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The Artificial Horizon
copyright Bob Graham 2002

When navigating at sea, the visible horizon serves as an astronomical horizon, but this is usually not the case on land. It is necessary to use an artificial horizon. In it's simplest form, this might be a puddle of water. The reflection of the object to be sighted is located in the puddle, and that reflection and the actual object are brought together in the sextant sights. The result is a double altitude.

Most often, a box is used and filled with mercury. The mercury, of course, presents a level reflective surface. To prevent the surface from being disturbed by wind, a roof-shaped glass cover made of optically flat glass is often added. Other liquids may be substituted. Frémont's mentor Joseph N. Nicollet reported in 1833 that when the mercury had been stolen "by some cunning fellow" from his horizon, he went on with his observations substituting "sugar water." My 1853 Bowditch suggest "Barbados tar," or "very clear molasses."

When making the reductions of the observations, the double altitude is divided by two, which also halves the reading error. And problems of horizon shifts due to refraction and thermal effects are eliminated, so that very accurate determinations may be made. This cannot be done at sea on a moving deck.

But the scale of the sextant takes in only 120 degrees (2 x 60). If the summer sun at noon is above 60 degrees, which it frequently is, the double elevation would be off the scale of the sextant. What then?

The instrument commonly used in the 19th century on these occasions, or whenever the angular distance between two objects was greater than 120 degrees, was the Reflecting Circle, or Circle of horizons. Having a scale that is a full circle, it is calibrated to 720 degrees (2 x 360).

Frémont carried the following instruments: a refracting telescope by Frauenhofer (150x) for observations of the satellites of Jupiter in time determinations); a reflecting circle by Gambey; two sextants by Troughton; two pocket chronometers by Goffe and by Brockbank; one syphon barometer by Bunten; one cistern barometer by Freye & Shaw; six thermometers, a spyglass, and a number of small compasses. Vouchered expenses also include several purchases of mercury. It was used in the artificial horizon, and also enabled him to make a remarkable field repair of his barometer in 1842.

From his expedition to the Rocky Mountains in 1842 Frémont had learned the superiority of the pocket chronometer over the boxed and gimbaled marine-type chronometer for rough traveling in extreme conditions. The pocket chronometer did not have nearly the potential for accurate rate maintenance, but it would keep running. He could make time checks on the rate by solar observations and observations of the satellites of Jupiter.

When I put my horizon on the ground, I found that the galloping of a horse five hundred yards off affected the mercury, and prevented a perfectly reflected image of the stars, and it was in vain to hope for these restless Maricopas to keep quiet. Lt. W. H. Emory, 1846

What about longitude? Click the chronometer

Related pages:

go AN EXPERIMENT IN THE DETERMINATION OF LATITUDE: This is a followup to the proceeding article, in which the conclusions made therein are put to practical test that may be repeated by anyone wishing to go to the trouble.
go A DAY AT THE COVE: An actual on-site demonstration of the determination of latitude with an astrolabe at Campbell Cove before a group of interested spectators.
How polaris has moved 2 degrees closer to the celestial pole during recorded California history, and why John C. Frémont got up at 3:00 a.m. to sight polaris in 1844--wasn't it there all night long?
go Re. the Drake landing site project, see a comparison of the 16th Century TABLES OF SOLAR DECLINATION by Martin Cortes with those of William Bourne. Pretty neat!
How easy it is today to carry the correct time about.

A Short Bibliography of Essential Reading:

Bourne, William, A Regiment For the Sea (1574), Cambridge, 1963.
Bowditch, Nathanial, The New American Practical Navigator: Any 19th Century Edition is most useful.
Sir Francis Drake (Bart.), The World Encompassed, 1628: Any edition, but especially The Argonaut Press, London, 1926.
Hanna, Warren L., Lost Harbor, University of California, 1979.
Kelleher, Brian T., Drake's Bay, Day Publishing, San Jose, 1997.
Wagner, Henry R., Sir Francis Drake's Voyage Around the World, John Howell, San Francisco, 1926.
Waters, D. W., The Art of Navigation, Yale University Press, 1958.
Wright, Edward, Certaine Errors in Navigation (1599), Walter Johnson, Norwood, N.J., 1974.

©1999, 2007
Bob Graham