2. EARTH-SUN G - Indiana University Bloomingtongeog109/topics/02_earth_sun/Earth_sun.pdfSep 09,...

G109: 2. Earth-Sun Geometry 1

Earth_sun.doc 9/9/03

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



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



ii) Revolution • As earth rotates it also revolves around the sun in

an elliptical orbit • 365¼ days (= a year) to complete an orbit

E

F2SunP A

P: Perihelion

Jan 3 147 x 106 km min distance

A: Aphelion Jul 4

152 x 106 km max distance

(F2: second focal point of ellipse)

Aphelion/Perihelion: ~ 6% change in distance

⇒ plays only a minor role in seasonal T° variations



zeni

th

solar altitude angle

zenith angle

α

βα + β = 90°

zeni

th

solar altitude angle

zenith angle

α

βα + β = 90°

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

• Altitude: angle of the sun above the horizon

• Zenith: angle of the sun from vertical (straight above)

• Summer - sun high above the horizon • Winter - 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



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



• 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 • 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

(see A&B: Figures 2-13 to 2-15)



• Length of day (sun above horizon) varies:

• seasonally • geographically (latitude)

Circle of Illumination - splits day and night ⇒ [lab 1]



• 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



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 Northern

Hemisphere Southern

Hemisphere Jun 21-22 Tropic of Cancer

23½°N

Sep 22-23 Equator 0°

Dec 21-22 Tropic of Capricorn23½°S

Mar 21-22 Equator 0°

• 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



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 [°])

2. EARTH-SUN G - Indiana University Bloomingtongeog109/topics/02_earth_sun/Earth_sun.pdfSep 09,...

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