PES 105        Fall 2001

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General Astronomy I

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Lecture Notes:

Earth: Atmosphere, Magnetic Field

text: Chapter 5, Sections 5.5 - 5.6 (Chapter 4 in 2nd edition)


From space Earth looks blue:

from : 1. scattered light in atmosphere
2. reflected light from oceans

Light scattering:

molecules and dust in atmosphere scatter light

which colors scatter best depends on particle size

in our atmosphere, size is right to scatter blue best

scattering makes sky blue, sunsets red


sky is blue

setting sun looks red

Atmosphere Composition


78 %

Nitrogen (N2)

from decay of biological stuff and volcanic eruptions

21 %

oxygen (O2)

from photosynthesis in plants (CO2 -> O2)

1 %

everything else

if no life on Earth => very different atmosphere

Ancient atmosphere had more hydrogen (which escaped), methane and amonia

Composition depends on:

  1. original formation of planet - what gasses were trapped
  2. chemical processes (including life)
  3. escape speeds of different molecules

escape speed: gas molecules can escape from planet if travelling fast enough

Air molecules are always in motion

speed depends on temperature and mass of molecules

hotter => faster => easier to escape

lighter => faster => easier to escape

If a planet's atmosphere is hot enough, light elements will escape

Earth is hot enough for hydrogen and helium to escape

Atmosphere and temperature

atmosphere absorbs energy from sunlight

X-rays and ultraviolet radiation are absorbed at different altitudes

table of altitude and temperature


average surface temperature is about 300 K

if no atmosphere, would be about 270 K (below freezing)

This temperature difference comes from the "greenhouse effect"

atmosphere traps heat near Earth's surface

Greenhouse effect

Figure 5.15 in the text is similar. [Link to Figure 5.15] (4.16 in 2nd ed.)

Air Motion

Two causes:

1. convection (hot air rises, cool air settles)
convection on Earth

2. rotation of Earth (coriolis effect) . . . . bends path of air

corriolis effect on Earth


combining thses produces cells

cells in atmosphere


Storms also arise from these effects

storms: highs and lows

air flows away from high pressure and toward low pressure

(hurricanes and tornadoes are Low pressure)

Magnetic Field

[Demo: bar magnet and iron filings] - see Figure 5.16 in text [Link to Figure 5.16] (Fig. 4.17 in 2nd ed.)


magnetic field around Earth

rotation axis and magnetic axis are separated by about 11.5 degrees


To have a magnetic field, planet needs:

  1. liquid layer inside ..... (carries charge)
  2. rotation of the planet
    1. faster rotation => stronger magnetic field

Since Earth has a magnetic field, and rotates, it must have a liquid region inside


  • magnetic poles are NOT aligned with the rotation axis
  • magnetic poles move
    • N pole moved about 800 km toward the rotation axis in last 150 years
  • magnetic poles flip
    • sometimes S magnetic pole is on "top"
    • poles typically stay for about 10,000 - 100,000 years
    • evidence from alignment in rocks


magnetic field traps charged particles from the Sun

Van Allen radiation belts

Figure 5.19 in the book also shows the Van Allen radiation belts. [Link to Figure 5.19] (Fig. 4.20 in 2nd ed.)

Aurora (northern lights) come from charge particles interactng with atmosphere

magnetic fields and aurora

see aurora near poles since magnetic field is strong closer to Earth

Download a movie of some spectacular aurora:

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