The sextant

A sextant measures the angle between two directions by bringing their images together in one mirror: the horizon seen directly, the body reflected by the index mirror. The arc reads degrees, the micrometer drum minutes and tenths. Before every session check the perpendicularity of the index mirror, the side error of the horizon mirror and the index error: with the arc at zero the horizon seen direct and reflected should be one line; if not, the drum reading that makes them one is the index error, subtracted when it is on the arc and added when it is off.

Hold the sextant vertical and rock it gently so the body swings like a pendulum, touching the horizon at the bottom of its arc. That touch is the altitude, and its time is the sight's time.

How is the index error found?
An index error on the arc is applied how?

Show the answers
  1. How is the index error found? With the arc at zero, by bringing the direct and the reflected horizon into one line At zero the two horizons should coincide; the drum reading that makes them coincide is the index error.
  2. An index error on the arc is applied how? Subtracted from the reading On the arc the instrument reads too much; off the arc, too little.

The twelve evenings

  1. The celestial sphere
  2. The sextant
  3. Time and the almanac
  4. Correcting the altitude
  5. The noon sight
  6. Latitude by Polaris
  7. The navigational stars
  8. The intercept method
  9. Plotting the position line
  10. The running fix
  11. The Moon and the planets
  12. The day's work

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