BYC CCYC Sextant Training

What is a sextant? An instrument for measuring angular distances used especially in navigation to observe altitudes of celestial bodies.

Olden Days

Objectives This course is designed to teach the basic sextant operation with emphasis on: Basic sextant operation and measurement Finding latitude using LAN Sun measurements Making corrections Local Apparent Noon Worksheets Intro to the Pole Star (maybe)

Teaching Method This course will be taught using: Lecture Video Demo/Practical Labs

Video 1 Getting Started

Basic Sextant Parts: Handle Telescope Shades Mirrors (Index and horizon) Slide Micrometer Drum

Basic Sextant Reading a Sextant: One degree of arc = 1 nautical mile 60’ 00.0” 30’ 30.0” 15’ 25.4”

Basic Sextant Taking a measurement: 1.Set to zero degrees 2.Set shades as needed, look at sun to check 3.Move drum 30 minutes to separate sun in mirror 4.Bring bottom of sun to horizon 5.Swing sextant in an arc to get best measurement

Questions

Video 2 The Noon Sight for Latitude

Noon Sight Geographic Position: An object at its 90’ position relative to earth – directly above! Zenith Distance: distance between your position and an objects geographic position. (degrees X 60) Declination: Latitude spot beneath an object

Vernal Equinox Can only be read between the two tropics Tropic of Cancer (North) Tropic of Capricorn (South)

Noon Sight Sun below equator Latitude Zenith DistanceDeclination =-

1.Shoot sun at 1800 UTC: 2.Sextant reading: Compute Zenith Distance (zd): 89’ 60.0” - sextant reading = Find declination (dec) from almanac: S Lat = zd – dec Lat = Noon Sight Sun below equator

Noon Sight Sun in lower latitude above equator LatitudeZenith DistanceDeclination=+

1.Shoot sun at 1530 UTC: 2.Sextant reading: Compute Zenith Distance (zd): 89’ 60.0” - sextant reading = Find declination (dec) from almanac: N Lat = zd + dec Lat = Noon Sight Sun in lower latitude above equator

Noon Sight Sun in upper latitude above equator Latitude Zenith DistanceDeclination =-

1.Shoot sun at 1618 UTC: 2.Sextant reading: Compute Zenith Distance (zd): 89’ 60.0” - sextant reading = Find declination (dec) from almanac: N Lat = dec - zd Lat = Noon Sight Sun in upper latitude above equator

Questions

Video 3 Precision

Index Error Sextant correction for built in error of the sextant 1.Find the horizon and make level in both mirrors 2.Read the micrometer drum 3.If the error is above zero subtract the reading 4.If the error is below the zero add the reading Makes sense if you understand you are trying to get back to zero!

Dip (Height of eye) Correction to adjust for height above sea level 1.Table in front page of the almanac (moon back) 2.Look up height of eye in ft or meters and subtract

Main Correction Correction to adjust to the center of the object 1.Table in the first page of the almanac 2.Look up month 3.Lower or upper limb 4.Use you apparent altitude - Apparent Altitude is the number after you have made your sextant sight and adjusted for index error and Dip.

Exact Declination Correction to refine declination 1.Find hourly declination on chart Actual reading Hourly declination – ex: Find the.d number bottom of page ex: d=.9 4. Look at the increments and corrections pages for 50 minutes 5. Fifth column is d corrections.9 (left)=.8 (right) 6. Look to see if declination is increasing or decreasing – make adjustment with.8

Local Apparent Noon Highest point of the sun Median between Sunrise and Sunset

Questions! Rick Bell

Video 3 The Pole Star