Earth-Sun Relationships
The Reasons for the Seasons
Solar Radiation
• The earth intercepts less than one two-billionth of
the energy given off by the sun.
• However, the radiation is sufficient to provide
99.9% of the energy that heats the earth / drives
our climate.
Radiation Receipt
• Solar radiation is received as
parallel rays of energy (“line of
sight, speed of light”)
– Earth’s diameter is 12,765 km
– Sun’s diameter is 1,390,000
km
(over 100 times that of the
Earth)
So is the receipt of radiation the same
everywhere on Earth?
Why is it hot in the tropics
and cold at the poles?
The Earth is curved and this
cause differing angles of
incidence.
Intensity of incoming solar radiation
(insolation) is related to angle of incidence.
Higher angles = higher intensity.
Why is it hot in the tropics and
cold at the poles?
• The Earth’s surface is curved relative to the
incoming solar radiation.
• Direct rays (most intense) are always in the
tropical latitudes (23.5 º S to 23.5º N)
• As a result, solar radiation is diffuse in the
polar regions.
What causes the seasons?
• rotation of the earth on its axis?
• revolution of the earth around the sun
• 23.5 O tilt of the earth axis from
perpendicular to the plane of the ecliptic
• polarity (axial parallelism)
Rotation of the Earth
• Rotate west to east or ‘counter-clockwise’
(when viewed from the North Pole)
– sunset and sunrise
• Rotation period is 24 hours per solar day
(23 hours 56 minutes with respect to other
stars)
Revolution of the earth
• Period of revolution is 365.25 solar days
• Elliptical orbit (plane of the ecliptic)
Perihelion -
Aphelion -
91.5 million
miles
94.5 million
miles
(Jan. 4)
(July 5)
93 million miles average distance
Tilt of the earth on its axis.
• The earth is tilted 23.5º from perpendicular to the
plane of the ecliptic.
• Tilt is currently constantly toward Polaris
SOLSTICES
SUMMER, JUNE 21
WINTER, DECEMBER 21
Where are the
overhead rays of the
sun on these days?
Which parts of the
earth are in darkness
and light? For how
long?
Notice these four
important parallels.
Where do they
occur? Why?
Tropic of Cancer at 23.5º N
Tropic of Capricorn at 23.5º S
Arctic Circle (66.5 º N)
Antarctic Circle (66.5º S)
EQUINOXES
VERNAL, MARCH 21
AUTUMNAL, SEPTEMBER 21
Effects of the Seasons
•
•
•
•
Changes
Changes
Changes
Changes
in
in
in
in
solar intensity
solar altitude
day length
temperature
All of these changes are most extreme
at high latitudes and minimized at the
equator.
Annual Changes in Daylength
1 of 2
Annual Change in Noontime
Solar Altitude
What is the change in height from the solstice to the equinox?
The total annual change?
Summer
Solstice
Equinox
Winter
Solstice
N
E
S
S
W
N
2 of 2
Annual Change in Noontime
Solar Altitude
Summer
Solstice
Total annual change:
47O
23.5O
Equinox
Winter
Solstice
N
E
S
23.5O
S
W
N
Key Points
•
•
•
•
Size and Shape of Earth.
Latitude and Longitude
Explain the basic earth-sun relationships.
Equinoxes, solstices and the relationship among
– seasons
– the latitude of the sun’s direct rays, and
– the intensity of solar radiation received
– changes in Daylight and Darkness
•23.5º N
Time
•Standard Time
•The International Date Line
• Daylight Savings Time
Year = 365 1/4 days or 365 days 5 hrs. 48 min.
45.685 seconds
Solar Day = 24 hours on average. Changes
slightly with the elliptical orbit around the sun.
Use analemma to calculate.
Standard Time
360o / 24 hr. = 15o standard zones (7.5o E and W from standard
meridian)
International Date Line
Cross W to E - gain day; E to W - lose day
International Date Line
A day is 24 hours,
but a date lasts for
48 hours!
At any given time on earth,
except Greenwich noon,
there are two dates on the
planet.
The new date starts at the IDL when midnight crosses the
IDL and moves westward around the planet for 24 hrs.
displacing the old date. Then it fades out as the “old” date
for 24 hrs. For more help on this see:
http://www.vistech.net/users/rsturge/dateline.html
Daylight Savings Time
•Day starts earlier April 1
- October 31
•Started during war time
to save energy (maximize
morning sun and increase
work day).
•Arizona and Hawaii
don’t use it.
•Navajo and Hopi do causes great confusion.