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
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