Course / Sights on land

Sights on land · Page 3 of 3

A sight from your backyard

The artificial horizon lets you practice real celestial navigation without a boat, and on land you can check your answer: a map app tells you exactly where you are.

What you need

  • A sextant. An inexpensive plastic one is fine to learn on.
  • A tray: a dark baking dish, filled with water, or better something thicker that ripples less, such as cooking oil or old motor oil.
  • Shelter from the wind: a sheet of glass or clear plastic over the tray, or set it in a box on its side. The slightest breeze ripples the surface.
  • The time: a phone clock set automatically is accurate to well within a second.

Protect your eyes. Never look at the Sun, or at its reflection in the tray, without the sextant’s shade glasses in place on both paths: the reflection is nearly as bright as the Sun itself.

Taking the sight

  1. Set the tray on firm, level ground, out of the wind, where you can see the Sun’s reflection in it. Sit or kneel a few feet back.
  2. Put the shades in. Set the sextant to roughly twice the Sun’s altitude, look down at the reflection through the telescope, and find the second Sun brought down by the mirrors.
  3. With the micrometer drum, bring that Sun to rest on top of the reflected one, edges just touching. Note the time to the second, then the reading.
  4. Take three to five sights a minute or two apart, and average them, as the course’s several sights page explains.

Working it, and checking it

  • Open Work your own sight, choose Artificial (on land) for the horizon, and enter the reading as read. The app applies the index correction, halves it, and leaves out dip.
  • For the DR, use your position from a map app, rounded a little. The intercept then tells you how far your line passes from where you really are.
  • Take two sights a few hours apart, or the Sun and the Moon, and plot a fix. A careful land fix lands within a mile or two.
  • A noon sight works on land too, as long as the Sun at noon is below about 60°: in winter at most latitudes, and all year only beyond about 54° north or south.