Chapter 2 Solar Energy to Earth and the Seasons Geosystems 6e An Introduction to Physical Geography Robert W. Christopherson Charles E. Thomsen

The Solar System, Sun, and Earth Solar system formation and structure   Gravity Mutual attracting force exerted by the mass of an object on all other objects Planetesimal hypothesis Explains the formation of planets and other celestial bodies A nebular cloud of dust, gas, and icy comets condensed to form universe

Dimensions and Distances Speed of light Light travels 6 trillion miles per year The distance light travels in a year is called a light year (ly) Milky Way Galaxy 100,000 ly across Our Solar System 11 light-hours across Moon is 1.28 light-seconds away

Milky Way Galaxy Figure 2.1

Our New Solar System

Solar Energy: From Sun to Earth The Sun Intercepted energy at the top of the atmosphere  The sun’s principle outputs Radiant energy and solar wind  

Solar Activity and Solar Wind Solar wind = clouds of electrically charged particles Sunspots are caused by magnetic storms. These cause changes in the solar output. Sunspots have activity cycle of 11 years Figure 2.2

Transmission of Energy Conduction: molecule to molecule transfer of energy as it diffuses through a substance. Hot pan + hand Convection: transfer of energy by physical mixing involving strong vertical motion. Steam rising from boiling water

Transmission of Energy Advection: transfer of energy by physical mixing involving strong horizontal motion. Air rushing in through an opened door Radiation: emission and propagation of energy in the form of EM waves sunlight

The Electromagnetic Spectrum EM Spectrum – all the radiant energy produced by the sun placed in an ordered range, divided by wavelength Wein’s Law – hotter objects emit shorter wavelengths Shorter wavelengths have higher energy Sun radiates shortwave energy Earth radiates longwave energy

Earth’s Energy Budget Figure 2.8

Distribution of Insolation Insolation – Radiation arriving at the Earth’s atmosphere and surface Solar constant – the amount of solar radiation received in the atmosphere Subsolar point – the point on Earth where the sun’s rays are perpendicular to the surface – maximum insolation received

Distribution of Insolation Tropics receive more concentrated insolation due to the Earth’s curvature Tropics receive 2.5X more than poles Solar rays pass through more atmosphere before reaching the poles, so more energy is lost to scattering, absorption, and reflection

Figure 2.9

Solar declination: latitudinal change of subsolar points

The Seasons   Seasonality: the variation of the sun’s position over the horizon; the changing daylength during the year   Seasons result from: variations in the sun’s altitude – angle between the horizon and the sun the suns’s declination – latitude of the subsolar point and daylength - duration of exposure to insolation 

Reasons for Seasons  Variations in the sun’s altitude, declination, and daylength are caused by: Revolution Rotation Tilt of Earth’s axis Axial parallelism Sphericity

Reasons for Seasons Revolution Rotation Earth revolves around the Sun One complete revolution is 365.25 days Orbit is elliptical, not circular Rotation Earth rotates on its axis once every 24 hours Earth rotates counter-clockwise

Revolution and Rotation Figure 2.13

Reasons for Seasons Tilt of Earth’s axis Axial parallelism Sphericity Axis is tilted 23.5° from plane of ecliptic Axial parallelism Axis maintains alignment during orbit around the Sun North pole points toward the North Star (Polaris) Sphericity

Axial Tilt and Parallelism Figure 2.14

Annual March of the Seasons Winter solstice – December 21 or 22 Subsolar point Tropic of Capricorn Spring equinox – March 20 or 21 Subsolar point Equator Summer solstice – June 20 or 21 Subsolar point Tropic of Cancer Fall equinox – September 22 or 23

Annual March of the Seasons Figure 2.15

Geosystems 6e An Introduction to Physical Geography End of Chapter 2 Geosystems 6e An Introduction to Physical Geography Robert W. Christopherson Charles E. Thomsen