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Kieran Kelly's contribution about on-land navigators was very relevant, and
of great interest.

It has to be remembered that the land navigator was in a much better
position than the mariner for using stars, in the days before position-line
navigation. He used an artificial horizon (impossible at sea), so star
altitudes could be measured at any time of night.

Conversely, at sea, a view of the illuminated sea-horizon was necessary, so
star altitudes could be observed only around dawn or dusk. During that
short observation period of 20 minutes or so, a mariner would be lucky if a
suitable bright star happened to pass through the meridian, so that he
could follow it up over the altitude peak, and then down a bit, with his
sextant. He would be fortunate indeed to find a second such star in that
short period.

Of course, once position-line navigation came to be adopted, a whole
collection of star altitudes could then be collected, whether the star was
passing the meridian or not, and a position line drawn for each, which
would intersect at, or near, the true position.

Kieran refers to a Mercury horizon, and on other occasions has mentioned
Gregory's use of black tea as a reflector. Lewis and Clark found that a
water-tray was an adequate reflector for observing the Sun, but found that
for stars it was necessary to use an adjustable mirror set by a spirit
level (with the levelling errors that are inherent in that process). It
seems to me that a Mercury tray must be the ideal, for all its
inconveniences for a traveller in the wilderness. What did Gregory use for
his star-altitudes, tea or Mercury? Afterwards, he could have drunk one,
but not the other...

Of course, the second big advantage of an artificial horizon is that a
doubled altitude was measured, effectively halving many of the errors.

The third advantage is that the sea-horizon, with all its imperfections of
waves, swell, heave of the ship, general fuzziness, and anomalous
refraction, didn't come into the picture at all .

And the fourth advantage, to a land-navigator, was that his deck was
perfectly steady under his feet.

One disadvantage of a reflecting horizon was that altitudes were limited to
60 deg because the 120 deg range of a sextant limited the doubled angle.

Nav-l contributors who have based their experiences entirely on
land-observations need to take these factors into account in assessing the
difficulties that mariners had to face.

George.

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