Thomas Jefferson Papers

Remarks on Observations with a Sextant, 18 April 1803

Remarks on Observations with a Sextant


By a practice of more than twenty years, I have constantly found water preferable to any other fluid for an artificial, or portable horizon.—The reflection of the Sun from the water it is true, will be fainter than that from the specula, unless the Telescope of the Sextant be directed nearly off the foliated part of the horizon speculum.—This direction can be easily given to it, by a screw for that purpose, and which carries the Telescope parallel to the plane of the Sextant.—

Altho the meridian altitude of the sun, when it exceeds 60.°, cannot be taken with a Sextant from the artificial horizon; yet the latitude may be accurately determined by using the altitude of the sun, and the horary angles formed in taking equal altitudes to ascertain the error, and rate of going of either a clock, or watch.—This method I have constantly used when the meridional altitude of the sun exceeded 60.°, and am convinced from a long experience, that the latitude may be deduced from such observations nearly, if not quite as accurately, as from the sun’s meridional altitude:—By many trials made at this place, the latitude in no case, differs so much as half a minute from that settled by the Zenith Sector.—

The equal altitudes ought always to be taken at least two hours before, and after noon.—

If the distance of the moon, from the sun, be taken immediately before the morning equal altitudes, or after those of the afternoon, or both if the position of the moon will permit, every requisite for determining with accuracy both the latitude, and longitude, will be had.

I do not find that this method has been practised by any person but myself, the theory has however been long understood.—

After the forenoon equal altitudes have been taken, the sextant should be carefully laid away, and the index not moved, till the afternoon ones are taken, and if the latitude is to be deduced from the observations, the altitude must be carefully counted off, but if the time only is wanted, the degrees, minutes, and seconds of altitude are of no importance.

It is rather better to have the vessel which contains the water for the artificial horizon unconnected with the talk, or isinglass cover, because the wind is sometimes so violent, as to shake the cover, and consequently if the two parts are connected, an undulatory motion will be communicated to the water.

It will be a necessary precaution, to have the Chronometer, with its case, tied up in a bladder when not in use,—it will prevent its being injured if by accident, it should be thrown in the water by the overturning of a canoe, or other accident.


MS (DLC: TJ Papers, 130:22461); in Ellicott’s hand, undated.

talk, or isinglass: Ellicott recommended using thin sheets of talc or mica (sometimes called isinglass) for the cover of an artificial horizon apparatus (APS description begins American Philosophical Society description ends , Transactions, 5 [1802], 207; OED description begins J. A. Simpson and E. S. C. Weiner, eds., The Oxford English Dictionary, Oxford, 1989, 20 vols. description ends ).

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