Objective – Students will understand how Earth’s tilt on its axis changes the length of daylight and creates the
seasons.
Earth-Sun Geometry
(Source: The South Carolina Department of Natural Resources-Southeast Regional Climate Center, Spring 2001, Volume 7, No. 1)
The Basics
The Sun is the center of our Solar System, and the planets in our Solar System revolve
around the Sun. Earth is the third planet from the Sun.
The planets orbit the Sun in elliptical paths. Because Earth’s path is elliptical, there are
times when the Earth is closer to the Sun and times when it is further from the Sun.
But this difference in distance is not the reason we experience seasons. Even though the
Earth is closest to the Sun on January 5 (it is 91 million miles away), and farthest from the Sun on
July 5 (it is 94 million miles away), the distance does not effect of the seasons in any way.
91 million miles
(January)
95 million miles
(July)
23.5o
Reasons for the Seasons
It is not distance from the Sun, but tilt of the Earth that causes the
seasons. Earth rotates counterclockwise (from an "above" view) on an
imaginary axis that extends from the North Pole to the South Pole. This
pole is not oriented straight up and down but is tilted about 23.5o from
straight up and down, as the diagram to the right illustrates.
The tilt causes the seasons because it allows the Sun’s rays to shine more directly and
for longer periods of time on some locations certain times of the year than others. The
Sun shines in parallel rays towards the Earth, and this directness of the Sun’s rays upon
the surface of the Earth is what warms the surface. Recall that the Northern
hemisphere refers to all latitudes located north of the equator, and the Southern
hemisphere refers to all latitudes south of the equator. Because different parts of the
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Objective – Students will understand how Earth’s tilt on its axis changes the length of daylight and creates the
seasons.
Earth are tilted towards the Sun as Earth revolves around the Sun, the Northern and
Southern hemispheres experience direct sunlight at the surface at different times of the
year.
You can see that during the summer solstice, the Northern Hemisphere is tilted
towards the Sun, while the Southern Hemisphere is tilted away. The Northern
Hemisphere is experiencing summer and the Southern is experiencing winter. As Earth
continues to revolve around the Sun throughout the year, we see that at the December
solstice, the Northern hemisphere is tilted away from the Sun, and the Southern
hemisphere is tilted toward it. The December solstice represents the beginning of
winter for the Northern Hemisphere and the beginning of summer for the Southern.
Day length
The amount of daylight we experience
will be greater in the Northern hemisphere
in June, but will be less in the Southern
hemisphere. In December, the Southern
hemisphere will have the longer days. The
following diagram illustrates hours of
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Objective – Students will understand how Earth’s tilt on its axis changes the length of daylight and creates the
seasons.
daylight that correspond to each hemisphere and each solstice:
Between the June and December solstices, there is a time that the Sun’s rays shine
directly at the equator, and the tilt of the Earth does not expose the Northern or
Southern hemisphere to more or less radiation. These times are called equinoxes, and
at an equinox all locations on Earth receive an equal amount of sunlight, 12 hours. The
vernal Equinox occurs on March 21-22 marking the beginning of spring in the
Northern hemisphere, and the autumnal Equinox occurs on September 22-23 marking
the beginning of fall.
The following table illustrates the differing hours of sunlight latitudes experience over
the course of the Earth’s revolution around the Sun.
Length of Time from Sunrise to Sunset for Various Latitudes
on Different Dates
Northern Hemisphere (Read Down)
Latitude March 20th
June 21st
Sept 22nd
Dec 21st
0o
12 hrs.
12 hrs.
12 hrs.
12.0 hrs.
10o
12 hrs.
12.6 hrs.
12 hrs.
11.4 hrs.
20o
12 hrs.
13.2 hrs.
12 hrs.
10.8 hrs.
30o
12 hrs.
13.9 hrs.
12 hrs.
10.1 hrs.
40o
12 hrs.
14.9 hrs.
12 hrs.
9.1 hrs.
50o
12 hrs.
16.3 hrs.
12 hrs.
7.7 hrs.
60o
12 hrs.
18.4 hrs.
12 hrs.
5.6 hrs.
70o
12 hrs.
2 months
12 hrs.
0 hrs.
80o
12 hrs.
4 months
12 hrs.
0 hrs.
90o
12 hrs.
6 months
12 hrs.
0 hrs.
Southern Hemisphere (Read Up)
Sunlight Intensity
Not only does the number of daylight hours a location receives change as Earth
revolves around the Sun, but also the intensity of the sunlight received. Think about it:
if you were to shine a flashlight straight at a piece of paper, a very intense circle of light
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Objective – Students will understand how Earth’s tilt on its axis changes the length of daylight and creates the
seasons.
would light up a small area. If you were to tilt the flashlight so the light shone on the
paper at an angle, the "circle" of light would not be a circle, but an ellipse. The ellipse
would be larger in area than the circle. Because the amount of light shining from the
flashlight had not changed, but area lit up had increased, the ellipse would be less
bright than the circle. The Earth’s surface is affected by the Sun’s rays in the same way.
The Sun’s rays shine on Earth with an intensity of about 1370 Watts per square meter.
Even though the Sun’s rays hit the Earth in parallel beams, the tilt of the Earth
towards the Sun causes the beams to hit more directly in some places than others.
