Re: False astronomical correlation

In article <1188795564.357656.194560@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Quadibloc <jsavard@xxxxxxxxx> wrote:

The Arctic region, just like *every* other part of the Earth, is in
sunlight exactly half the time over the course of a year.

:-) ....no!

If we assume that all of the Earth is completely cloud free at all
times, then e.g. the North Pole will receive sunlight during a longer
time than the South Pole. Why? Because the Earth is at aphelion
during the northern hemisphere summer, and in that part of its orbit
the Earth moves a little bit slower. But during the southern
hemisphere summer, the Earth is at perihelion, and in that part of its
orbit the Earth moves a little bit faster.

Due to the refraction in the Earth's atmosphere, most of the Earth is
in sunlight a little bit more than half the time over the course of
the year. Only parts of Antarctica (that parth which is southward of
approximately 75 deg S latitude) will be in sunlight slightly less
than half the time.

The South Pole is in sunlight 182 days per year, while the North Pole
is in sunlight 189.5 days per year. But the part of the Earth which
receives sunlight for the longest time is the northern arctic circle,
which is in sunlight 191.5 days per year if we sum it all up. The
southern arctic circle receives sunlight 186.5 days per year.

At northern midlatitudes, the Earth is in sunlight some 185 days per
year. At the equator this has dropped to 183.5 days per year, and at
southern midlatitudes to 183 days per year.

So the northern arctic circle will receive sunlight almost 10 days
longer than the South Pole, over the course of a year. That's
a 2.6% difference in the time of being in sunlight, and it's
quite a large discrepancy from your claim:

# The Arctic region, just like *every* other part of the Earth, is
# in sunlight exactly half the time over the course of a year.


Be careful when using the word "exact"! Even of the Earth moved in a
perfectly circular orbit, subjected to zero perturbation from other
celestial bodies, and even if the Earth lacked an atmosphere (so that
atmospheric refraction was absent), and even if the Sun was a point
light source (so we wouldn't have to worry about whether sunrise and
sunset should refer to the center of the solar disk or its upper
limb), and even if the rotation of the Earth was perfectly uniform,
every region of the Earth would still not be in sunlight *exactly*
half the time! Every region would then be in sunlight slightly less
than half the time. Why? Because of the solar parallax, which makes
the Sun appear to be almost 9 arc seconds lower in the sky. Near the
equator this would make the day only some 2.5 seconds shorter than the
night - but that difference is enough to remove the exact match. To
make all regions of the Earth be in sunlight exactly half the time we
would also have to move the Sun to an infinite distance -- and if we
did that, we'd get serious problems in making the Earth orbit the Sun,
since the orbital period would then be infinitely long.... in addition
we'd get serious interference from all those stars which are at finite
distances, i.e. much closer than this imaginary point-like sun at
infinite distance....



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e-mail: pausch at stockholm dot bostream dot se
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