Bike Trails and Compasses

By Bob Gariano

One of the pleasures of Chicago’s North Shore communities is our network of bike and running trails. The 18 mile Green Bay bike trail connects Wilmette with Lake Bluff. At Lake Bluff the rider can turn west onto the 16 mile North Shore trail. This trail is the longest east west route in suburban Chicago and runs parallel to Route 176 all the way to the Des Plaines River trail. Alternatively, a rider can continue north bound out of Lake Bluff onto the Robert McClory trail and then onto the Kenosha County trail as it crosses the state border into Wisconsin.

Experienced riders often carry a compass as routes can be confusing and it is easy to get lost. This reminds the rider of a question encountered years ago on a Northwestern University physics examination. Two astronauts get lost on the moon when they wander out of sight of their landing module. One remembers he brought his Boy Scout compass and begins using it to plot a return before their air supply runs out. The two wander aimlessly for sometime and are finally saved when they catch sight of their space craft. What went wrong?

It is a trick question. Compasses are useless on the moon. Unlike our earth, the moon generates no magnetic field and so there is no magnetic north pole to attract the compass needle. Actually, the north pole of the compass needle is attracted to the earth’s south magnetic pole, but that is another story. Back to our exam question, none of the other planets in our solar system generates a magnetic field like we have here on earth, so why the apparent anomaly?

Geologists have determined that the earth is about 5½ times as heavy as water and this in spite of the fact that rock and soil is only about 2 to 3 times as heavy as water. Our planet must be made up of something heavier than rocks. Scientists are now convinced that the center of our planet is largely made up of iron. If one were to drill towards the center of the earth, after going through a relatively thin crust, the drill would travel about one third of the way to the center through the rocks of the mantle.

Then the drill would enter a denser layer of molten iron, heated by both residual heat when our planet was formed and from continuing radioactive decay within the core. This molten layer was discovered by scientists in 1936 as they were studying seismic waves from earthquakes. Finally, the drill would transition into yet another layer. About one third of the way from the earth’s center the drill would hit the planet’s iron inner core, kept solid by the enormous pressures at that level.

As the outer core of molten iron circulates around the solid iron inner core, a magnetic field is generated just like what happens in an electric motor. The earth generates this magnetic field as these two electrically conductive regions of iron pass each other. Even more, as the molten iron swirls around and changes direction, the north magnetic pole changes position. This has been shown to have happened repeatedly in prehistoric times. Compared to the cold, rock like moon, the earth is a seething cauldron of molten metal with shifting properties and uncertain orientations.

The bicycle rider enjoying an afternoon ride on the quiet trails of Chicago’s North Shore is not aware of these hot turbulent changes going on thousands of miles beneath the pavement. But the dynamics of our planet make a difference. Naturalists theorize that magnetic sensitivity helps guide migratory birds on their yearly trips. The little compass needle on the bicycle handle bar that points the way home is also indicating a magnetic field produced from gigantic and cataclysmic movements deep inside the planet.