ManIsNotLost

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Man is not lost : 

Man is not lost Simon Thompson ASC Vice Commodore

Celestial Navigation... : 

Celestial Navigation...

... by Sextant : 

... by Sextant

Man is not lost : 

Man is not lost The Celestial Bodies How do we define where they are? The Sextant How do we use it? Sun Sights Lets do the math! Sun-Run-Sun Other methods

Celestial Bodies : 

Celestial Bodies

Celestial Body Location : 

Celestial Body Location Body on inside surface of Celestial Sphere Stretch String Centre of Earth Centre of Body String exits Earth at a point Geographic Position

Celestial Body Location : 

Celestial Body Location This points Latitude is the body's Declination. This point's Longitude is the body's Greenwich Hour Angle (measured in degrees West of Greenwich)?

The Sextant : 

The Sextant

Parts of a Sextant : 

Parts of a Sextant WARNING Looking Directly at the Sun can damage your sight!

Parts of a Sextant : 

Parts of a Sextant WARNING Looking Directly at the Sun can damage your sight!

Parts of a Sextant : 

Parts of a Sextant Direct Image of Horizon

Parts of a Sextant : 

Parts of a Sextant Reflected Image of Celestial Body

Use a Sextant : 

Use a Sextant Note the exact time of the sight!

Sextant Errors : 

Sextant Errors Mirrors not correctly aligned Measure offset - correct Eye not at Sea height Correct from table Not measuring to centre of Body Correct from table

PRACTICAL TIME : 

PRACTICAL TIME

Calculate Observed Altitude : 

Calculate Observed Altitude Sextant Altitude Add or Subtract Index Error Error of sextant Subtract DIP Correction Eye is not at 0m Add or Subtract SUN Correction Reading from top or bottom – not centre = Observed Altitude

Our Scenario : 

Our Scenario You're out-of-soundings and unable to see land. GPS isn't working, but you can Dead-reckon your position from log and tide. Shortly after lunch you decide to shoot the sun. 14/01/2009 12:25:19 UTC Sextant Alt: 32° 36' (Sun LL)? Sextant Index Error: + 1' Height of eye: 2m DR Longitude: 003° 30' W DR Latitude: 36° 00' N

Calculate Observed Altitude - EXAMPLE : 

Calculate Observed Altitude - EXAMPLE

Doing the Math! : 

Doing the Math!

1) Estimate Position : 

1) Estimate Position Dead-Reckoning. Latitude and Longitude.

1) Estimate Position - EXAMPLE : 

1) Estimate Position - EXAMPLE 36° 00' N 03° 30' W

2) Calculate GMT in degrees : 

2) Calculate GMT in degrees Time of Sight in UTC From Watch Correction for Watch gaining/losing time? Use table to convert to degrees and minutes

2) Calculate GMT in degrees - EXAMPLE : 

2) Calculate GMT in degrees - EXAMPLE

3) Calculate GHA of Sun : 

3) Calculate GHA of Sun Obtain GHA(0000UTC) and v From tables Modify v using tables GHA at observation time = GHA(0000UTC)? +/- v + Time (in degrees and minutes)?

3) Calculate GHA of Sun - EXAMPLE : 

3) Calculate GHA of Sun - EXAMPLE

4) Calculate LHA of Sun : 

4) Calculate LHA of Sun LHA = Local Hour Angle LHA = difference in Longitude between observer position and GHA If Longitude = WEST LHA = GHA - Longitude If Longitude = EAST LHA = GHA + Longitude

4) Calculate LHA of Sun - EXAMPLE : 

4) Calculate LHA of Sun - EXAMPLE

5) Calculate Declination of Sun : 

5) Calculate Declination of Sun Obtain Dec(0000UTC) and d From tables Modify d using tables Dec at observation time = Dec(0000UTC)? +/- d

Slide 29: 

5) Calculate Declination of Sun - EXAMPLE

6) Calculate Altitude – Method 1 : 

6) Calculate Altitude – Method 1

6) Calculate Altitude – Method 2 : 

6) Calculate Altitude – Method 2 Nelson didn't use Excel! Using Sight Reduction Tables: Find the LHA in Degrees and seconds – note value in LHA Column (A)? Find the Latitude in Degrees and seconds – note value in Lat Column (B)? Find the Declination in Degrees and seconds – note value in Dec Column (C)? Sum A, B and C Degrees top – minutes left Degrees bottom – minutes right

6) Calculate Altitude – Method 2 cont : 

6) Calculate Altitude – Method 2 cont Using Sight Reduction Tables: Find nearest value to this Sum in the Sum Column (Anywhere in the tables)? Note Result to right in Res column (D)? Evaluate L~D, find value from tables (E)? Sum D and E Find this Value in the ALT column, read off Computed Altitude in Degrees and Minutes.

Note on L~D : 

Note on L~D If Declination is North and Latitude South, or vice versa add values else take smallest from largest

6) Calculate Altitude - EXAMPLE : 

6) Calculate Altitude - EXAMPLE

7) Calculate Azimuth – Method 1 : 

7) Calculate Azimuth – Method 1

7) Calculate Azimuth – Method 2 : 

7) Calculate Azimuth – Method 2 Nelson didn't use Excel! Using Weir Diagrams: Mark a point on the graph axis corresponding to Declination Mark a point on the LHA scale corresponding to that of the body Move in until at a point corresponding to latitude. Draw a line parallel to these two points through centre – read off Azimuth.

7) Calculate Azimuth - EXAMPLE : 

7) Calculate Azimuth - EXAMPLE

What next? The Plot : 

What next? The Plot The intercept is the difference between observed and calculated altitudes. It represents the distance from the dead-reckoned position to the Line of Position (LoP)? 1' = 1 NM LoP is a Line at 90° to the azimuth on the chart representing the circumfurence of distance from the sun GOAT – Greater Observed Altitude Towards

The Plot - EXAMPLE : 

The Plot - EXAMPLE

Sun-Run-Sun : 

Sun-Run-Sun Run on desired course From any position on the initial LoP plot a new dead-reckoned position transfer initial LoP to new point Draw line parallel to initial LoP through DRP Take new sextant sight Draw new LoP You are where they cross WARNING Not as accurate as GPS!

Other Positioning Ideas : 

Other Positioning Ideas

Triple Shot : 

Triple Shot Three Celestial Bodies Moon, Jupiter, Betelgeuse etc. NOT Polaris Approx 60° Azimuth Apart Draw LoP for each You're within the Triangle (roughly)

The Meridian Passage a.k.a. MERPASS a.k.a. The Noon Sight : 

The Meridian Passage a.k.a. MERPASS a.k.a. The Noon Sight Measure the highest altitude of the sun at local noon Work out the Zenith distance, Zd 90° - observed altitude If dec is North and Zd South or Vice versa latitude = sum of dec and Zd else latitude = difference between dec and Zd

Questions : 

Questions ?