What Causes Earth’s Seasons?
• Earth’s axis is tilted 23.5 degrees – it always points in
the same direction (Polaris, the North Star) as we
orbit our Sun once a year
• This tilt causes the hemispheres to alternate in the
amount of our Sun’s light and heat they receive
through the year
This image shows the reason Earth experiences seasons. Points we discuss
using this image are:
1) Earth’s orbit around the Sun is only slightly elliptical
2) Earth’s path around the Sun brings us closer to the Sun in January. Many
students think we have seasons because Earth is sometimes closer and
sometimes farther from the Sun. This is correct, however, we actually are
closer to the Sun in January in the Northern Hemisphere!
3) Earth’s seasons are caused by Earth’s tilt on its axis (~23 degrees). Earth’s
axis essentially is fixed - it always points to the same place in the sky (on the
celestial sphere) – towards Polaris. As we orbit the Sun each year, first one
polar region is tilted toward the Sun, and then the other is tilted toward the
Sun. When the north polar region is tilted toward the Sun (summer) the south
polar region is tilted away (winter).
Notes: Earth’s tilt does change over very long time periods, but for the most
part, it moves between 22 and 23 degrees. Earth’s axis also wobble a bit, but
over time periods of thousands of years, pointing toward different stars.
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Northern Hemisphere Summer
More daylight hours, more direct sunlight
At this point, we have participants use styrofoam balls with sticks and a bright lamp to model the seasons on the Earth,
with the axis of the “Earth” tilted toward a “North Star” that has been placed high in the corner of the room.
For part of our orbit the northern half of Earth is tilted toward the Sun. This is summer in the northern hemisphere;
there are longer periods of daylight, the Sun is higher in the sky, and the Sun's rays strike the surface more directly,
giving us warmer temperatures. The north pole is in constant daylight!
When the northern half of Earth is tilted toward the Sun, the southern hemisphere is tilted away. People in the southern
hemisphere experience the shorter day lengths and colder temperatures of winter.
During winter in the northern hemisphere, our northern axis continues to point to the North Star, but, because we have
moved in our orbit around the Sun, our northern hemisphere now points away from our Sun. The north pole is
completely dark and other places in the northern hemisphere experience the shorter day lengths and colder
temperatures of winter as the Sun traces a lower arc across the southern sky and the Sun's rays strike the surface at a
lower angle. When it is winter in the northern half of Earth, the southern hemisphere, tilted toward our Sun, has
summer.
During fall and spring, some locations on Earth experience similar, milder, conditions. Earth has moved to a position
in its orbit where its axis is more or less perpendicular to the incoming rays of the Sun. The durations of daylight and
darkness are more equally distributed across all latitudes of the globe.
Solstices occur when Earth's axis is pointed directly toward our Sun. This happens twice a year during Earth's orbit.
Near June 21 the north pole is tilted 23.5 degrees toward our Sun and the northern hemisphere experiences summer
solstice, the longest day of the northern hemisphere year. On that same day, the southern hemisphere is tilted 23.5
degrees away from our Sun and the southern regions of Earth experience the shortest day of the year — the winter
solstice.
The second solstice occurs on December 21 or 22 when the north pole is tilting 23.5 degrees away from our Sun and
the south pole is inclined toward it. This is the shortest day of the year in the northern hemisphere — the northern
hemisphere winter solstice.
Twice each year, during the equinoxes (“equal nights”), Earth's axis is not pointed toward our Sun, but is
perpendicular to the incoming rays. During the equinoxes every location on our Earth (except the extreme poles)
experiences 12 hours of daylight and 12 hours of darkness. The vernal or spring equinox occurs in the northern
hemisphere on March 21 or 22 (the fall equinox of the southern hemisphere). September 22 or 23 marks the northern
hemisphere autumnal or fall equinox.
As Earth orbits our Sun, the position of its axis relative to the Sun changes. This results in a change in the observed
height of our Sun above the horizon. For any given location on Earth, our Sun is observed to trace a higher path above
the horizon in the summer, and a lower path in the winter. During spring and fall, it traces an intermediate path. This
means that our Sun takes a greater amount of time tocross the sky in the summer and a shorter amount of time in the
winter. This effect is greater as you move toward the poles; people living near the equator experience only small
changes in daylight during the year. The change is more extreme toward the poles.
From the National Maritime Museum
During the northern hemisphere summer solstice, Earth is tilted such that the Sun's rays strike perpendicular to the
surface at the Tropic of Cancer (23.5 degrees north latitude, corresponding to the tilt of Earth's axis). At (solar) noon,
our Sun is directly overhead in this location (and at a decreasing height above the horizon north and south of the
Tropic of Cancer). At locations north, our Sun will be at its highest position above the horizon and will take the
greatest amount of time to cross the sky. All northern locations have more than 12 hours of daylight. All locations
south experience less than 12 hours of daylight. Locations above the Arctic Circle (north of 66.5 degrees latitude; 90
degrees minus the tilt of Earth's axis) receive 24 hours of sunlight. Locations below the Antarctic Circle (66.5 degrees
south latitude) experience 24 hours of darkness.
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Earth’s orbit is almost a perfect circle
• Earth is CLOSEST to our Sun (91 million
miles) in winter—January 3
• Earth is farthest from on our Sun (94
million miles) in summer –July 4
More information at http://www.physicalgeography.net/fundamentals/6h.html
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Height of Sun for USA
W inter: The Sun rises in the southeast,
stays low in the sky, and sets in the
southw est.
Sp ring : The Sun rises d ue east, m oves
hig her in the sky than in w inter, and sets
d ue w est.
Sum m er: The Sun rises in the northeast,
travels hig h ( near zenith) , and sets in the
northw est.
Fall: The Sun rises d ue east, travels to a
m ed ium -heig ht in the sky, and sets d ue
w est.
During this section, we demonstrate physically, using a planetarium or the
horizon or the walls of the classroom, the location of the Sun’s path across the
sky for each of the seasons, and ask the participants to predict how high the
Sun rises in the sky and where it will set.
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