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.
Why is it hot in the tropics
and cold at the poles?
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 diffused in the
polar regions.
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.
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)
Seasons
Seasons on Earth are
created by The tilt of Earth Axis
Relative to The Plane of the Ecliptic
Earth’s tilt allows the Subsolar
Point to Migrate during the year
Earth’s Tilt Relative to Orbit
Earth’s Axis is tilted 66.5 degrees from its Plane
of Ecliptic (Plane of Earth’s Orbit)
Sometimes this
Is expressed as
23.5 degrees
from a line
Perpendicular
To The Plane
of the Ecliptic
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
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.