What Are Those Things For?

Why we have monuments for the Tropic of Cancer

In case you didn’t know, we have two roadside monuments here in Baja California Sur dedicated to the Tropic of Cancer. The first is a tall white statue that sits along Highway 19, the highway between Cabo and Todos Santos. On the other side of Baja is much larger, more impressive Tropic of Cancer monument, located just outside of Santiago, on Highway 1. That monument has two statues, a plaza and a small chapel.

So, what exactly are these monuments marking? The Tropic of Cancer is one of three of the most significant imaginary lines running across the surface of the Earth, along with the equator and the Tropic of Capricorn. While the equator is the longest line of latitude on the Earth (the line where the Earth is widest in an east-west direction), the tropic lines are both based on the sun's position in relation to the Earth at two points of the year.

All three lines of latitude are significant in their relationship between the Earth and the sun. The equator is located at zero degrees latitude, and runs through Indonesia, Ecuador, northern Brazil, the Democratic Republic of the Congo, and Kenya, among other countries. It is 24,901.55 miles long. On the equator, the sun is directly overhead at noon on the two equinoxes - near March 21 and September 21.

The equator divides the planet into the Northern and Southern Hemispheres. On the equator, the length of day and night are equal every day of the year - day is always twelve hours long and night is always twelve hours long.

The Tropic of Cancer and the Tropic of Capricorn each lie at 23.5 degrees latitude. The Tropic of Cancer is located at 23.5° north of the equator and runs through Mexico, the Bahamas, Egypt, Saudi Arabia, India, and southern China. The Tropic of Capricorn lies at 23.5° South of the equator and runs through Australia, Chile, southern Brazil (Brazil is the only country that passes through both th equator and a tropic), and northern South Africa.

The tropics are the two lines where the sun is directly overhead at noon on the two solstices - near June 21 and December 21. The sun is directly overhead at noon on the Tropic of Cancer on June 21 (the beginning of summer in the Northern Hemisphere and the beginning of winter in the Southern Hemisphere) and the sun is directly overhead at noon on the Tropic of Capricorn on December 21 (the beginning of winter in the Northern Hemisphere and the beginning of summer in the Southern Hemisphere).

The reason for the location of the Tropic of Cancer and the Tropic of Capricorn, at 23.5° north and south, respectively, is due to the axial tilt of the Earth. The Earth is titled 23.5 degrees from the plane of the Earth's revolution around the sun each year. The area bounded by the Tropic of Cancer on the north and Tropic of Capricorn on the south is known as the "tropics." This area does not experience vast seasonal differences because the sun is always high in the sky. Only higher latitudes, north of the Tropic of Cancer and south of the Tropic of Capricorn, experience significant seasonal variation in climate.

Since ancient times, people have tried to come up with reliable systems with which to measure their location on Earth. For centuries, both Greek and Chinese scientists attempted several different methods but a reliable one did not develop until the ancient Greek geographer, astronomer and mathematician, Ptolemy, created a grid system for the Earth. To do this, he divided a circle into 360°. Each degree comprised 60 minutes (60') and each minute comprised 60 seconds (60''). He then applied this method to Earth's surface and located places with degrees, minutes and seconds and published the coordinates in his book Geography which he published about 150 AD.

Although this was the best attempt at defining the location of places on Earth at the time, the precise length of a degree of latitude was unresolved for around 17 centuries. In the middle ages, the system was finally fully developed and implemented with a degree being 69 miles and with coordinates being written in degrees with the symbol °. Minutes and seconds are written with the symbols ', and '', respectively.

Today, latitude is still measured in degrees, minutes and seconds. A degree of latitude is still around 69 miles while a minute is approximately 1.15 miles. A second of latitude is just over 100 feet. San Jose is at 23° 3' 41" N / 109° 42' 29" W. In addition to degrees, minutes and seconds, latitude can also be measured using decimal degrees. San Jose’s location in this format looks like 23.3.410 / 109.42.290 Both formats are correct, although degrees, minutes and seconds is the most common format for latitude. Both however can be converted between each other and allow people to locate places on Earth to within inches.