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Title: Marine Sextants: History & Technology (Coastal Navigation) Grade(s): 6-8 Introduction: Today’s low cost metal sextants offer high accuracy and ease of use. Plastic models are perfect for lifeboat provisioning. Older sextants tend to have smaller mirrors and scopes which make them harder to use. For all practical purposes, metal sextants are error free when compared to the many uncontrollable errors which may exist from such things as refraction, oblateness of the earth, and data tabulation. Generally, a minute of arc (one mile) is about the best anyone can hope to achieve. Plastic sextants commonly exhibit errors in excess of 5 minutes, even when great care is exercised. The size of the mirrors on a sextant generally vary directly with the quality of the instrument. Large index and horizon mirrors are desirable. Sextants are available with their major metal parts of either aluminum or brass. The alloys of each metal are both suitable for use at sea. A 3.5 (or 4) x 40 scope is a good choice for stars. The specified power magnification helps you find and maintain stars in view in both calm or pitching seaways. Many sextants have an option of either the traditional (half-silvered) horizon mirror or what is called a “whole horizon mirror”. With the traditional mirror, the horizon glass is divided vertically into two halves producing a “split image”. The half nearest the frame is a silvered mirror and the other half is clear glass. A later development in sextant technology is the whole horizon mirror. This superimposes both the horizon and the celestial body on the entire mirror with no split image. Sextant lighting is the least needed feature on a sextant, since a flashlight should normally be available in any event for recording observations. Learner Objective(s):
Florida Sunshine State Standards: Science: SC.H.1.3.5 Math: MA.B.3.3.1 Competency Based Curriculum: Science: M/J-I-3.A Math: M/J-I-III-5-A; Math: M/J-3-II-13-C Materials:
Activity Procedure(s):
Student Assessment: Allow the student to answer critical thinking skills questions assigned by the teacher.
Student participation is assessed during group activity. Activity Extension(s): Before the sextant was invented, navigators such as Columbus, Magellan, and Drake used an instrument known as the cross-staff. How did they use this instrument for their travels? (Social Studies) Latitude and longitude are systems of coordinates on the surface of a sphere. Rene Descartes, a French scientist and philosopher, devised a systematic way of labeling each point on a flat plane by a pair of numbers. How did the so- called “Cartesian coordinates” facilitate navigators in locating destinations? (Math) Read the following literary piece, Sea Fever, by John Masefield and react to the author’s message (Language Arts):
Is there any association to the use of a sextant in the piece? Home Learning Activity: Explain how the sun and the stars aided the use of the sextant. Vocabulary: sextants, refraction, oblateness, tabulation, horizon References/Related Links:
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