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,

* solar declination—the latitude at which the sun is directly overhead at solar noon * zenith angle—the angle between a point directly overhead and the sun at solar noon * solar elevation (sun) angle—the angle of the sun above the horizon at solar noon Zenith direction Zenith angle S@--cceeaen Figure 2-6. ienith and solar elevation angles. Since we can assume that the sun'’s rays . strike the earth in straight, parallel paths, we sce that the zenith angle of any location is the sarf‘l ] as the number of degrees separating the loca-tlo and the place receiving direct solar rays. 1\10;11: in Figure 2-6 that the zenith angle A at 40 the same as the latitude difference betv\./een 40° N and where the sun’s rays strike directly e (22.3/21n813;‘gure 2-6, how many latitude degr'ee.s sezfzmie the person at 40° N and the place receiving :t.zc solar rays? _ 3, 5 _° What is the date of this example? t. December &7 Notice that the zenith angle plus the solar eleva- tion angle sum to 90°. Solar elevation always equals 90° minus the zenith angle. . . 3. What is the solar clevation angle in Figure 2- 13.5% Sun’s rays ‘ i le e A'(%liillllllfil: gfniglemt = solar declination) 1§ B. Solar elevation angle i " - i (qoo -Zfl\ith angle) Zenrth N\ale = (n5 q0- b .5=135.

90° in the center. The angle labels on the outside of the outer circle measure the azimu'th angle relative to north (i.c., 90° is east, 180° is south, etc.). Each black curve shows the course of the sun during an individual day. For example, Figure 2-10 shows that on the equinoxes at 30° N the sun rises directly in the east (90°), sets directly in the west (270°), and reaches its peak sun angle (60°) at solar noon. Figure 2-11.Solar angle, azimuth, and daylight hours at 30° N . North Deceimber21.; 2100 ot 21 5 — South G ectest — Jvne g st Figure 2-12, Solar angle, azimuth, 5“’\6\‘”65'{' > DGCGMber QIJT and daylight hours at 60° N. Figures 2-11 and 2-12 can be used to calcu- 15. a. Describe the seasonal difference in the direction late the solar elevation angle and daylength at in which the sun rises and sets at 30° N, two latitudes (30° N and 60° N) for the solstices and equinoxes. The circles in solar elevation angle, from ¢° Mace rgus ot 309 n these graphs show b. How does it differ at 60° N? on the outside to | Less rq(\js at 0o N