Latitude and Seasons
Annual Variations in Solar Energy
What’s your favorite season of the year? Do you
like summer more than winter? Do you know why
we have seasons? Do you know that you can go
from one season to another just by changing your
location?
Our seasons are caused by the relationship
between Earth and the Sun. The Sun plays a very
important part in our lives. Life on Earth is
supported by energy from the Sun. Our language
shows how important the Sun is to our everyday
lives. We have Sun roofs, Sunglasses, and Sun
screens. We need the Vitamin D that the Sun
provides to keep our bodies strong.
Earth-Sun Relationships
Most of Earth’s energy comes from the Sun. This
type of energy is called solar energy and reaches
the Earth as light and heat. All life on Earth
depends on solar energy. Solar energy affects
weather, plants, animals and human activities. It
influences the clothes we wear, the homes we live
in, the foods we eat, and even which sports we
play. Three different relationships between Earth
and the Sun control how much solar energy is
received at different locations. Do you know what
these are?
Solar energy strikes only the half of the Earth
facing the Sun. If Earth did not rotate on its axis –
creating day and night – only the half facing the
Sun would receive solar energy. That side of Earth
would be very hot. The half of the planet facing
away from the Sun would always be dark and cold.
Earth’s rotation allows the entire planet’s surface to
receive the warming effects of daylight and the
cooling effects of darkness.
Revolution
Rotation
In addition to rotating on its axis, Earth revolves
around the Sun. It makes one elliptical orbit, or
revolution, every 365¼ days -- one Earth year.
Each time you celebrate your birthday we have just
completed another orbit around the Sun. For
convenience, our calendars have 365 days in a
year. To account for the one-fourth day gained
each year, an extra day – February 29 – is added
to the calendar every four years. This year, one
day longer than the previous three, is called leap
Imagine that Earth has a rod running through it
from the North Pole to the South Pole. This rod
represents Earth’s axis, and the planet spins
around on it. One complete spinning of Earth on
its axis is one rotation, which takes 24 hours.
Earth rotates in a west-to-east direction. From the
North Pole the rotation appears to move in a
counter-clockwise fashion (see diagram at top
right). We see the effects of Earth’s rotation as the
Sun “rising” in the east and then “setting” in the
west. To us, it appears that the Sun is moving
across the sky. Actually, it is only Earth rotating on
it’s axis.
year.
Tilt
If Earth’s axis always pointed straight up and down
in relation to the Sun, daylight would be the same
at every location on Earth. Each day would consist
of 12 hours of daylight and 12 hours of darkness.
This would be true throughout the year. However,
this is not the case because Earth’s axis is tilted in
relation to the Sun.
The Earth makes a complete rotation once every 24
hours. The Earth’s rotation is responsible for the
daily cycles of day and night. At any one moment
in time, one half of the Earth is in Sunlight, while
the other half is in darkness.
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As Earth revolves around the Sun, its axis points
toward the same spot in the sky. The North Pole
points to a star known as the North Star. The
position of the axis is fixed in respect to the North
Star. Yet it is not fixed in relation to our Sun. As
Earth revolves around the Sun, the North Pole
points at times toward the Sun at at times away
from the Sun (see diagram below). The tilt of
Earth on its axis affects the amount of solar energy
that different places receive during the year.
Solar Energy and Latitude
When the North Pole points toward the Sun, direct
rays strike the Northern Hemisphere. Thus, the
Northern Hemisphere receives more concentrated
solar energy, making temperatures warmer. The
length of time between Sunrise and Sunset also
grows longer. At this time, the Southern
Hemisphere receives more indirect rays and is
cooler.
As you know, different places on Earth receive
different amounts of solar energy. Areas near the
equator receive a lot of solar energy all year.
These places are generally warm. We call these
warm low-latitude areas near the equator the
tropics. Other places get very little solar energy.
These areas are at high latitudes and are cold most
of the time. Because these areas surround the
North and South poles, we call them the polar
regions. The areas between the tropics and the
polar regions are called the middle latitudes.
The amount of solar energy reaching these areas
changes greatly during the year. They may be
warm or cool, depending on the time of the year.
When the North Pole tilts away from the Sun, the
most direct rays strike the Southern Hemisphere.
Now, the Southern Hemisphere receives more solar
energy, experiences longer days, and has warmer
temperatures. At this time, the Northern
Hemisphere receives less solar energy and has
cooler temperatures.
