The Earth's place in the universe
Location: Milky Way
one of millions of galaxies…
THE MILKY WAY
•Spiral-shaped galaxy
•100,000 light years across
•400 billion stars
•Solar system on a trailing edge of the Orion Arm
(Fig 2-1)
•Earth ~150 000 000 km from sun
•8 min, 20 s from sun, 1.28 sec from moon (light)
•Solar system is 11 light hours across
Earth's Formation
Planetesimal/nebula hypotheses
(dust-cloud hypotheses)
Basis: observations of other systems
1.
2.
3.
4.
5.
6.
Large star in Milky Way exploded
Nebula (cloud of dust and gas) results
H and He condense into Sun
Other elements form disk of matter around sun
Disk slowly accretes into clumps (planetesimals)
planetesimals  planetoids  planets and
satellites
Solar Energy
High pressure and
temperatures in Sun's
dense interior
SUNSPOTS
H atoms forced together;
fusion of 2 H nuclei
Energy is released as heat, solar wind and
electromagnetic energy
Solar wind
•Stronger during sunspot activity
•Sunspot cycle ~11 years
•Solar wind does not reach
Earth's surface
Deflected by magnetosphere
Some absorbed by atmosphere
near poles (Aurora borealis and
Image source:
Aurora borealis
Radiant Energy:
Electromagnetic Spectrum
Wavelength - distance to same point on next wave
(Fig 2-5)
Frequency - number of waves in one second
Solar maximum in visible wavelengths (Fig 2-6, 2-7)
The hotter the object, the shorter the wavelengths
Earth emits longer wavelengths (infrared)
a
nt
Radiation emitted from Earth is of
a much longer wavelength and is of
much lesser energy
Top-of-Atmosphere Energy
Thermopause - outer boundary of Earth's energy system
Insolation at TOA: the solar constant, 1372 W/m2
Reduced at surface by angle of incidence, atmospheric
absorption and reflection (Fig 2-8)
Seasonality
Seasons respond to changes in solar altitude and
daylength
Solar declination
•Varies over 47 from Tropic of Capricorn to Tropic
of Cancer
Daylength
•Always 12 hours at the equator
•8 hr variation at Lethbridge
•24 hour variation near poles
•Total darkness and 24 light occur within 23.5 bands
from poles (at time of solstices)
For the current circle of illumination, see:
http://www.jgiesen.de/sunshine/index.htm
Revolution determines length of year (365.24 d)
•How ? Axial Parallelism (Fig. 2-13)
Earth axis always “points to Polaris”, but…
our planet moves to opposite side of the sun
Rotation determines daylength (24 hr)
Lit portion of day determined by latitude and
solar declination on that day
December 21 and June 20 - solstices
March 20 and September 22- Vernal and autumnal
equinoxes (everywhere gets 12hr day)
Energy surpluses dominate in the tropics
(incoming energy exceeds outgoing loss)
In the middle-latitudes, surpluses and deficits occur
seasonally, but deficits dominate (Fig. 2-9)
In polar regions, there is a deficit
(outgoing loss exceeds incoming energy gain)
This results in a net poleward transport of the energy surplus
through atmospheric and oceanic currents (more later)
The Atmosphere
•The sum of all inhalations and exhalations from
the Earth over time
•Mixture of gases that filter sun's rays
•480 km in height - exosphere beyond to 32000 km
(scarce H and He atoms)
•Air pressure due to weight of atmosphere above
(1 kg/cm2 or 101.32 kPa)
•Atmosphere is most dense in the troposphere
(90% of total mass)
Atmospheric Composition
Heterosphere (above 80 km)
•unevenly mixed gases
Homosphere (surface to 80 km)
•more evenly mixed (except ozone layer and
near surface trace gases)
•Current composition reached about 500 million
years ago
•78% nitrogen, 21% oxygen, almost 1% argon,
some CO2 (0.037%) and trace gases
Atmospheric Temperature
Thermosphere
•250 - 550 km
•temperature rises sharply with height due to
impact of intense sun on N2 and O2 molecules
•not ‘hot’ because kinetic energy of motion not
transferred to particles easily (not sensible heat)
Mesosphere
•temperature falls with height from 0C to
-90C at outer boundary
Stratosphere
•ranges in elevation from 18 km to 50 km
•ozone  temperature rises (-57C to 0C)
•Increasing temperature with altitude prevents
mixing with troposphere (except for some mixing
along jet streams)
Troposphere
•altitude from surface to 8-18 km
•contains water vapour, clouds, weather, air
pollution and life
•Upper limit (tropopause) is -57C
•Normal lapse rate is 6.4C per km, but
environmental lapse rate varies
Atmospheric Function
Ionosphere
•absorbs cosmic rays, gamma rays, X-rays and
short UV wavelengths
•atoms are changed to positive ions
•auroral lights occur here
Ozonosphere
•Contains elevated levels of ozone (O3)
•Absorbs UV light (0.1-0.3 m)
•Has been stable for several hundred million
years but has been destabilized