Learning Goals
During the Revolutionary War at sea, sailors had to process a firm command of the science, technology, engineering and math skills and understandings necessary to successfully navigate the Atlantic Ocean. This lesson reinforces how timeless the possession of practical knowledge relative to geography, cardinal direction, latitude and longitudinal measurement and the measurement and identification of celestial beacons is to the art of navigation.
Navigation at Sea and the North Star
Eighteenth century sailors relied on their navigational skills to guide them to their destinations and keep them from getting lost or entering known areas where hidden reefs or other hazards could ground or sink their wooden vessels. Onshore landmarks were helpful in determining a ship’s location while sailing along the coast; navigators used a tool called a spyglass to spot islands, mountains and rivers in the distance (and also to identify neighboring ships as allies or enemies and read signal flags). On the open ocean however, where there are no discernable landmarks, early nav gators needed to know their latitude and longitude to tell precisely where they were on earth. To stay on course, they also needed to know their cardinal direction (north, east, south or west). To get their bearings relative to both direction and global positioning, the seafarers looked to the sky—specifically to the North Star.
To determine direction and course, sailors could use a compass, which points in a northerly direction called “magnetic north” from almost anywhere on earth, but compasses at sea easily become demagnetized and are therefore unreliable. When visible, the North Star is the most reliable reference point for determining direction. While all other stars appear to travel in circular arcs across the night sky (because of the Earth’s rotation on its axis), the North Star remains in a fixed position directly above the Earth’s axis at the North Pole, representing “true north.”
To locate the North Star, which is also called Polaris and is typically one of the brightest stars in the night sky, a sailor would first find the Big Dipper. Polaris is located in a straight line across from the two stars making up the spoon of the Big Dipper. Polaris is also the last star on the handle of the Little Dipper.
In the eighteenth century, after a ship’s navigator located the North Star, an instrument called a sextant was used to measure the angle of the horizon to determine the ship’s global location. The word sextant is derived from the Latin term for “one sixth,” because a sextant is equipped with an arc which is usually one sixth of a circle, or sixty degrees. Invented in 1731, the sextant has a movable radial arm with a fixed index mirror which pivots at the center of the arc. The other end of the sextant’s arm is extended to the rounded scale. The sextant features a telescope which a navigator looks through to “sight” the ship’s horizon. The navigator then moves the index mirror using the radial arm to make the North Star (or any star including the sun) appear exactly on the horizon. Traditional sextants (like the one in our traveling trunk) have a half-horizon mirror mounted to the telescope, which divides the field of view in two. On one side is a view of the horizon, and on the other, a view of the celestial object. Most sextants also have filters which can be used singly or in combination to reduce haze and the sun’s brightness. This process of locating the horizon and the navigation star, called “sighting” or “taking a sight,” and results in an angular measurement along the sextant’s rounded scale. A vessel’s latitude simply equals the angle measured.
Sextants can be useful for other measurements as well. Sighting the height of a landmark with a sextant can give a measure of distance off, and, held horizontally, a sextant can measure angles between objects for a position on a chart.
Determining longitude was trickier in the eighteenth century in that it required a precise clock which could keep time reliably at sea given the motion of a ship. It was not until the early nineteenth century that most ships were outfitted with marine chronometers which could provide longitudinal accuracy. Using a sextant to measure the lunar distance between the moon and another celestial object could roughly estimate Greenwich Mean Time, which provided relative longitude.
Today, sextants and other instruments used to determine a ship’s global position at sea during the eighteenth century remain eminently practical back-up navigation tools as they are not dependent upon electricity or reliant upon satellite signaling.
Materials
- protractors
- rulers or straws to use as sights
- string (cut to approximately six inches in length)
- heavy tape (postal packing tape or duct tape)
- simple metal nuts or washers to use as weights
Making Classroom Sextants
- Distribute materials (protractors, rulers/straws, string segments, heavy tape and weights) to students working as individuals or pairs.
- Tape a ruler or straw to the protractor so that it lies along both the 90-degree and center mark of the protractor, bisecting the half-circle.
- Tape a six-inch piece of string to the center of the protractor (it is best to tie it through the center hole if the protractor has one), making sure the string can swing freely.
- Tie a weight to the end of the string.
- The string should hang along the zero-degree mark of the protractor when the ruler or straw is held parallel to the ground.
Lesson, Procedure and Assessment
- Read and discuss the “Navigation at Sea” part of this lesson.
- Examine the sextant and compass included with the traveling trunk.
- Make primitive sextants to use in measuring distances within the classroom using the “Making Classroom Sextants” part of this lesson.
- Place a “North Star” in the classroom, and have students determine the relative latitude of their desks by positioning their eye, along the with their quadrant level (ruler or string), with the edge of their desk.
- Allow the sextant weight to swing freely until it stops with the string pointing straight down.
- Assign a partner to read the measurement of a student’s “sight” to determine the angle (or “latitude”) of their desk in relation to the classroom Polaris.
- As an extension activity, ask students to predict how the measurements change the closer or further one’s desk is to “Polaris.” As a class, plot and discuss the differences in students’ desk “latitudes.”
Additional Links
Institute Of Navigation (ION). “Navigation Education Materials.”