Visualizing Change In Axis Tilt
Name: Loli. A
Date: December 10, 2002
How do you visualize or show the seasons if the earth's
tilt changed -say 90degrees? I am trying to visualize this myself and
am stumped - maybe because it seems catastrophic!
If the tilt of the earth were to change from its current 23¢ to 90¢ would
be catastrophic to all life on earth. This question has been asked and
answered on another board:
How does the earth's tilt affect the changing of the seasons, and what
different angles cause those different seasons?
The bottom line for the changes from season to season is the average
daytime temperature. This depends on the amount of heating that the earth
receives in a single day throughout the year, and this depends on how many
hours the sun is above the horizon and exactly how long it spends at its
highest elevation above the horizon. For every square meter on the surface
of the earth, it will be heated by the sun at a rate that depends on the
'cosine' of the angle of the sun above the horizon. The higher the sun
gets, the less slanted the rays of light are that intercept each square
meter, and so the efficiency with which these slanted rays can deliver
energy to the surface gets better and better the higher up the sun gets.
When you add up during the daylight hours just how much heating this
surface gets, it receives most of its heating from those times during the
day when the sun is the highest above the horizon. For a tilted earth,
there will be some days during the year at a given latitude, where this
heating rate is the highest and we call this summer. There will be other
days when the sun never gets very high above the horizon and so its
heating ability is very low, and we call this winter. The details of just
how hot and cold we get, and the exact dates, depend also on whether we
are near water, or in the interior of a continent.
So, seasonal changes depend on the tilt of the earth's axis because they
lead to changes in the amount of heat delivered to a square meter of
surface, and the fact that there are a changing number of hours in the day
when the sun is above the horizon and high enough up that it can
efficiently heat the surface over the course of a typical day. (from Ask a
Space Scientist )
There is an activity on our StarChild website that illustrates how the
earth's tilt affects the change in the seasons. Take a look at
Jim Lochner and Maggie Masetti
for Ask a High-Energy Astronomer
Questions on this topic are no longer responded to by the "Ask a
High-Energy Astronomer" service. See
imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html for help on other
astronomy Q&A services.
The easiest way to visualize is to picture (or use a model of) the geometry
of sun and earth, and pick a point on earth for different orientations of
the axis and locations of the earth in its orbit around the sun.
In the extreme case, imagine the case where the earth's axis was in the
same plane as the earth's orbit. Once a year, the north pole would
directly face the sun, and 6 months later, the south pole would directly
face the sun. At those times, the sun would stay almost exactly in the
same spot for the whole day for someone at the pole, and people in the>
northern or southern hemisphere would see the sun travel in a circle at the
same height over the horizon.
The effect on season would be most extreme at the poles- when facing the
sun, that pole of earth would get direct sun continuously, about equal to
being on the equator, but having no nighttime to cool off. When facing
away from the sun, it would be similar to winters in the arctic or
antarctic circle, a continuous night.
It would be catastrophic for life as we know it. But not to fear, the type
of event that could change the axis that severely would be so catastrophic
that the change in seasons would be the least of our concerns. If the earth
had always had this different axis tilt, life on earth would be very
different, depending on where you lived.
Catastrophic would be the word for it. It is hard to guess what would
develop if the earth were to spin directly on its side. One of the other
planets - Uranus - does move like this. Because its year is 84 earth years
long, a point on Uranus has about 42 years of daylight and 42 years of
night. For the earth that would be about 180 of our present days for one
"on-the-side" day. It would likely mean the end of life on earth as we know
The earth is actually changing its inclination to the sun, but it is very
slight. We say it is 23.5 degrees and that is pretty close. There was a
theory about 50 years ago that the earth would tip periodically and suddenly
to assume a new axis of rotation... but the new axis would be inclined
roughly 90 degrees. I have heard nothing of this theory for many years.
That is likely for failing under scientific scrutiny.
Well, it WOULD make the weather quite unpleasant. Just as now, the poles
would be in light for half the year and darkness for the other half.
However, the intensity of the sunlight would vary much more dramatically
than it does now. At the solstice, the pole near the sun would experience
the direct overhead rays of the sun, as the tropics do now.
The rest of the planet would be both tropical and arctic; tropical because
the sun would appear to the north and to the south at different times of the
year, and arctic because the extreme periods of the year would involve
continuous sunlight and continuous darkness, respectively. At two times
during the year, the sun would be directly overhead. So the temperature
differences between summer and winter would be dramatic. Every place would
receive intense sunlight when the sun is overhead.
The most consistent location to be would be at the equator. Things would be
odd even here. Only here would there be both day and night every day of the
year. Day and night would be of equal length throughout the year, as it is
now. At the equinoxes, the sun would be directly overhead. At the two
solstices, the sun would be just at the horizon all day long, both day and
The best way to visualize this is with a globe. You'll have to take it out
of its stand, because globe stands typically are fixed at the natural angle
of the earth's tilt. Place the globe so that its polar axis is horizontal
to the ground, and place a light source at the same height as the globe.
The light shining on the globe will then model the sunlight illuminating the
earth. As the earth rotates about its polar axis and revolves around the
sun, you can see how the day/night pattern in various locations depends on
the time of year.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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