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Lunar Methods For ‘Longitude Without Time’

Published online by Cambridge University Press:  23 November 2009

D. H. Sadler
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Extract

In 1966, in a note in the Forum entitled ‘Longitude without Time’, Francis Chichester described a method of determining longitude, or time (G.M.T.), from observations of the altitude of the Moon; and this gave rise to considerable interest. Recently a number of papers, of varying standards, have been published on the same subject; some, ignoring fundamental principles, have described techniques that are erroneous or misleading. Those principles, which have been well known for centuries, are here restated. In general, techniques of observation, calculation and plotting are not given; they are matters for the choice of the individual.

Type
Research Article
Information
The Journal of Navigation , Volume 31 , Issue 2 , May 1978 , pp. 244 - 249
Copyright
Copyright © The Royal Institute of Navigation 1978

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References

NOTES AND REFERENCES

1This note was written at the request of the Editor.Google Scholar
2This Journal, 19, 106.Google Scholar
3In particular, the following:Google Scholar
Ortlepp, B. (1969). Longitude without time, Navigation, Vol. 16, 29.CrossRefGoogle Scholar
Ortlepp, B. (1969). Longitude without time, Nautical Magazine, Vol. 210, 276.Google Scholar
Wright, F. W. (1971). Examples of Moon sights to obtain time and longitude, Navigation, Vol. 18, 292;CrossRefGoogle Scholar
Kerst, D. W. (1975). Longitude without time, Navigation, Vol. 22, 285.CrossRefGoogle Scholar
Luce, J. W. (1977). Longitude without time, Navigation, Vol. 24, 112.CrossRefGoogle Scholar
Ortlepp, B. (1977). Improved plotting solution to longitude without time, Nautical Magazine, Vol. 218, 334.Google Scholar
4The recommended terminology (according to a Resolution of the International Astronomical Union adopted at Grenoble in September 1976—Transactions of the I.A.U., Vol. XVI B, 218) is now Universal Time, with abbreviation U.T. 1 in order to distinguish it from Co-ordinated Universal Time (U.T.C.) which may also be included in the general term Universal Time (U.T.). G.M.T. is here used, instead of U.T. 1, in order to emphasize its relationships with L.M.T. and G.H.A.Google Scholar
5But, since the Moon's ecliptic latitude is always small, its declination at new Moon differs little from that of the Sun. The full Moon thus ‘rides high’ in winter months.Google Scholar
6It can be performed on all ‘mathematical’ models, but the amount of intermediate recording required depends on the sophistication (storage, programmability, &c.) of the calculator.Google Scholar
7Interested readers might note how these differences vary through a lunar month; the longitude of the node of the Moon's orbit is 180°, and thus the inclination, /, to the equator is a minimum in July 1978.Google Scholar
8Deliberately vague: (B-C) varies between the extremes (90° − ϕ)+/ cos θ and − (90° − ϕ) + / cos θ, but the optimum value cannot always be chosen.Google Scholar
9The method, as described by J. W. Luce (loc. cit., see note 3), appears in fact to be ‘new’ since it is erroneous; he ignores the Moon's motion in declination.Google Scholar
10See the following papers presented at the Royal Observatory Tercentenary Symposium held at the National Maritime Museum in July 1975:Google Scholar
Forbes, E. G. (1976). The origins of the Greenwich Observatory, Vistas in Astronomy, Vol. 20, 39.CrossRefGoogle Scholar
Sadler, D. H. (1976). Lunar distances and the Nautical Almanac, loc. cit., 113.Google Scholar
11Board of Longitude Papers, Vol. 7 (confirmed Minutes 1802–1823) page 10. At the Royal Greenwich Observatory.Google Scholar
12Raper, H. (1840). The Practice of Navigation, first edition. There was a twentieth edition in 1914!Google Scholar