OCEANOGRAPHY • CONSTRUCTION & USE OF A SIMPLE SEXTANT

Title: Construction and Use of a Simple Sextant for Determining Position

(Historical Perspectives)

Grade Level(s): 6-8

Introduction: Today, with the advent of GPS (Global Positioning Systems), navigators and sailors can determine their position on anywhere on the earth within moments. However, before the invention of the GPS, LORAN, and other radio signaling devices, sailors and navigators used the celestial sky for determining position. For many years the sextant was the instrument of choice, where position was determined by measuring the altitudes of celestial bodies. Experienced navigators could determine the position using a sextant in less than ten minutes.

A simple sextant can easily be constructed and a latitude calculation can easily be determined using the North Star, or Polaris, and the celestial body of reference. Longitude cannot be determined using the simple sextant.

Learner Objectives:

• The student will be able to construct and use a simple sextant to determine latitude.
  
  
• The student will be able to understand the use of stars and their positions in the celestial sky for navigation purposes.

Florida Sunshine State Standards: Science: Sci.E.2.3.0 Math: MA.B.3.3.1

Competency-Based Curriculum: Science: M/JO-IA2 Math: M/J-I-III-5-A, M/J-3-II-13-C

Materials:

Protractors - one for each student (a compass rose can also be used)
Ice cream sticks (or pencils) - 3 per student
Rubber bands or string

Activity Procedures:

1. Give each student 3 ice cream sticks and rubber bands or string.
   
   
2. Instruct the students to attach the three sticks into a triangle, using the rubber bands or string to fasten the sticks together.
   
   
3. Each student will then hold the triangle to their eye. They will sight the horizon with one stick, and the other will be aimed at Polaris.
   
   
4. Move the upper stick of the triangle so that the student can sight Polaris along one stick, and without moving their heads, also sight the horizon.
   
   
5. The student will then measure the angle created by the sticks sighting the horizon, and Polaris, using the protractor or compass rose.
   
   
6. The resulting angle will be the latitude (in degrees) of where the students are performing the exercise

Student Assessment:

Allow students to answer critical thinking skills questions assigned by the teacher.

1. Using a GPS, determine a position assigned by the teacher. Then, compass the latitude reading from the GPS to the latitude reading calculated using the simple sextant that was constructed. How do the two latitudes compare? How close were the estimations using the simple sextant?
   
   
2. Explain different sources of error that may have caused the latitude reading determined using the simple sextant to be different from the latitude obtained from the GPS?
   
   
3. How can the model of the simple sextant be modified to be more effective?

Activity Extensions:

Before the advent of GPS, Loran, and the sextant, ancient mariners such as Magellan, Drake and Columbus used a cross-staff to determine position during their voyages. Discuss how accurate their determination of position was using the rudimentary instruments that they used. Furthermore, discuss how their original voyages across the oceans could have been more effective if they had such technology and advances in science during their era. (Social Studies, Technology)

Home Learning Activity:

Research the web, or other sources for instructions on how to construct other simple instruments to determine position.

Vocabulary: GPS, celestial bodies, sextant, latitude, longitude

References/Related Links:

Callahan, Steven (1986) Adrift: seventy-six days lost at sea. Boston: Houghton Mifflin Company.

www.northernlight.com
http://celestaire.com
http://home.earthlink.net/~nbrass1/cardart.htm
http://trimble.com/gps/index.html

Conversion Table for FCAT Questions: