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Download Earth-Sun Geometry - Indiana University Bloomington
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Earth-Sun Geometry Readings: A&B: Ch. 2 (p. 42-50) CD Tutorial: Earth-Sun Geometry www: 2. EarthSun 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 G109: Weather and Climate 2: Earth-Sun Geometry 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 G109: Weather and Climate 2: Earth-Sun Geometry 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 • G109: Weather and Climate 2: Earth-Sun Geometry Orbital Geometry: Revolution • Earth’s revolution is an elliptical orbit P: Perihelion = Closest Approach – 147x106 km, January 3 A: Aphelion = Furthest Distance – 152x106 km, July 4 • Aphelion/Perihelion ~6% change in distance G109: Weather and Climate 2: Earth-Sun Geometry 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: G109: Weather and Climate 2: Earth-Sun Geometry Seasons G109: Weather and Climate 2: Earth-Sun Geometry Seasons: Equinoxes and Solstices • Based on the annual “migration” of the direct rays of the sun – a yearly cycle Sun directly overhead Northern hemisphere Southern hemisphere June 21-22 Sept. 21-22 Dec. 21-22 Mar. 21-22 • • Equinoxes: Weather (Meteorological Seasons) doesn’t fall neatly into these Astronomical Seasons G109: Weather and Climate 2: Earth-Sun Geometry 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 G109: Weather and Climate 2: Earth-Sun Geometry 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 G109: Weather and Climate 2: Earth-Sun Geometry Seasons: Solar Altitude i. Concentration/intensity of sun's ray When rays overhead (90°) energy is concentrated on small area Æ intense light When rays are at a lower (oblique) angle, a larger area illuminated Æ less intense G109: Weather and Climate 2: Earth-Sun Geometry Seasons: Solar Altitude ii. 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 G109: Weather and Climate 2: Earth-Sun Geometry 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 → G109: Weather and Climate 2: Earth-Sun Geometry 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. G109: Weather and Climate 2: Earth-Sun Geometry 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 ° G109: Weather and Climate 2: Earth-Sun Geometry
