| The Tropic of cancer is a parallel that circles around
the Earth at a latitude of about 23°22'N. What makes
this latitude special is that is the northernmost point on
the globe where the Sun can stand directly overhead; this
happens at the summer solstice, which is in June at the
Northern Hemisphere. Hence, it marks the north boundary of
the tropics.
On a globe, this tropic cuts through Mexico, passes under
Florida, the Atlantic Ocean, cuts the Sahara almost in
half, passes through the Middle East, India, goes through
the south of China, and passes
over the Pacific Ocean through Hawaii to arrive again in
Mexico. If you were to make this tour, you'd encounter high
temperatures, lush vegetation, desert, and, of course,
oceans.
Okay, this is what most high school kids know of the Tropic
of Cancer. But did you know it moves? Creeping around
on the surface of the Earth, like a slug, only
invisible and larger? You see, the point where the Sun is
directly overhead of course depends on the tilt of the
Earth's axis. This tilt varies from day to day, and means
the location of the tropics moves by some meters per day,
with all sorts of complicated patterns. This is caused by
nutation, and is principally caused by the wobbling of the
moon. There is also a long term tendency for the tilt to
vary between 21.5° and 24.5°. This change
takes place over a period of 41000 years, and currently, the
average range of change is 0.5" per year. This corresponds
to an average movement of this tropic by 15 meters per
year, reducing the size of the Tropics by 1100 square
kilometers per year, if the like movement of the Tropic of
Capricorn is also taken into account.
To make matters worse, not only is the location not known,
the name is also dead wrong. You might expect that the
Tropic of Cancer is named after the constellation of
Cancer, and you would be right. When it was named, the Sun
was in the constellation of Cancer at the summer solstice.
Nowadays, it is in the constellation of Taurus, due to
precession of the Earth's axis.
In short, the Tropic of Cancer represents the north
boundary of the Tropics. Its location is not fixed, but is
around 23°22'. See also: Tropic of Capricorn.
Sources:
- http://en.wikipedia.org/wiki/Tropic_of_Cancer
- http://www.crystalinks.com/precession.html
- http://www.starlink.rl.ac.uk/star/docs/sun67.htx/node206.html
- http://www.pietro.org/Astro_Util_StaticDemo/MethodsNutationVisualized.htm
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