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Margetts's Horary Tables

Published online by Cambridge University Press:  18 January 2010

D. H. Sadler
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Extract

IN the course of a systematic search for tables or graphs using the principles appealed to in the Experimental Astronomical Navigation Tables (see p. 333), I examined a copy of the above tables, whose full title follows:

‘Margetts's Horary Tables for shewing by Inspection the Apparent Diurnal Motion of the Sun, Moon and Stars, the Latitude of a Ship and the Azimuth, Time or Altitude corresponding with any Celestial Object.

‘London. Printed for and sold by the Author. “Published as the Act directs 1st July 1790.”’

These tables (or diagrams) do not appear to be well known and they seem to possess sufficient interest to warrant notice. The author's own description is so precise that I can do no better than to repeat it verbatim with a few added notes of explanation. The accompanying photograph, reproduced from the copy in the collection of navigational books and tables in the Admiralty Library by kind permission of the Admiralty Archivist, shows a typical opening of the tables.

I have been unable, in the course of an admittedly hurried and incomplete search, to find any biographical information about Margetts. Perhaps some reader of the Journal can supply the deficiency.

Type
Research Article
Information
The Journal of Navigation , Volume 6 , Issue 4 , October 1953 , pp. 403 - 404
Copyright
Copyright © The Royal Institute of Navigation 1953

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References

1 The distance between the horizontal lines for declinations of N. 30° and S. 30° is 10·6 inches or 0°177 inches for each degree.

2 The 15′ horizontal rules are omitted for the odd degrees of latitude, presumably for economy in engraving the copper plates. For the even degrees they are finely drawn and an accuracy of 5′ in interpolation is easily possible.

3 If h is the hour angle then the abscissa is proportional to 1 – sin (6h – h), being very compressed for small hour angles. It is, of course, almost linear for hour angles greater than about three hours.

4 The horizontal distance allowed for the whole six hours of hour angle is 17·8 inches, and near six hours the scale is such that Im = 0·078 inches, or 5′ = 0·026 inches−which precision is easily attainable.

5 The point of this remark is difficult to comprehend.

6 The scale of altitude naturally varies from that for declination to that for hour angle, according to azimuth; interpolation to 5′ is feasible.

7 The azimuth lines are not given for some of the odd degrees and it would appear that the plates were not completed. All those signed ‘George (or G.) Margetts inv. et del.’ are complete in every particular.

8 Charts are given for every degree of latitude from the equator to N. 66°.