(1). Radiation and the Electromagnetic Spectrum

• solar energy/radiation is part of the electromagnetic spectrum of radiant energy

• which travels at the speed of light

• it exhibits both electrical and magnetic properties

• all objects emit electromagnetic energy (EMR)

• the EMS is made up of many different forms of electromagnetic energy: radio waves, microwaves, gamma waves, etc.

• EMR travels in waves and does not need molecules to propogate them - thus, they can move through a vacuum (i.e. space)

they can be characterized by their wavelength - the distance measured along a wave from one crest (trough) to the adjacent crest (trough)

                                                             
   Electromagnetic Spectum            Electromagnetic Spectum
 

(2) . Earth's Orbit

• earth has an elliptical orbit about the sun

• average distance to the sun is ~ 150 million km

• plane on which the earth orbits about the sun is called the "plane of the ecliptic"

• "perihelion" - Earth is closest to the Sun - January 3

• "aphelion" - Earth is farthest from the Sun - July 4

• only 3.4% difference in solar energy receipt

• earth's orbit changes over a 100,000 yr. cycle, by ~ 18 million km - may contribute to glacial and interglacial periods

(3). Direct Versus Indirect Rays from the Sun

• direct rays provide more heating (more concentrated energy) than do indirect rays

                                                                
     Direct vs. Indirect Insolation            Rays Must travel Through More Atmosphere
 
 

(4). Reasons for the Seasons
 

Terms and Important Dates and Geographic Locations:

• Seasonality - a reference to the seasonal variations in the Sun's rays above the horizon, & the changes in daylength throughout the year

• Insolation - incoming solar radiation

• Sun's Altitude - the angular distance between the horizon and the Sun

• Declination - the latitude that receives direct (at a 90 degree angle) rays from the sun (migrates between 23.5⁰N & 23.5⁰S)

• Summer Solstice (June 20-21) - @Tropic of Cancer (23.5 N) - Sun's rays directly overhead

• Winter Solstice (Dec 21-22) - @ Tropic of Capricorn (23.5 S) - Sun's rays directly overhead
• Vernal Equinox ( March 20-21) - Sun's rays directly overhead at the equator

• Autumnal Equinox (Sept. 22-23) - Sun's rays directly overhead at the equator

• Daylength varies throughout the year depending on latitude
 
• except at the equator and on the two equinoxes, all latitudes experience uneven daylength throughout the course of the year

• @ equator: 12 hrs of daylight & 12 hrs night

• @ 40 N/S: ~6 hours difference in daylight

• @ 50 N/S: ~8 hours difference in daylight

• Altitude (sun angle above the horizon) changes as well.  What causes the changes in daylength and altitude?

• They occur because Earth's orientation to the Sun continually changes as it travels along its orbit.

• because of the Earth's curved surface, the angle at which the Earth receives radiant energy from the sun varies with latitude

• the lower latitudes receive rays that are more direct (closer to the perpendicular) than do the higher latitudes

                                
                   Earth-Sun Relationships