Earth-Sun Geometry Readings: A&B: Ch. 2 (p. 42-50) CD Tutorial: Earth-Sun Geometry www: 2. Earth- Sun Geometry Topics 1. Introduction 2. Orbital Geometry a) Rotation b) revolution 3. Seasons a) Solstices & Equinoxes b) Sun s altitude c) Length of Day 4. Noon Sun Angle G109: Weather & Climate Introduction: Earth-Sun Geometry The Sun is Latitudinal and seasonal changes of the amount of radiation reaching the surface drive atmospheric circulations and winds How much solar energy is received on Earth depends on: Distance traveled and angle of incidence vary during the Earth s orbit around the Sun Two principal motions: and
Orbital Geometry: Rotation Each day the Earth rotates on its axis Axis: imaginary line through the planet between the North (N) and South (S) poles Rotation gives us Day and Night Looking down on the North Pole, the Earth rotates Sun Orbital Geometry: Revolution Each year the Earth revolves in an orbit around the sun, on the Ecliptic Ecliptic: imaginary plane which intersects the sun, on which the earth orbits
Orbital Geometry: Revolution Earth s revolution is an elliptical orbit P: Perihelion = Closest Approach 147x10 6 km, January 3 A: Aphelion = Furthest Distance 152x10 6 km, July 4 Aphelion/Perihelion ~6% change in distance Seasons Earth s axis is tilted 23.5 relative to the ecliptic Axis remains pointed in the same absolute direction (to the North Star) as it journeys around the sun Orientation relative to the sun changes June: December:
Seasons Seasons: Equinoxes and Solstices Based on the annual migration of the direct rays of the sun a yearly cycle June 21-22 Sept. 21-22 Sun directly overhead Northern hemisphere Southern hemisphere Dec. 21-22 Mar. 21-22 Equinoxes: Weather (Meteorological Seasons) doesn t fall neatly into these Astronomical Seasons
Seasons Tilt of the Earth on the Ecliptic causes i. Variations in Solar Altitude = angle of sun above horizon Variations in ii. Variations in Length of Day Variation in Seasons: Solar Altitude Altitude : angle of the sun above the horizon Zenith : angle of the sun from vertical (straight above) In Summer In Winter Variations in solar altitude influence the amount of energy received at Earth's surface in 2 ways: i. Energy concentration / intensity ii. Atmospheric path length
Seasons: Solar Altitude i. Concentration/intensity of sun's ray When rays overhead (90 ) energy is concentrated on small area intense light When rays are at a lower (oblique) angle, a larger area illuminated less intense ii. Seasons: Solar Altitude Angle of sun determines When sun is lower in sky Longer path (up to 15 times longer) More chance for Reduces intensity of radiation at the surface
Seasons: Length of Day Length of day (sun above horizon) varies: Circle of Illumination: splits day and night Summer at high latitude Sun is at lower altitude (relative to mid-latitudes) Length of day is longer 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 Sample Problem: What is the altitude of the sun at noon in Bloomington on June 21? Assume Bloomington is at 40 N.
Calculating Noon Sun Angle Problem: What is the altitude of the sun at noon in Bloomington (40 N) on June 21? Calculation 3 steps a. At what latitude is the sun overhead at the given date? b. How many degrees of latitude separate that location from the place of interest? (Note: may need to cross equator) Bloomington: Sun overhead: Difference: c. Subtract the answer of (b) from 90º noon sun angle The maximum solar angle for Bloomington is Note: the result has units of angle-degrees