Category: Navigation

The Sextant and its Corrections


The Sextant is a very accurate and technical device for doing something very simple - measuring angles. You could use a protractor and a piece of string, though accuracy would not be very good!


The Sextant is designed to measure angles very precisely, to an accuracy of Degrees, Minutes and 10th’s of minutes (we call 1/10th of a Nautical Mile a Cable). We normally use the Sextant to measure the angle between the horizon and a vertical object.


In Coastal Navigation we could measure the angle between our horizon and the top of a lighthouse. This can tell us how far we are from the Lighthouse. For those interested, it’s the Tangent Rule, though you don’t need to know this as a table called ‘Vertical Sextant Angles’ is readilly available in many marine publications such as ‘Reeds’ Nautical Almanac.

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