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XIV. Method of determining the Latitude, by a Sextant or Circle, with simplicity and accuracy, from Circum-meridian Observations, taken near Noon
Published online by Cambridge University Press: 17 January 2013
Extract
As it very frequently happens that an observation may not be obtained for the latitude, at the precise instant of noon, it becomes a most desirable object to supply that loss by every possible means. The method which I am about to detail, I have long practised, and from the experience of many hundred trials, I can recommend it, as combining much simplicity with the greatest accuracy; since one day's observations may be equal to those derived from the chances of three weeks of the ordinary course of weather in our climate. This method consists in merely reducing to noon these observations, the same as if made when the sun's centre was on the meridian, by the means of a very simple calculation, which I shall detail, and illustrate with the observations for two days, in order to shew the accuracy of the results thus obtained. Having previously ascertained the time of noon, either by equal altitudes, or from simple ones, in the manner I had the honour to detail in a former communication to the Society, “On the Mode of determining Time with the Sextant” I begin nearly 10′ from noon to observe the sun's altitude, from an artificial horizon of oil, or quicksilver, and continue making as many observations as I can accomplish until the sun has nearly the same altitude as when I began, which will be the case about as long past noon, during which an expert observer will easily take 20 altitudes, which, in most cases, will be sufficient to enable him to retain all those that appear to be consistent, and to reject those that differ much from the mean.
- Type
- Research Article
- Information
- Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 9 , Issue 1 , 1823 , pp. 227 - 231
- Copyright
- Copyright © Royal Society of Edinburgh 1823
References
page 228 note * At sea, or where it is not required to proceed with the greatest accuracy, the mean refraction may be used, without having recourse to the correction for atmospherical pressure and temperature.
page 228 note † If the place of the observer is west of the first meridian, it becomes necessary to add the proportional part to the noon declination, for the difference of meridians, from the Vernal Equinox until the Summer Solstice, and from the Autumnal-Equinox to the Winter Solstice, but for the rest of the year it must be subtracted. If the observer is situated east of the first meridian, it must be subtracted during the spring and autumn, but added during summer and winter. Had the sun no motion in declination during the time that the observations continue, it would be unnecessary to apply any correction, as in the case of observing by the stars; but although the quantity is very small during so short a period of time, still it is requisite to to it. For this purpose the proportional part of the declination, corresponding to the horary angle of the observation, is to be added to the declination at noon, in order to have it for the instant of observation, when the observation is made before noon, and the declination is diminishing. If the observation is made in the afternoon, the proportional part must be subtracted.
page 230 note * No astronomer should be without this little work, or Callet's tables, both of which, from their value, and universal use in calculations, have been stereotyped in France, and abridge, in a most wonderful degree, the proportions required in all astronomical computations. As the one term is invariaby constant, viz. 24 hours, the change that takes place in declination, the sun's longitude, or AR, serves for all the calculations of one day; so that in fact there are but two logarithms to look out, in the most complicated proportion.