Earth

1
Earths RevolutionSeasons and Retrograde Motion
of Planets
2
Earths Revolution

• Revolution is the motion of a body that travels
  around another body in space.
• Earth takes 365 ¼ days to revolve around the sun.
  
• Earth does not revolve in a perfect circle.

3
Aphelion and Perihelion
4
Tilt of Earths Spin Axis
23.5o tilt
The Earths spin axis is tilted by 23.5 degrees
off vertical with respect to the ecliptic plane
(plane of the Earths orbit around the sun)
5
Amount of Light

• Changes in the angle at which the suns rays
  strike Earths surface cause the seasons.
• When the Northern Hemisphere is tilted away from
  the sun there is less direct sunlight and less
  total daylight which leads to cooler
  temperatures.
• When the Northern Hemisphere is tilted toward the
  sun there is more direct sunlight and more total
  daylight which leads to warmer temperatures.

6
The Seasons
7
Summer Solstice June 21 or 22
tropic of Cancer
arctic circle
sunlight
equator
antarctic circle
sunlight
8
Winter Solstice December 21 or 22
tropic of Cancer
arctic circle
sunlight
equator
antarctic circle
sunlight
tropic of Capricorn
9
Vernal Equinox March 21 or 22
arctic circle
sunlight
tropic of Cancer
equator
tropic of Capricorn
sunlight
antarctic circle
10
Autumnal Equinox September 21 or 22
arctic circle
sunlight
tropic of Cancer
equator
tropic of Capricorn
sunlight
antarctic circle
11
Seasons
12
Other Seasonal Extremes

• The sun never rises for observers north of the
  arctic circle on the day of the winter solstice.

• The sun never sets for observers north of the
  arctic circle on the day of the summer solstice.

• This is opposite for the antarctic circle.

13
Tilt of Earths Spin Axis
W
N
S
E
winter solstice
summer solstice
equinox
The sun rises on the east point and sets on the
west point on the days of the equinoxes,
giving equal periods of day and night.
The sun is in the sky for the longest duration on
the summer solstice and illuminates the northern
hemisphere most directly.
14
Temperature Effect

• Summer days are longer and the sun is more
  intense (due to the more direct illumination
  angle). Thus summer is hotter than winter.

• There is a lag of the seasons when comparing the
  dates of the solstices with the actual extremes
  in temperature because it takes time to heat up
  the oceans and atmosphere at the onset of summer
  and to cool them off at the onset of winter.

• If the Earths spin axis were not tilted by some
  angle, we would have no seasons.

15
Motions of the Wanderers The Planets
retrograde motion
normal direct (eastward) motion
Mars
West
East
The night sky facing south
16
Retrograde Motion of Planets

• The apparent backward motion of other planets as
  Earth revolves around the sun.
• Early Models of the Solar System
• Aristotle Earth centered geocentric
• Copernicus Sun centered heliocentric
• Aristotles model could not explain retrograde
  motion. Galileo then observed moons of Jupiter
  going around Jupiter which supported Copernicus
  sun centered theory.

17
Aristotles View
18
Copernicus View