Mariner's Compass & Periscopes: Definition & Use

Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Among the many devices a mariner uses, there is a compass. If the vessel a mariner is on is a submarine, he might use a periscope too. This lesson will explain what these devices are, and how they are used.

Mariner's Compass and Periscope

You are bored because the power is out, and your cellphone's battery is drained to 1%. Desperate for something to do, you go to the bookshelf, and recognize your last name on one of the books. It is titled, My Days Hunting Nazi Submarines. You pull it off the shelf, blow the dust off of the cover, and open it. The author is your great grandfather, the captain of the USS Javelin 1 during World War II. You open the book and begin reading:

''The sonar located the Nazi U-boat to be 100 yards away, on the surface. It was bearing 50o. I pulled out my compass, and gave directions on how to align our boat so we could fire upon the U-boat that tried to sink us last month. When we were in position, I raised the periscope. My gut told me it would be okay to do this because it was at night, and the U-boat showed no indication that its crew knew we were so close, and ready to unleash our torpedoes. The view through the periscope showed we were in prime position to attack.''

This sounds interesting, but you want to understand more about the compass, and periscope. What are they, and how are they used?

The Compass

A compass is a device that detects Earth's magnetic field. Global positioning systems (GPS) are the most advanced way to determine your position on Earth, and which way you are heading, which is called the heading or bearing. GPS utilizes satellites in orbit around Earth. Before GPS, the only way to have any inkling of which way you are sailing when at sea was to use star constellations or a compass. Submariners only had the compass when they were underwater.

An antique compass

It is a simple device that has a few parts:

  • A round case holding water or air
  • A magnetic needle with its north end marked with paint mounted on a frictionless pin so it can spin freely
  • An angular grid ranging from 000o to 360o indicating direction

A navigator would be part of your great grandfather's submarine crew. He could hold the compass in line with the front of the boat. The marked end of the arrow always points to the north magnetic pole so he could determine the boat's exact heading, or direction of motion relative to North. Using the heading, the speed of the boat, the time of travel, and charts, a navigator could track the course of the vessel.

Charting a course

The Periscope

The advantage of a submarine is stealth. They can travel for extended periods of time underwater to avoid being seen by anything above the water's surface. A periscope is a device used on submarines to view above the water's surface when submerged.

A periscope is a more complicated instrument than a compass. The most basic periscope contains:

  • Two flat mirrors set at 45o and parallel to each other; one at the top and one at the bottom
  • A long tube in which the mirrors are placed

Periscope setup

When the captain of a submarine wants to see above the surface while remaining underwater, he would push up the periscope, and he could turn it around 360o. Light would enter the opening, reflect straight down towards the second mirror striking it at 45o , and reflect into the eyes of the observer inside the conning tower.

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