CelNav Eleven - Calculator Sight Reduction Form

[HenryC]

This is a form to be used as a guide to CelNav Six in the celestial navigation course.  Using sight reduction tables like Pepperday requires a lot of practice and effort, it's a pain in the butt at the best of times, and in emergency conditions it can be a real nightmare.  The alternative is to use a pocket calculator to reduce the sight, and in CelNav Six I do the class example using the calculator method described in page 279 of the Nautical Almanac. This form guides you through that procedure  step by step, and keeps your work organized.

In today's era of cheap and reliable GPS, the whole idea of deriving a position from scratch is really for old Luddites like myself, or hobbyists who do all their navigation in their den at home.  But if you are really looking for a sextant backup to GPS with a lot less aggravation than a sight reduction, the pocket calculator is an excellent compromise, and in my opinion, a very practical one.  If you're going offshore, full expertise in Celestial is not essential, but it would be a very good idea to become familiar with this method  just in case your equipment broke down or the GPS system itself was knocked out.

You still need to use the sextant and timepiece, make your corrections and derive the GHA and Dec, but the calculator procedure here can be substituted for the Pepperday sight reduction step to generate  the Azimuth and Intercept that make up your Line of Position.

<Begin Form Here>

INPUT DATA

Ho      =   ___________

DEC    =  __________ LAT  =______________GHA   =  ___________  LON   =   ___________



INTERMEDIATE VALUES
                                                                                                                 
LHA = GHA + LON         =       ___________   If 0 > LHA > 360, + or - 360 until 0 < LHA < 360     

Cos LHA    =  _________________ 
 
Cos DEC = ___________                         Cos DEC  x Cos LHA     =  _________________  = C

Cos LAT = ___________,   Sin  LAT = __________,    Sin DEC     = _________________  = S



INTERCEPT CALCULATION 

Hc = INVsin (         S          x      Sin Lat      +            C         x      Cos Lat       )

Hc = INVsin ( __________ x  _________  +  __________ x ____________)

Hc = INVsin ( ______________________  +  _______________________ )

Hc = INVsin (_________________________________________________)

Hc =             ____________________________________           Cos Hc =   ______________

Intercept =  Ho - Hc  = ______________

Hc > Ho, the intercept is drawn  AWAY from celestial body
Ho > Hc, the intercept is drawn TOWARD the celestial body.



AZIMUTH CALCULATION

X = (        S            x    Cos Lat         -          C         x    Sin Lat      )    /  Cos Hc

X = (  _________  x  ________       -   _______    x  ________   )    /  Cos Hc

X = ( _______________________  -   ____________________ )   /  Cos Hc

X = ( _______________________________________________)  / _____________

X= ________________________________________________       

If  1 < X or < -1, (due to truncation error), just reset it to +1 or -1.

A = INVcos ( X ) = INVcos (_______________)  =  ___________________

If LHA > 180 degrees, then Azimuth Z = A
Otherwise, Z = 360- A

<End Form Here>