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If
the reason for the seasons was solely due to our proximity to the Sun, then it
should be warm in both the northern and southern hemispheres at the same time
of year - that doesn't happen - it's really the tilt that is the main reason we
have seasons
By John
P. Millis, Ph.D
The change of seasons is one of those phenomena
that people take for granted. They know it happens in most places, but don't
always stop to think about why we have seasons.
The answer lies in the realm of astronomy and
planetary science.
Think of the orbital plane of the solar system
as a flat plate. Most of the planets orbit around the Sun on the
"surface" of the plate.
Rather than having their north and south poles
point directly perpendicular to the plate, most planets have their poles at a
slant. This is particularly true of Earth, whose poles are tilted 23.5 degrees.
Earth may have a tilt because of a large impact
on our planet's history that likely caused the creation of our Moon.
During that event, infant Earth was smacked pretty
heavily by a Mars-sized impactor. That caused it to tip over on its side for a
while until the system settled down.
.
Eventually, the Moon formed and Earth's tilt settled to the 23.5 degrees it is today. It means that during part of the year, half of the planet is tilted away from the Sun, while the other half is tilted toward it.
.
Eventually, the Moon formed and Earth's tilt settled to the 23.5 degrees it is today. It means that during part of the year, half of the planet is tilted away from the Sun, while the other half is tilted toward it.
Both hemispheres still get sunlight, but one
gets it more directly when it's tilted toward the Sun in summer, while the
other gets it less directly during winter (when it is tilted away).
This diagram shows Earth's axial tilt and how it affects the hemispheres that are tilted toward the Sun through different parts of the year. |
The solstices and equinoxes are used mostly in
calendars to mark the beginning and end of seasons but are not themselves
related to the causes of the seasons.
Seasonal Changes
Our year is divided up into four seasons:
summer, fall, winter, spring. Unless someone lives at the equator, each season
delivers different weather patterns.
Generally, it's warmer in spring and summer, and
cooler in autumn and winter.
Ask most people why it is cold in the winter and
warm in the summer and they'll likely say that Earth must be closer to the Sun
in the summer and farther away in the winter.
This seems to make common sense. After all, as
someone gets close to a fire, they feel more heat. So why wouldn't closeness to
the Sun cause the warm summer season?
While this is an interesting observation, it
actually leads to the wrong conclusion.
Here's why: Earth is farthest from the Sun in
July each year and closest in December, so the "closeness" reason is
wrong.
Also, when it is summer in the northern
hemisphere, winter is happening in the southern hemisphere, and vice versa.
If the reason for the seasons was solely due to
our proximity to the Sun, then it should be warm in both the northern and
southern hemispheres at the same time of year.
That doesn't happen. It's really the tilt that
is the main reason we have seasons. But there is another factor to consider.
All planets have an axial tilt, including the
gas giants. The Uranus tilt is so severe it "rolls" around the Sun on
it side.
It's Hotter at High Noon Too
Earth's tilt also means that the Sun will appear
to rise and set in different parts of the sky during different times of the
year.
In the summertime the Sun peaks almost directly
overhead, and generally speaking will be above the horizon (i.e. there will be
daylight) during more of the day.
This means that the Sun will have more time to
heat the surface of the Earth in the summer, making it even warmer. In the
winter, there's less time to heat the surface, and things are a bit chillier.
Observers can generally see this change of
apparent sky positions quite easily.
Over the course of a year, it's fairly easy to
note the position of the Sun in the sky. In the summertime, it will be higher
up and rise and set at different positions than it does in the wintertime.
It's a great project for anyone to try, and all
they need is a rough drawing or picture of the local horizon to the east and west.
Observers can glance out at the sunrise or
sunset each day, and mark the positions of sunrise and sunset each day to get
the full idea.
Back to Proximity
So, does it matter how close Earth is to the
Sun? Well, yes, in a sense, it does, just not the way people expect.
Earth's orbit around the Sun is only slightly
elliptical. The difference between its closest point to the Sun and the most
distant is a little more than three percent.
That isn't enough to cause huge temperature
swings. It translates to a difference of a few degrees Celsius on average.
The temperature difference between summer and
winter is a lot more than that. So, closeness doesn't make as much of a
difference as the amount of sunlight the planet receives.
That's why just simply assuming that Earth is
closer during one part of the year than another is wrong. The reasons for our
seasons are easy to understand with a good mental image of our planet's tilt
and its orbit around the Sun.
Key Takeaways
Earth's axial tilt plays a large role in creating
seasons on our planet.
The hemisphere (north or south) tilted toward
the Sun receives more heat during that time.
Closeness to the Sun is NOT a reason for the
seasons.
John
P. Millis, Ph.D
Professor
of Physics and Astronomy
Education
Ph.D.,
Physics and Astronomy, Purdue University
B.S.,
Physics, Purdue University
Introduction
Associate
Professor of Physics, Anderson University
Chairman,
Department of Physical Sciences and Engineering, Anderson University
Conducts
astronomical research at the VERITAS observatory
Experience
John P.
Millis, Ph.D., is a former writer for ThoughtCo, where he contributed articles
on space and astronomy for three years. He has taught physics and astronomy at
the college level since 2001 and is currently the chair of the Department of
Physical Sciences and Engineering at Anderson University in Indiana. He teaches
a wide variety of courses while maintaining an active research program in high
energy astrophysics.
Dr.
Millis's research focuses on pulsars, pulsar wind nebulae, and supernova
remnants. Using the VERITAS gamma-ray observatory in southern Arizona, he
studies the very high energy radiation from these dynamic sources to extract
information about their formation and emission mechanisms. In 2010, he
co-founded a small consulting business, Aurum Consulting, LLC, assisting with
biological testing, chemical formulations, and product development.
Education
Dr.
Millis received a Ph.D. in physics and astronomy and a B.S. in physics with a
mathematics minor from Purdue University.
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