thirteen Towers are part of the Chankillo temple complex built in coastal peru over 2000 years ago, the oldest known solar observatory in the Americas. sunrise aligns with certain towers at different dates during the year. research and preservation of the monument are ongoing; see www.wmf.org/project/chankillo. [(a) Ivan ghezzi/reuters. (b) World monument Funds.]
of sites in North America, demonstrating an ancient awareness of seasons and astronomical relations. Many seasonal rituals and practices persist in this modern era.
Seasonality
Seasonality refers both to the seasonal variation of the Sun’s position above the horizon and to changing day- lengths during the year. Seasonal variations are a response to changes in the Sun’s altitude, or the angle between the horizon and the Sun. At sunrise or sunset, the Sun is at the horizon, so its altitude is 0°. If during the day, the Sun reaches halfway between the horizon and directly over- head, it is at 45° altitude. If the Sun reaches the point di- rectly overhead, it is at 90° altitude.
The Sun is found directly overhead (90° altitude, or zenith) only at the subsolar point, where insolation is at a maximum, as demonstrated in Geosystems Now and the chapter-opening photo. At all other surface points, the Sun is at a lower altitude angle, producing more dif- fuse insolation.
The Sun’s declination is the latitude of the sub- solar point. Declination annually migrates through
47° of latitude, moving between the Tropic of Cancer and Tropic of Capricorn latitudes. Although it passes through Hawai‘i, which is between 19° N and 22° N, the subsolar point does not reach the continental United States or Canada; all other states and provinces are too far north.
The duration of exposure to insolation is daylength, which varies during the year, depending on latitude. Day- length is the interval between sunrise, the moment when the disk of the Sun first appears above the horizon in the east, and sunset, that moment when it totally disappears below the horizon in the west.
The equator always receives equal hours of day and night: If you live in Ecuador, Kenya, or Singapore, every day and night is 12 hours long, year-round. People liv- ing along 40° N latitude (Philadelphia, Denver, Madrid, Beijing), or 40° S latitude (Buenos Aires, Cape Town, Melbourne), experience about 6 hours’ difference in day- light between winter (9 hours) and summer (15 hours). At 50°N or S latitude (Winnipeg, Paris, Falkland, or Malvinas Islands), people experience almost 8 hours of annual daylength variation.
At the North and South poles, the range of daylength is extreme, with a 6-month period of no insolation, beginning with weeks of twilight, then darkness, then weeks of pre- dawn. Following sunrise, daylight lasts for a 6-month pe- riod of continuous 24-hour insolation—literally, the poles experience one long day and one long night each year!
CRITICAlthinking 2.1
A Way to Calculate Sunrise and Sunset
For a useful sunrise and sunset calculator for any location, go to www.esrl.noaa.gov/gmd/grad/solcalc/sunrise.html, select a city near you or select “enter lat/long” and enter your coordinates, enter the difference (“offset”) between your time and UTC and whether you are on daylight sav­ ing time, and enter the date you are checking. Then click “Calculate” to see the solar declination and times for sun­ rise and sunset. give this a try. make a note of your finding, then revisit this site over the course of a full year and see the change that occurs where you live. •
Reasons for Seasons
Seasons result from variations in the Sun’s altitude above the horizon, the Sun’s declination (latitude of the subso- lar point), and daylength during the year. These in turn are created by several physical factors that operate in concert: Earth’s revolution in orbit around the Sun, its daily rotation on its axis, its tilted axis, the unchanging orientation of its axis, and its sphericity (summarized in Table 2.1, page 53). Of course, the essential ingredient is having a single source of radiant energy—the Sun. We now look at each of these factors individually. As we do, please note the distinction between revolution—Earth’s travel around the Sun—and rotation—Earth’s spinning on its axis (Figure 2.12).