The Reason for the Seasons
The Earth’s seasons are caused by the tilt of the Earth’s axis, NOT the differences in
distance from the Sun, which are extremely small. The Earth’s axis is tilted 23.45
degrees from the perpendicular to the plane on which the earth revolves around the
Sun. The tilting of the axis is responsible for the four seasons of the year: spring,
summer, autumn (fall), and winter. The tilt combined with the spherical shape of the
Earth causes different parts of the Earth to receive varying concentrations of the Sun’s
light energy over the year.
As shown in Figure 1, each hemisphere’s summer is warmer than in winter because
(1) during the summer the Sun’s rays hit that hemisphere at a more direct angle
than during the winter and
(2) in the summer the days are much longer than the nights (the length of day
and night is also caused by the Earth’s tilt; more on this later).
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http://www.bbc.co.uk/schools/ks3bitesize/science/images/position_of_earth_around_sun.gif
How the Earth’s Tilt Causes the Seasons – Part 1: The Spread of the Sun’s Rays
on the Earth
When the Sun’s rays do not hit a location directly, they are more spread out and hence
less concentrated. Think of the Sun’s light like the light from a flashlight. The flashlight
gives out a set amount of light. If you point the flashlight directly at the wall, you get a
concentrated circle of light like the picture on the left, whereas if you point the flashlight
at an angle to the floor, then you get a spread out ellipse shape of less concentrated
light (Figure 2). The ellipse covers a larger area than the circle, but contains the same
amount of light. The direct light in the circle is more concentrated and intense because
it is less spread out.
Figure 2
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The Solstices
The summer solstice is shown in the picture on the left and occurs on June 21. It marks
the beginning of summer in the Northern Hemisphere and is the longest day (and
shortest night) of the year. Also, during this time, the northern hemisphere is maximally
oriented towards the sun.
The winter solstice occurs December 21 or 22 (it varies a little because it doesn’t take
the Earth exactly 365 days to make one trip around the Sun; leap years cause the date
to vary). It marks the beginning of winter Northern Hemisphere and is the shortest day
(and longest night) of the year. Also, during this time, the Northern Hemisphere is
maximally oriented away from the sun.
Figure 3. The Solstices
http://www.thecuriousg.com/blog/wp-content/uploads/2010/06/thecuriousg-summer-winter-solstice-positions.jpg
The Equinoxes
The equinoxes (think equal) are days of equal duration day and night (Figure 4). During
these periods, the equator receives the Sun’s rays most directly.
The vernal equinox (the word vernal also means “fresh, new, or youthful”) occurs in late
March and marks the beginning of spring in the Northern Hemisphere. The rays of the
Sun hit the equator directly (0° tilt) while the poles have a 90° tilt and receive the least
amount of light.
The autumnal equinox occurs in late September and marks the beginning of autumn/fall
in the Northern Hemisphere.
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Figure 4. The Vernal and Autumnal Equinox
http://etc.usf.edu/clipart/72700/72740/72740_equinox_lg.gif
How the Earth’s Tilt Causes the Seasons – Part 2: The Lengths of Days and
Nights
The second cause of seasonal changes is the length of day, which is also caused by
the tilt of the Earth’s axis. During the summer for a hemisphere, more of that
hemisphere is in light than darkness (Figure 5). The exact amount by which day is
longer than 12 hours depends on how far north one is from the equator, with length of
day increasing the further north you go.
For example, when it is summer solstice for the Northern hemisphere, daytime in
Chicago, Illinois lasts about 15 hours (and night only 9 hours).1 In Barrow, Alaska,
daytime lasts a full 24 hours during summer solstice2 (this is true because above a
certain latitude, 180 degrees around the earth are bathed in light)!
Figure 5. Summer in the Northern Hemisphere:
Night
Day
Red Line – Earth’s Axis
Blue Line – Earth’s Equator
Dashed Line – the perpendicular to the plane on which the Earth travels
around the Sun.
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The Equator
The equator is warm all year round because, on average, it receives the most direct
light from the sun. Also note that the equator has equal length days and nights no
matter the time of the year. Exactly half the equator is in light and half is in darkness no
matter where the Earth is in its course around the sun. The means that seasons at the
equator are only caused by one factor: the varying spread of the suns rays on the earth
caused by the tilt of the earth. As you get further away from the equator, the more day
and night varies over the year and the more pronounced the seasons become.
More things to think about…
The two hemispheres experience opposite seasons; when it is summer for the Northern
Hemisphere, it is winter for the Southern Hemisphere, and vice versa. When it is spring
for the Northern Hemisphere, it is autumn/fall for the Southern Hemisphere, and vice
versa. Think about it – when the Northern Hemisphere is maximally oriented towards
the Sun (23.45 degrees), the Southern Hemisphere is maximally oriented away from the
Sun, (and vice versa, of course!).
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