The following image helps explain the relationship between beam angle, area covered,
and intensity:
Notice that it is the same amount of sunlight hitting the surface, but the angle of the
sunlight is what changes the way that sunlight is spread out. Because the Earth is
round, we can see the different angles that sunlight makes as it hits the Earth.
Notice in the above diagram that the Earth is receiving
sunlight at a 90o angle at about 23.5o North in latitude.
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Objective – Students will understand how Earth’s tilt on its axis changes the length of daylight and creates the
seasons.
What made Earth a Tilt-a-World?
What made Earth a Tilt-A-World? I'm Bob Hirshon and this is Science Update.
Every 24 hours, the Earth spins around once on its axis—an axis that's tilted 23.5o
with respect to Earth's orbit. Vijay Thurimella of Denver, Colorado, wants to know
how Earth got that way.
We consulted Clark Wilson, a Geophysicist at the University of Texas at Austin. He
says Earth's tilt came about early in the formation of the solar system, four and a half
billion years ago. Back then, a lot of dust and rocks were floating around and crashing
into each other. That debris eventually stuck together to form the planets.
Wilson:
That process is a little messy, and in the case of the Earth, probably led to some big
impacts that eventually tilted the axis to what it is, 23.5o now.
One of those big impacts, for example, ejected a lot of debris that eventually coalesced
to form the moon.
But Wilson says Earth doesn't get knocked around much anymore.
Wilson:
Most of those objects are gone, and we're left with sort of little pieces, asteroids and so
on, that would not significantly change the Earth’s orbit or rotational axis direction
now.
For the American Association for the Advancement of Science, I'm Bob Hirshon.
Making Sense of the Research
First of all, let's be sure we're clear on what the Earth's axis is. It's an imaginary stick
going through the center of the Earth. Picture the Earth spinning like a top, straight up
and down. Now picture a stick going right through the center of the Earth. If the
Earth weren't tilted, it would rotate like that as it revolved around the Sun. We
wouldn't have seasons—only areas that were colder (near the poles) and warmer (near
the Equator).
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Objective – Students will understand how Earth’s tilt on its axis changes the length of daylight and creates the
seasons.
But the Earth is tilted, and that's why the seasons happen. When the Northern
Hemisphere is pointed toward the Sun, it gets more hours of sunlight. Temperatures
rise, and you get summer in New York. At the same time, it's darker and cooler “down
under” in Australia. Six months later, the reverse is true. It's the Southern Hemisphere
that experiences summer. The 23.5o degree tilt also explains why changes in daylight
during the seasons are very dramatic near the poles (which are flooded with sunlight all
day long in summer and get no light in mid-winter) but barely perceptible near the
equator (where the Sun shines more or less equally throughout the year).
The axis exists as a result of the rough-and-tumble environment of the early Solar
System. Objects collided and clumped together, which increased their gravitational
pull, which in turn drew more objects in, which made the object even more
gravitationally powerful. Today, the Solar System looks like a Sun and eight fairly neat
planets. There is not much stray junk flying around.
Occasionally these forming objects were hit by rocky objects big enough to knock it
off-kilter. That's what probably happened to the Earth, after it was already large
enough to start rotating. Actually, Wilson says it probably took several impacts to
whack the Earth into the position it's in today.
Back in the “old days,” the Earth used to rotate a lot faster—once every 6 to 10 hours.
The Moon's gravity has played a big role in slowing us down to 24 hours. It's a good
thing too, because a 6-hour day would certainly lead to an awfully hectic work week.
What do you think?
(source: NASA Space Place)
Many people believe that Earth is closer to the Sun in the summer and that is why it is
hotter. And, likewise, they think Earth is farthest from the Sun in the winter.
Although this idea makes sense, it is incorrect.
It is true that Earth’s orbit is not a perfect circle. It is a bit lop-sided. During part of
the year, Earth is closer to the Sun than at other times. However, in the Northern
Hemisphere, we are having winter when Earth is closest to the Sun and summer when
it is farthest away! Compared with how far away the Sun is, this change in Earth's
distance throughout the year does not make much difference to our weather.
There is a different reason for Earth's seasons.
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Objective – Students will understand how Earth’s tilt on its axis changes the length of daylight and creates the
seasons.
Earth's axis is an imaginary pole going right through the center of Earth from "top" to
"bottom." Earth spins around this pole, making one complete turn each day. That is
why we have day and night, and why every part of Earth's surface gets some of each.
Earth has seasons because its axis doesn't stand up
straight. Long, long ago, when Earth was young, it is
thought that something big hit Earth and knocked it
off-kilter. So instead of rotating with its axis straight up
and down, it leans over a bit.
By the way, that big thing that hit Earth is called Theia.
It also blasted a big hole in the surface. That big hit
sent a huge amount of dust and rubble into orbit. Most scientists think that that
rubble, in time, became our Moon.
As Earth orbits the Sun, its tilted axis always points in the same direction. So,
throughout the year, different parts of Earth get the Sun’s direct rays.
Sometimes it is the North Pole tilting toward the Sun (around June) and sometimes it
is the South Pole tilting toward the Sun (around December).
It is summer in June in the Northern Hemisphere because the Sun's rays hit that part
of Earth more directly than at any other time of the year. It is winter in December in
the Northern Hemisphere, because that is when it is the South Pole's turn to be tilted
toward the Sun.
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