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 ?