File:Intercept Sight Reduction-00.png

Schematic diagram of the basic computation process for sight reduction by the intercept method.

• Green lines and keys indicate only addition and subtraction and can be done with pencil and paper using an appropriate form.
• Red lines and keys indicate more complex calculations but in most of them simple tables can be used or a handheld calculator.
• Hs: the height read off the sextant.
• IC: the sextant's Index Correction also called Instrument Correction.
• He: the height of eye above the water from which Dip correction can be obtained using lookup tables or the simple formula mentioned in the diagram.
• Ha: (Height apparent) is obtained by adding algebraically the three previous. Ha = Hs - IC - He
• UTC Universal Time, read from a chronometer at the instant the sight was taken.
• Body: the name of the celestial body observed.
  

With the body name and UTC as arguments the four following are extracted from the Nautical Almanac

• GHA: Greenwich Hour Angle.
• dec.: Declination (GHA & dec. are the coordinates of the body)
• SD: SemiDiameter (only applicable to Moon and Sun).
• HP: Horizontal Parallax (mostly Moon only).
  

This step is the most complex in the sense that it cannot be done by any simple calculations locally. One needs to extract the information from the almanac or use a computer.

• lat: assumed latitude.
• lon: assumed longitude.
• Refr.: Refraction can be found using lookup tables or the formula provided.
• PA: Parallax in Altitude = HP * Cos(Ha)
• Aug: Augmentation is a very minor correction used for the Moon only and which can mostly be ignored unless very exact results are desired.
• Ho: height observed.

All the previous steps are common to all the different variants of the intercept method. We now come to the resolution of the spherical triangle which can be done by many different ways. The formulas which permit the calculation of Hc and Z are the two lower ones below the diagram. They can be easily calculated using a handheld calculator with trig functions or lookup tables such as H.O. 229 can be used. Lookup tables were the only method before the advent of calculators. Today, a cheap, programmable calculator will do the calculation instantly by just entering the three arguments: LHA (Local Hour Angle), dec., and lat. .

With those three arguments we obtain:

• Hc: height computed
• Z: Azimuth angle

The difference between Ho and Hc is the "Intercept distance" or error and Zn is Z reduced to the correct quadrant.

This process can be done quite mechanically using an appropriate form where the appropriate boxes are filled sequentially.