The Longitude


An excellent and satisfying introduction to the measurement of time, the determination of longitude at sea, the meaning of the Greenwich meridian, and Standard Time is Derek Howse's Greenwich Time (Oxford: Oxford University Press, 1980). The book is meant for the general reader, but is accurate and informative. The coverage is very well chosen, and several appendices explain the more technical aspects. These appendices, explaining longitude and clocks among other topics, are a valuable part of the book. The subject of The Longitude has recently been the subject of a television program, as well as a book. These efforts are commendable, but do not surpass this small volume. There are few topics more interesting and rewarding of study for the curious than this one, which combines so many different threads. They do not seem to write books of this quality any more.

In these notes, I record some notes and observations for my own information, and explain some topics that I think are not fully elucidated in the book. The reader is referred to the book for everything else, which will not be summarized here.

The Greenwich Observatory

Flamsteed appointed Astronomical Observator, 4 March 1675. Longitude was in the air. Wren suggested Greenwich Castle, tower built 1437 by Humphrey Duke of Gloucester, rebuilt 1526, used by Henry VIII and Elizabeth I, garrisoned by Parliament, standing in 1662, down by 1675, and the observatory was built on its foundations. Flamsteed moved in 1676. Two Tompion clocks with 13' pendulums above works. [did they resonate?]

Flamsteed verified that the rotation of the earth was uniform, to the best accuracy possible at the time, so that it could be used as the basis of a uniform time. There was previously no proof of this assumption.

Longitude at Sea

The methods of determining the difference in longitude between two places are the following: (1) direct measurement of the distance (not possible at sea); (2) observation of lunar eclipses, which permits observations of local times at two points simultaneously; (3) observation of phenomena of Jupiter's satellites, which are more frequent than the preceding; (4) using the motion of the Moon with respect to the stars to give time; (5) carrying a reliable clock from one place to the other; (6) using radio messages to determine the time at a standard location. All methods but the first compare simultaneous local times.

The extreme difficulty of making astronomical observations at sea is probably not evident to the casual reader. In a telescope of any power, the incessant movement of the ship causes the field of view to dance wildly. Try using binoculars from a moving car. Any telescope of a power that would allow observations of the phenomena of Jupiter's satellites (say 30X) would be hopeless on deck. Consider measuring the angular distance between two objects. Even with low powers, or no optical aid at all, it is necessary to set the sights first on one object, note the reading, then on the other, note the reading, and subtract the two readings. On shore, this is possible. At sea, the instrument moves between the two readings, making the procedure useless. The cross-staff is a partial solution, but incapable of accuracy. The solution was found only with the double-reflection quadrant or sextant, where both objects can be seen at once.

Accurate star catalogue, accurate lunar ephemeris, some way of measuring lunar distance required. Longitude Act 1714. Quadrant: John Hadley (1682-1744) 1731, and Thomas Godfrey (1704-1749) 1730. Newton had designed a double-reflection instrument in 1700 but Halley rejected it. 1757--John Bird and Capt. John Campbell design the sextant. Nautical Almanac, 1766 (for 1767).

The International Meridian Conference, Washington DC, 1884

It stands to reason that a first-class astronomical observatory should stand on the principal meridian. This is, however, completely absurd, though within the mental grasp of politicians. The correction from any longitude is purely arithmetic. Any constant can be added to the longitude. The sole reason for adopting Greenwich is that it was already used on a large number of charts. If one said that the Greenwich meridian was 20° east of Ferro, and adopted the Ferro meridian, it would only be necessary to add 20° to all longitudes. Ferro, of course, would actually determine nothing--it would still be the meridian circle in Greenwich. In fact, the meridian instrument was moved 0.417" east in 1851. In 1911, Standard Time was finally adopted in France as Paris Mean Time (less 9 minutes 21 seconds to agree with GMT). Holland used Amsterdam Mean Time until 1940.

Another stupidity is the 'decimal measurement' of angle and time by redefining the degree and the second. There is no more advantage to giving a right angle 100 degrees (grads) than giving it 83 or 217. Simply divide the degree decimally, and all the convenience is there. The sexagesimal division actually has an advantage in practical use, since it is more difficult to misread or miswrite than a string of digits. With modern pocket calculators, it is dead easy to use. Time is similar, though it is hard to believe even Frenchmen wanted a 10-hour day in 1884. Metric units are arbitrary, not natural. Incidentally, the metre is the length of the seconds pendulum. The ten-millionth of the meridian is a contrived, useless definition, on a par with the kilogram as the mass of a cubic decimetre.

The little-remembered Nautical Day is mentioned. This solar day was kept at sea, and ended at noon, 12 o'clock, when the clock was adjusted to the local time. Whether apparent or mean is not stated. Monday ended at noon, while the civil Monday went on until midnight. This reckoning avoided changing the day during the night watches. In port, normal civil reckoning was used. One can appreciate that this calls for care when interpreting log entries. The Nautical Day ended in the Royal Navy on 11 Oct 1805, and in the East India Company in the 1820's, but persisted here and there later. Even then, local time was used with the day beginning at midnight. Standard Time was not finally used until after the First World War.

Standard Time

Charles F. Dowd (1825-1904) of Saratoga, pamphlet in 1870. Suggested time zones. 1872 based on Greenwich instead of Washington meridian. Adopted 18 Nov 1883. Not legal time until 1918, but immediately adopted by the public.


Some Astronomers Royal
John Flamsteed 1676-1719
Edmond Halley 1719-1742
James Bradley 1742-1762
Nathaniel Bliss 1762-1764
Nevil Maskelyne 1764-1811
John Pond 1811-1835
George Airy 1835-1881
W. H. M. Christie 1881-1912

Return to Astro Index

Composed by J. B. Calvert
Created 7 July 2000
Last revised