Chapter 2Solar Energy to Earth and

the Seasons

Geosystems 6eAn Introduction to Physical Geography

Robert W. ChristophersonCharles 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 hypothesisExplains the formation of planets and other celestial bodies

A nebular cloud of dust, gas, and icy comets condensed to form universe

Dimensions and DistancesSpeed 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 outputsRadiant energy and solar wind  

Solar Activity and Solar WindSolar 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 EnergyConduction: 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 EnergyAdvection: 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 SpectrumEM 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 InsolationInsolation – 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

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

Distribution of Insolation

Figure 2.9

Solar declination: latitudinal change ofatitudinal change of subsolar pointssubsolar 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 sunthe suns’s declination – latitude of the subsolar pointand 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

Earth revolves around the Sun

One complete revolution is 365.25 days

Orbit is elliptical, not circular

RotationEarth 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

Axis is tilted 23.5° from plane of ecliptic

Axial parallelismAxis 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 SeasonsWinter solstice – December 21 or 22

Subsolar point Tropic of Capricorn

Spring equinox – March 20 or 21Subsolar point Equator

Summer solstice – June 20 or 21Subsolar point Tropic of Cancer

Fall equinox – September 22 or 23Subsolar point Equator

Annual March of the Seasons

Figure 2.15

End of Chapter 2

Geosystems 6eAn Introduction to Physical Geography

Robert W. ChristophersonCharles E. Thomsen