The amount of solar energy that a place receives
relates to the angle at which the Sun’s rays strike
Earth. Direct vertical solar rays heat Earth’s
surface more than angled or indirect rays. This is
because the amount of solar energy in a direct ray
is concentrated on a smaller area. The same
amount of energy in an angled ray is spread over a
larger area.
The Seasons
When it is summer in the Northern Hemisphere it is
winter south of the Equator in the Southern
Hemisphere. For example, if you are baking in the
heat and direct Sun of an Arizona summer, you can
fly to somewhere south of the Equator and enjoy
cold winter weather.
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The Solstices
We refer to the times of greater and lesser heat as
the seasons. There are four general seasons:
winter, spring, summer and fall. Some regions,
particularly the tropics, are warm year-round but
have alternating wet and then dry seasons.
Twice during the year, Earth’s poles are tilted
towards or away from the Sun more than any other
time. When Earth’s poles point at their greatest
angle towards or away from the Sun is called a
solstice. Solstices occur each year around
December 21 and June 21.
In each hemisphere, the Sun’s energy is strong
during the summer. Daytime lasts longer. In the
winter, daytime is shorter, and the Sun’s energy
weaker. During spring and fall, the Sun’s energy is
more evenly distributed. At these times, daylight
and darkness are closer to equal length. The tilt of
Earth’s axis causes the Northern and Southern
Hemispheres to have opposite seasons at the same
time of the year.
In the Northern Hemisphere, the December solstice
has the fewest daylight hours of the year and is the
first official day of winter. The Southern
Hemisphere on the same day has its greatest
number of daylight hours, and it is the first day of
summer.
During the December solstice, the Sun’s most direct
rays strike Earth in the Southern Hemisphere along
a parallel 23.5o south of the equator known as the
Tropic of Capricorn. At this time the South Pole
is tilted toward the Sun and all areas south of the
Antarctic Circle have 24 hours of daylight.
What would happen if the Earth was not tilted and
all areas received an equal amount of solar energy
during all times of the year? There’d be no
seasons. This comes closest to happening near the
Equator where temperatures remain hot all year
around.
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Meanwhile, the area around the North Pole
experiences constant darkness and is very cold.
The parallel beyond which no Sunshine shines on
this day is known as the Arctic Circle located
66.5o north of the equator.
Equinoxes
An equinox occurs twice each year when Earth’s
poles are neither pointed toward or away from the
Sun. Equinox means “equal night” in Latin. At this
time, the direct rays of the Sun strike the equator,
and both poles are at a 90o angle to the Sun. At
this time both hemispheres receive an equal
amount of Sunlight – 12 hours each.
On the June solstice in the Northern Hemisphere,
it’s the first day of summer and the longest day of
the year. On this day, the Sun’s most direst rays
are at their most northerly position, striking Earth
at a line known as the Tropic of Cancer located
23.5o north of the equator. If you traveled to
Australia on the June solstice, it would be the first
day of winter and be the shortest day of the year.
During the June solstice, the Sun never sets north
of the Arctic Circle and daylight lasts 24 hours. At
this time the opposite occurs south of the Antarctic
Circle, where darkness lasts 24 hours.
Equinoxes occur on or about March 21 and
September 23. In the Northern Hemisphere, the
March equinox marks the beginning of spring. To
people living in the Southern Hemisphere, however,
the March equinox marks the beginning of fall. The
opposite situation occurs about September 23.
Days between the solstices and equinoxes
gradually become warmer or cooler, and daytime
becomes longer or shorter, depending on where
you live. This cycle is repeated each year, creating
the four seasons.
Angle of the Sun’s Rays Hitting Earth
March
Equinox
June
Solstice
March 21
June 21
North Pole (90oN)
0o
23.5o
0o
- 23.5o
Antarctic Circle (66.5oN)
23.5o
47o
23.5o
0o
Tropic of Cancer (23.5oN)
66.5o
90o
66.5o
43o
Equator (0o)
90o
66.5o
90o
66.5o
Tropic of Capricorn (23.5oS)
66.5o
43o
66.5o
90o
Antarctic Circle (66.5oS)
23.5o
0o
23.5o
47o
South Pole (90oS)
0o
- 23.5o
0o
23.5o
Location's
Latitude
September
Equinox
December
Solstice
September 23 December 21
The table above shows the changes in the angle of the Sun for the two solstices and equinoxes. All
measurements are in degrees. The horizon is 180o from true north to true south. When the Sun is
directly overhead it would be at 90o.
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