BYC CCYCSextant 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 MethodThis course will be taught using:

• Lecture

• Video

• Demo/Practical Labs

Video 1Getting Started

https://www.youtube.com/watch?v=DrAkrgZRb9Y

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 2The Noon Sight for Latitude

https://www.youtube.com/watch?v=BWGOlpj4YwE

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 SightSun below equator

Latitude Zenith Distance Declination= -

1. Shoot sun at 1800 UTC:

2. Sextant reading: 49 15.0

3. Compute Zenith Distance (zd):

89’ 60.0” - sextant reading = 40 45.0

4. Find declination (dec) from almanac: S 17 16.0

Lat = zd – dec

Lat = 23 29.0

Noon SightSun below equator

Noon SightSun in lower latitude above equator

Latitude Zenith Distance Declination= +

1. Shoot sun at 1530 UTC:

2. Sextant reading: 53 24.7

3. Compute Zenith Distance (zd):

89’ 60.0” - sextant reading = 36 35.3

4. Find declination (dec) from almanac: N 08 09.7

Lat = zd + dec

Lat = 44 45.0

Noon Sight Sun in lower latitude above equator

Noon SightSun in upper latitude above equator

Latitude Zenith Distance Declination= -

1. Shoot sun at 1618 UTC:

2. Sextant reading: 86 53.3

3. Compute Zenith Distance (zd):

89’ 60.0” - sextant reading = 03 06.7

4. Find declination (dec) from almanac: N 17 08.7

Lat = dec - zd

Lat = 14 02.0

Noon Sight Sun in upper latitude above equator

Questions

Video 3Precision

https://www.youtube.com/watch?v=uNWTN2LQ-TU

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 1150

2. Hourly declination – ex: 8 28.7

3. 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 Bell361 815 3111

richardebell@hotmail.com

Video 3The Pole Star

https://www.youtube.com/watch?v=sa_2qF1iCqk