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Mark 3 Sextant A simple plastic sextant from Davis Instruments based on the original US Navy Lifeboat sextant. It has no telescope, no horizon glass, and no micrometer drum. The sextant altitude is read from the arc using a vernier scale. The scale can be read to a precision of 2'. This is a dependable and fairly rugged instrument that can guide a vessel across the ocean. For coastal piloting, the Mark 3 is superior to metal sextants costing $800 to several thousand dollars. We have a book on the use of the instrument called GPS Backup with a Mark 3 Sextant. Sextant Piloting in covered in depth in our book How to Use Plastic Sextants.

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