Principles of Celestial Navigation

The Intercept Method » Observed Altitude (Ho)

schematic illustrating the basic sextant corrections

The first step in using the Intercept Method is to determine the "Observed Altitude" of the star. This is also called "Height observed," with a notation of Ho. Ho is important to remember.

By its name, you may expect that the observed altitude is what we read from a sextant. The sextant reading is the major portion, but Ho is actually what we would measure with a sextant if:

  • the sextant were perfect;
  • and if our eye were at sea level;
  • and Earth's atmosphere didn't bend light;
  • and if we observed the exact center of the object.

These corrections to our sextant reading sound like they are difficult, but they are not.

dip and altitude corrections for celestial bodies, from the U.S. Naval Observatory Nautical Almanac

They are simple additions or subtractions, and they have been pre-computed in tables in The Nautical Almanac, which is an annual publication by the U.S. Naval Observatory and Her Majesty's Nautical Almanac Office. Also, each step is laid out in a page, called a strip form, used by navigators. You can think of a strip form as a paper spreadsheet that details each step in the sight reduction process.

blank sight reduction strip form. the strip form provides a worksheet for working through the sextant corrections and sight reduction for the intercept method.

The strip form acts as a worksheet to step you through the sight reduction process. A blank strip form is available for printing here (PDF).