tropic of cancer

Tropic of Cancer, parallel of latitude at 23°30' north of the equator; it is the northern boundary of the tropics. This parallel marks the farthest point north at which the sun can be seen directly overhead at noon; north of the parallel the sun appears less than 90° from the southern horizon at any day of the year. The sun reaches its vertical position over the Tropic of Cancer at about June 22, the summer solstice in the Northern Hemisphere. When the Tropic of Cancer was named, the sun was in the constellation Cancer at the time of the summer solstice.

Where is it?

All of us who have looked at a map have noticed a little dotted line called the Tropic of Cancer. In fact, here in Mazatlan, this little dotted is less than 15 miles north of us. But what is it, and what if anything does it have to do with Cancer? The answer is both simple and complicated. The simple answer is that the Tropic of Cancer is 23.5 degrees north latitude. So what? The real answer is a bit more complicated, and very interesting.

Some History Back in the old days, before television, cellular phones, and the internet, people who had a lot of free time on their hands used to look up at the stars and the sky. They noticed a bunch of remarkable things. One of the things they noticed was that the sun and the stars did not appear in the same place at the same time every day. In the northern hemisphere, from winter until summer, the sun gradually rose higher and higher in the sky each day, while from the summer until winter, the opposite happened. By carefully measuring how high the sun rose at mid day, they discovered that around June 21 (in our current calendar, but that is another story) the sun got as high as it was ever going to get, and that around December 21 the sun stayer lower in the sky than on any other day. To make matters worse, how high and how low the sun gets depends on where you live. For the folks living near the equator, on June 21 the sun was up in the northern sky, their day was long and hot, but things cooled off at night, while for the folks living up near the north pole, the sun was to the south and moved in a small circle. In fact they were in the middle of 6 months of constant daylight. Something funny is clearly going on.