← Sextant Lab

Sight reduction, step by step

A sextant reading on its own is just an angle. To turn it into a position you correct it, compare it with the altitude you would have measured from an assumed position, and draw the difference on the chart.

1. Correct the reading. Apply index error, dip of the horizon (it depends on your height of eye), atmospheric refraction, and for the Sun or Moon the semi-diameter. The result is the observed altitude, Ho.
2. Look up the body. From the almanac, for the exact time of the sight, read the Greenwich hour angle and declination of the body.
3. Choose an assumed position close to where you think you are, chosen so the local hour angle comes out in whole degrees.
4. Compute the expected altitude Hc and the azimuth Zn, from tables or from sin Hc = sin L · sin d + cos L · cos d · cos LHA.
5. Compare. The intercept is Ho − Hc in minutes of arc, and one minute is one nautical mile. Positive means "towards" the body.
6. Plot. From the assumed position, go along the azimuth by the intercept and draw a line at right angles. You are somewhere on that line. Two or three stars give a fix.

The arithmetic is unforgiving but simple. Most mistakes come from step 1 and from the clock.