G109: 2. Earth-Sun Geometry 1 2. EARTH-SUN GEOMETRY Reading Assignment: A&B: Ch. 2 (p. 53-61) CD: Tutorial 1 Solar Geometry LM: Lab. 5 www: Earth-Sun Geometry Introduction The Sun is the only important source of energy for the Earth/Atmosphere system On global scale: motions of the atmosphere are a direct response to latitudinal and seasonal changes of radiation reaching the surface Primary influence on how much solar energy is received on Earth: distance the solar radiation needs to travel angle at which solar radiation hits Earth composition of atmosphere (see Ch. 1) Earth: part of the solar system (planets, asteroids, etc) moves regularly around the sun gravitational attraction to the sun earth's orbit affected by the "gravitational pull" of other bodies within the solar system
G109: 2. Earth-Sun Geometry 2 1. Orbital Geometry Earth has two Principal Motions: i) Rotation - each day the earth rotates on its own axis Axis - imaginary line through the planet between the North (N) and South (S) poles Looking down at the N pole Earth rotates counter-clockwise This rotation gives us day & night 24 h period to complete rotation
G109: 2. Earth-Sun Geometry 3 ii) Revolution As earth rotates it also revolves around the sun in an elliptical orbit 365¼ days (= a year) to complete an orbit E P Sun F 2 A P: Perihelion Jan 3 147 x 10 6 km min distance A: Aphelion Jul 4 152 x 10 6 km max distance (F 2 : second focal point of ellipse) Aphelion/Perihelion: ~ 6% change in distance plays only a minor role in seasonal T variations
G109: 2. Earth-Sun Geometry 4 2. Seasons (see CD rom: Tutorial 1) Tilt of Earth on the ecliptic (23.5 ) leads to variations in solar position (solar altitude = angle of sun above the horizon) Tilt of Earth on the ecliptic leads to variations in daylength amount of energy accumulating over time A) Sun's Altitude - key to change in seasons zenith zenith angle α β solar altitude angle α + β = 90 Altitude: angle of the sun above the horizon Zenith: angle of the sun from vertical (straight above) Summer - Winter - sun high above the horizon sun low on the horizon Sun s altitude variations: influence on amount of energy received at Earth's surface in 2 ways: i) energy concentration / intensity ii) atmospheric path length
G109: 2. Earth-Sun Geometry 5 i.) Concentration/intensity of sun's ray When rays overhead (90 ) energy is concentrated on small area (intense) Lower angle (oblique) - larger area illuminated but less intense ii) Angle of sun determines the amount of atmosphere the sun rays have to traverse
G109: 2. Earth-Sun Geometry 6 Longer path by up to 15 times direct route Longer path greater chance for absorption, reflection, scattering by the atmosphere reduces intensity of radiation at the surface B) Length of Day - energy accumulation (see A&B: Figures 2-13 to 2-15) If we look at earth on any given day only places at a particular latitude will receive vertical rays (90 ) As move N or S the sun's ray will strike at ever decreasing angle
G109: 2. Earth-Sun Geometry 7 Length of day (sun above horizon) varies: seasonally geographically (latitude) Circle of Illumination - splits day and night [lab 1]
G109: 2. Earth-Sun Geometry 8 Daylength: important for accumulation of solar energy at surface. Summer high latitudes: o sun is at lower altitude (compared to mid-latitudes) intensity is reduced o length of day is longer accumulation of energy over longer periods Causes of variations in sun angle & length of day Earth's orientation to sun continually changes Earth's axis is tilted at 23½ Axis remains pointed in the same absolute direction (North Star, Polaris) as it journeys around the sun orientation of the earth's axis relative to the sun's rays always changing
G109: 2. Earth-Sun Geometry 9 4 Days in Year of Special Significance based on the annual migration of the direct rays of the sun yearly cycle (see A&B Fig. 2-12) Date Sun directly Overhead Jun 21-22 Tropic of Cancer 23½ N Sep 22-23 Equator 0 Dec 21-22 Tropic of Capricorn 23½ S Mar 21-22 Equator 0 Northern Hemisphere Southern Hemisphere Jun 21/22: NH - longest day (NH summer solstice) SH - longest night (SH winter solstice) Equinoxes: 12 h day/night (worldwide!) Seasons Winter solstice Spring Equinox Fall equinox Summer solstice Weather we experience doesn't fall neatly into these astronomical seasons: meteorological seasons do not usually correspond to astronomical (calendar) seasons
G109: 2. Earth-Sun Geometry 10 3. Calculating Noon Sun Angle Principle: For every 1 of latitude we move away from the location where the sun is directly overhead, the solar altitude drops by 1 Problem: What is the altitude of the sun at noon in Bloomington on June 21? See additional problems on the web: geog109 page Earth-Sun Geometry Calculation in three steps: 1. At what latitude is the sun overhead at the given date? (somewhere between 23.5º N 23.5º S; Fig. 2-14) 2. How many degrees of latitude separate that location from the place of interest? (Note: may need to cross equator) 3. Subtract the answer of (2) from 90º noon sun angle (Note: the result has units of angle-degrees [ ])