Lab 2

Lab 2 EARTH~SUN GEOMETRY Materials Need » calculator (W.?: . : Earth-Sun Relationships * ruler ¥ et fimCtionS) The distance between the earth and the sun averages about 150 million kilometers (93 mil- . lion miles). Because of this distance and the Introduction earth’s relatively small size compared with that of the sun, it is reasonable to assume that the sun’s rays strike the nearly spherical earth in straight paths. geometry influences these variables. We exam- The earth’s axis of rotation is tilted 23%° ine the earth-sun relationship early in ourstudy from the perpendicular to the plane of the eclip- of weather and climate because most atmos- tic—the plane on which the earth revolves pheric processes are ultimately driven by spatial around the sun. This tilt is oriented in the same variations in solar energy. direction throughout the year, with the North Sun’s rays Figure 2-1. Parallel rays striking the spherical earth.

Solar declination: 23;* § Orgy; June21-22 oY Solar declifation: 23'2" N Equinox March 21-22 Solar declination: equator o R e S : Figure 2-2. The earth’s revolution around the sun. Notice that the axis tilts in the same direction throughout the year. Pole presently pointing toward the North Star, Polaris. Figure 2-2 (not to scale) shows that the Northern Hemisphere is tilted toward the sun during its summer months and away from the sun during its winter months. Our seasons occur because of this tilt. As the earth revolves around the sun, the sun’s direct rays strike different latitudes. When the Northern Hemisphere is tilted toward the sun, it receives the more direct and, therefore, more intense rays of the sun. Locations in the Southern Hemisphere receive less direct solar radiation. Six months later, when the Southern Hemisphere is tilted toward the sun, it receives the more direct solar radiation. Figure 2-3 shows how the sun’s rays strike the earth on December 22, the Northern Hemisphere’s winter solstice. At solar noon on this date, the sun’s rays are perpendicular to the carth’s surface at 23)%4° S (location D). As we move away from 23) ° S we see that the rays of the sun strike the earth’s surface at progressively lower angles. Location C is at the equator, loca~ tion B is at 30° N latitude, and location A is at 66%° N latitude. How does it look to us at the earth'’s surface? A profile (or side) view, like that in Figure 2-4, shows how the angle of the incoming sun strikes the earth at different latitudes. Study the differ= ences in sun angle among the locations from the two perspectives,