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MCA Celestial Navigation Exam prep
Article 2
Calculating the time of Twilight and Sunrise / Sunset
Looking into the ‘Daily Pages’ of the Nautical Almanac, you will note on the right hand side, the times of Nautical, Civil twilight as well as Sunrise and Sunset (Sunrise at the top of the page, Sunset at the bottom of the page). Times are given for a variety of Latitudes (North and South).
The daily pages give 3 consecutive dates, but the times for twilight etc. are for the ‘central’ day. If you are using the middle day, you can simply pull the times from the table, but if you are using either the day at the top or the bottom of the page, you will need to interpolate between the 2 central days.
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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.