Physics and Astronomy
Earth's Axis


A few days ago I was playing videogames with my cousin at home and our game was
suddenly interrupted. Sunlight, entering through a window situated on the south side
of our home, cast a bright glare on the TV screen.

We had just recently moved the TV to this new location and I was distressed at the
possibility of this becoming a daily occurrence. I realized that it was possible for the
situation to grow increasingly worse as the season progressed and the path of the
Sun changed. I wanted to know how my video game playing was going to be affected
so I took a moment to think.

If you look up in the sky and are able to see the sun directly overhead, then you are
seeing the sun at its brightest. Areas near the equator experience overhead sunlight
or near overhead sunlight, around noon, throughout the year. This is why countries
that are near to the equator are very hot. They are not significantly hotter, however,
just because they are closer to the sun. They are hotter because the sunlight that is
warming them has to travel through the least amount of atmosphere to reach them
and thus is much stronger. This is the first factor necessary in understanding how the
sun would move in relation to the window in my home.

The earth completes one revolution around the sun once every 365.25 days, it also
rotates about its axis once every 24 hours. Both of these changes are instrumental in
but not the cause of seasonal variation. The changes in sun exposure and heat that
we notice on a seasonal basis, like the difference between summer and winter, are
due to the tilt of the earth’s rotational axis with respect to its orbit.

The axis on which the Earth rotates is actually tilted 23.4 degrees away from the
direction perpendicular to the earth’s orbital plane. The Earth maintains the
orientation of this tilt, with respect to the stars, throughout the year.

During the winter time the areas in the northern hemisphere, the top half of the
globe, are tilted away from the sun.  So, as the days go by and the autumn turns into
winter, the southern hemisphere gets more sun and the Northern Hemisphere gets
less. Yet when the Earth is near the other side of its orbit, in the summer time, the
northern hemisphere is tilted towards the sun and experiences more sun exposure.
The seasons are always reversed in the opposite hemisphere and the further from
the equator you live the more pronounced the seasonal effects are.

This is why people who live in areas in the southern hemisphere, like Australia, South
Africa and South America celebrate the winter holidays with long warm days instead
of short cold days.

In Southern California the sun is lower in the sky, further south, than it would be for
an area closer to the equator.  Also in Southern California the sun is especially low in
the winter (again this is because of the 23.4 degree tilt of the Earth’s axis). The sun is
even lower in the skies for people living in Northern California. Going even further
north, at the North Pole the sun is at its lowest, very near to the horizon (at the south
pole the sun is always close to the horizon as well, the only difference is that the sun
sits further “north”). In fact the North Pole goes from early October to early March in
complete darkness. The second half of the year though the sun never sets.   

Considering many of these factors in my determination of the sun’s future path, I
assumed that if in the coming winter my location moves north with respect to the sun
than the sun will move south in my daytime skies.  Because of this I reasoned that the
sun should enter through our southernmost window for increasingly prolonged
periods of time as the winter season progresses. This was bad news for me because
it meant that the glare on the TV would get continually worse.

Maybe I should be making time for reading instead of playing video games anyway.


Continually: adj.
Reoccurring frequently with breaks and irregularity

Continuously: adj.
Not interrupted, steady, without break or irregularity

Hemisphere: noun
1)  A half of a symmetrical, approximately spherical object as divided by a plane of
symmetry.
2)  Either the northern or southern half of the earth as divided by the equator or the
eastern or western half as divided by a meridian.

Orbital Plane: noun
(astronomy) the plane on which a body is orbiting
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