Transcript The Reason for Seasons - Somers Public Schools

The Reason for Seasons
It’s the Tilt, Not the Distance
created by Mrs. Bodine-Donahue
12/2006
What will you learn?
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How is Earth’s tilt on its axis related to
the seasons?
Why is summer in the Northern
Hemisphere warmer than winter?
How are solstices and equinoxes related
to the position of Earth in its orbit?
created by Mrs. Bodine-Donahue
12/2006
Our Four Seasons
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Earth’s revolution
around the Sun takes
365.25 days. During
that one-year period,
Earth’s weather
patterns change in a
regular, predictable
cycle. Spring, summer,
autumn and winter are
the four divisions of the
year that we call
seasons.
created by Mrs. Bodine-Donahue
12/2006
You might think that the seasons are caused by Earth’s changing distance from
the Sun due to Earth’s elliptical orbit. In other words, some people believe it is
warmer in the summer because we are closer to the Sun, and colder in the winter
because we are farther away. But this is not true! The difference in the
distance to the Sun at different points on the ellipse is very small, not enough to
cause a major change in the weather.
created by Mrs. Bodine-Donahue
12/2006
Polaris;
the North Star
We have different seasons because Earth’s axis is tilted 23.5 degrees.
This tilt is called Earth’s axial tilt. As Earth travels in its orbit around
the Sun, Earth’s tilt stays the same. This makes Earth’s North or South
Poles lean towards or away from the Sun at different times of the year.
The North Pole is always pointed towards the star Polaris, also known as
the North Star.
created by Mrs. Bodine-Donahue
12/2006
Did you notice that when the northern [top] half of the Earth
is pointing away from the Sun, the southern [bottom] half is
pointing toward it?
As the Earth revolves around the Sun, the northern and
southern hemispheres have opposite seasons.
created by Mrs. Bodine-Donahue
12/2006
As Earth moves around the Sun, its northern half, or hemisphere,
receives more or less sunlight than the southern one. These
changes in direct or indirect sunlight create the seasons.
In the diagram above, the Sun’s rays hit the southern hemisphere
almost directly, so days are warm and bright. It is experiencing
summer. In the northern hemisphere the Sun’s rays hit are
hitting at an angle, so the sunlight is indirect and not as strong.
Days are cooler and dimmer. The northern part is having winter.
created by Mrs. Bodine-Donahue
12/2006
The Sun’s rays strike less directly on the part of the Earth that tilts
away from the Sun. That part gets less energy. A place that gets less
energy from the sun becomes colder.
The Sun’s rays strike more directly on the part of Earth that tilts toward
the Sun. That part of Earth gets more energy. A place that gets more energy
is warmer.
created by Mrs. Bodine-Donahue
12/2006
DIRECT AND INDIRECT ENERGY: Imagine the flashlights to be the sun. The
energy coming from each flashlight is the same, but the way the light is striking
the ground is different. The two flashlights on the left are allowing their energy
to strike the ground DIRECTLY in a concentrated manner. The flashlight on the
right is tilted so that when its energy strikes the ground, the energy is spread
over a much larger area. The energy from the tilted flashlight is striking the
ground INDIRECTLY, and its energy is less concentrated.
created by Mrs. Bodine-Donahue
12/2006
Near Earth’s equator, the Sun is never far from directly overhead
at noon. Temperatures stay warm all year at the equator because
the Sun’s rays always hit it almost directly.
Earth has two solstices a year. The solstices are the two times of
the year when the Sun’s direct rays strike earth the farthest north
or south of the equator.
created by Mrs. Bodine-Donahue
12/2006
Around June 20, Earth passes through a point in its orbit called the
summer solstice. At this time, the Northern Hemisphere is tilted directly
towards the Sun. It is the first day of summer. It is the longest period
of daylight. The Southern Hemisphere is tilted directly away from the
Sun at this time, so it is the first day of winter.
Around December 21, Earth reaches the winter solstice. At this time, the
Northern Hemisphere is tilted directly away from the Sun. It is the
Northern Hemisphere’s first day of winter. It is also the day with the
shortest period of daylight. But in the Southern Hemisphere, it is the day
with the longest period of daylight and the start of summer.
created by Mrs. Bodine-Donahue
12/2006
Twice a year, between the solstices, the Sun’s rays strike the equator
directly. When this happens, day and night are equal in length everywhere
on Earth. Both hemisphere have 12 hours of daylight and 12 hours of
darkness. These two days are called equinoxes, from the Latin word for
“equal night.”
The autumnal equinox occurs around September 23, marking the beginning
of autumn in the Northern Hemisphere.
The vernal equinox occurs around March 21, marking the beginning of
spring in the Northern Hemisphere.
created by Mrs. Bodine-Donahue
12/2006
It’s the Tilt, Not the Distance!
The Sun is not quite at the center of Earth’s elliptical orbit.
So, the distance between Earth and the Sun changes as Earth
moves throughout the year. In some parts of its orbit, Earth is a
little closer to the sun. In other parts of its orbit, it is a little
farther away. Earth’s distance from the Sun does not affect the
seasons. We know this to be true because if it were the distance,
every place on Earth would have the same season at the same
time!
created by Mrs. Bodine-Donahue
12/2006
http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/en
ergy/earth_sun_relations_seasons.html
created by Mrs. Bodine-Donahue
12/2006