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Chapter 1 The Earth as a Rotating Planet This chapter deals with the way solar radiation drives energy and matter flows in the atmosphere and oceans and how these flows are linked to weather and climate. This chapter introduces you to some basic ideas about the Earth, its rotation, and revolution. The Earth is shaped as an oblate ellipsoid because the Earth's rotation causes it to bulge slightly at the equator. The Earth rotates on its axis in an eastward (retrograde) direction and revolves (counterclockwise) around the sun . The Earth's rotation has three important environmental effects: 1. It imposes a daily, or diurnal, cycle of daylight, air temperature, air humidity, and air motion. 2. It produces the Coriolis effect which deflects the flow of fluids (air and water) to the right or left of their original path in the northern or southern hemisphere’s, respectively. 3. As the Earth rotates, the combined effects of both the moon’s and suns’ gravitational pull on different sides of the Earth creates a rise and fall of ocean water known as tides. The Geographic Grid provides a system for locating features on the Earth’s surface using parallels of latitude and meridians of longitude. Latitude is the angular distance of a point north or south of the equator. It increases from a minimum of 0° at the equator to a maximum of 90° at the north and the south poles. Lines of latitude are parallel to each other and describe circles that decrease in circumference away from the equator. Longitude is the angular distance of a point east or west of the prime meridian located at Greenwich, England. It increases to the west and the east away from the prime meridian (0°) to a maximum of 180°. Lines of longitude are farthest apart at the equator and converge at the poles. All circles described by meridians of longitude have the same circumference. A map projection is a system that allows for the representation of a curved/spherical geographic grid on a flat grid surface. Map scale relates measured distance on a map to actual distance on the Earth’s surface. The polar projection (centered on either the north or south pole) produces a map with true relative shapes and sizes of small areas. The Mercator projection portrays a rectangular grid of meridians as straight vertical lines intersected by parallels as straight horizontal lines which represent the true compass direction of any straight line on the map. Meridians are equally spaced but parallels at higher latitudes. This map projection is particularly useful for showing the flow of winds and ocean currents as well as lines of equal air temperature and pressure. The Goode projection uses two sets of mathematical curves (sine curves and ellipses) to depict meridians. This projection indicates the true sizes of geographical areas on the Earth’s surface. The standard time system is based on twenty-four time zones that keep time according to standard meridians that are spaced 15° apart and represent a time difference of one hour. The international date line is located near 180° longitude. Crossing this line in a westward direction requires the calendar to be advanced by one day. Daylight savings time is an adjustment of the clock during the longest daylight period of the year (high sun season) to transfer the morning light to the early evening (to allow society to benefit from longer summer evenings). Clocks are set one hour ahead in the spring and returned to standard time in the autumn (set back by one hour). The Earth revolves counterclockwise around the sun every 365¼ days in an elliptical orbit. The Earth is closest to the sun at perihelion (~ January 3) and farthest from the sun at aphelion (~ July 4). The Earth’s axis of rotation is tilted 23½° away from the perpendicular (to the plane of the ecliptic) and its north pole always points towards the North Star, Polaris. The Earth’s axial tilt in combination with the its revolution around the sun produce the progression of the seasons. At an equinox (vernal equinox-March, autumnal equinox-September), everywhere on Earth experiences a 12-hour day and a 12-hour night. At a solstice (June, December), polar regions experience either a 24-hour day or a 24hour night. The maximum solar radiation is received at the subsolar point which crosses the equator twice in the course of a year as it moves between the Tropic of Cancer (23 ½°N) to the Tropic of Capricorn (23 ½°S). Two important facts about the Sun-Earth energy flow system: 1. Half of the Earth is always receiving solar energy. 2. Solar energy is not received uniformly over the Earth’s surface.