The Navigational Utility Of Lunar Eclipses

Britain's rule of the waves from Halley's time onwards came about not only from its strong navy, but also through its scientists providing accurate navigational charts and methods for determining position at sea.

This did not happen overnight. The measurement of one's geographical longitude was a long-term problem. Deduction of the latitude was relatively easy, from the minimum angle achieved each day between the Sun and the overhead point (called the zenith). This minimum occurs at noon. At night, various stars can be used. Tables of Sun and stars were available allowing a ship's latitude to be ascertained in that way, but longitude is a different story.

As you sail east or west the time according to the position of the Sun alters. If you had an accurate clock that maintained the time at some reference point, say back in London, then by comparing the clock time with the time according to the Sun in the sky, the longitude might be determined. Unfortunately the pendulum clocks used in churches and observatories would not work on a tossing and rolling ship at sea.

In 1714 the British government offered a very large prize—

£20,000, worth about $3 million today—to anyone who could solve this general problem and enable ships to be navigated more safely. Prospective solutions fell into two camps. One approach involved constructing mechanical clocks that would function accurately on board ship, and this led to many advances in timekeeping. (The identity of the word for a time period spent maintaining a lookout, and a small timepiece that will fit in a pocket or strap to your wrist, did not come about by accident. I refer, of course, to a watch.) The problem was eventually solved this way by a skilled artisan, John Harrison, although there was much wrangling over the award of the prize (he never received the cash and credit which was his due) continuing for several decades.

Harrison was an outsider to the scientific establishment, which favored a different method: using astronomical objects as natural clocks. In principle, for instance, the positions of the four giant moons of Jupiter might be read as the hands on a clock, showing the same time whether viewed from anchor in the Thames estuary or from Port Royal in Jamaica.

Jupiter, though, could not be seen for much of the year when lost in the solar glare and was also difficult to observe telescopi-cally from a ship in the mid-Atlantic. The Moon provided a better target. It could be seen at some stage during the day for all except about 72 hours straddling conjunction each month, and in principle its position could be used to give the time.

The problem was that the location of the Moon in the sky, from a theoretical basis, was not known with sufficient precision. The best available set of positions for the Moon computed in advance was derived from the lunar theory published by Sir Isaac Newton in 1702, but observations showed these to be inaccurate. Halley examined this question and, realizing that the eclipse grid allowed a major refinement, he suggested a solution that effectively used the saros.

Some decades before, John Flamsteed (1646-1719) had been Astronomer Royal and had made measurements of the lunar positions, these showing varying discrepancies from the positions according to Newton's theory. Between 1722 and 1740 (a complete saros) Halley, by then Astronomer Royal himself, made 2,200 observations of the same parameter, and discovered that the discrepancies charted against the theoretical positions simply repeated those displayed by Flamsteed's measurements from 18 and 36 years earlier. This indicated that Newton's theory could be numerically corrected using the saros in quite a simple way.

In the middle of his observations, in 1731, Halley recognized the potential of this method to provide a solution to the navigation problem, but failed to publish the results during his lifetime. By the time Halley's analysis appeared in 1749, better lunar theories had been developed. Unbiased observers also had realized by then the accurate and practical use of Harrison's clocks. This did not, though, stop the establishment astronomers from fighting a continuing rearguard action.

Edmond Halley's lunar observations were never used in the practical matter of navigation, but his earlier investigations did lead to the rediscovery and naming of the saros. Halley recognized not only that eclipses repeat on that cycle, but also that the eclipse characteristics recur. To that extent he is the true father of eclipse prediction as we have received it.

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