Correcting the altitude

The sextant altitude becomes the observed altitude by a short chain of corrections. The index error first. Then dip: the sea horizon lies below the true horizontal because your eye is above the sea, by an angle that grows with the height of eye; a table gives it. Then the altitude corrections proper: refraction, which lifts a body's image and is largest near the horizon, and for the Sun and the Moon the semi-diameter, since you touch the lower or the upper limb to the horizon, not the centre. The Moon has parallax too, large enough to need its own table.

The almanac's altitude correction tables combine refraction and semi-diameter for the Sun by month and limb. Write the corrections in a column, sign them, and add them once.

Dip is applied because of what?
Refraction is largest for a body where?

Show the answers
  1. Dip is applied because of what? The height of the eye above the sea The sea horizon dips below the horizontal from a raised eye; refraction and semi-diameter are the other corrections.
  2. Refraction is largest for a body where? Near the horizon Light from a low body crosses more air at a shallower angle.

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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