NavList:

A while ago someone asked how chronometers were checked for accuracy.

I found a passage in Susan P. Howell's "Practical Celestial Navigation."

It stated, "By the early twentieth century, time ball signals activated by telegraph and low power audible signals in major ports enabled the navigator to check the accuracy of the chronometer."

The above is about 150 years after its invention, and approx. 100 years after "... the instruments were constructed with more reasonable size, weight, and price."

Frank had an interesting post many years ago proposing if a ship at anchor knew both its lat and long, a reverse time sight could be used for LHA/GHA and time established from that. Accurate enough for a chronometer?

The reverse time sight would need an almanac. Based on Frank's recent history of the Nautical Almanac the NA&AE was born in 1767 (chronometer in 1735). "The origin of the British "Nautical Almanac and Astronomical Ephemeris", the first almanac widely used by mariners at sea. It was known almost universally as "The Nautical Almanac". It is hereafter refered to as the NA&AE to distinguish it from the modern Nautical Almanac. The first volume contained ephemeris data for the year 1767 (calculated during most of 1766 and published in January, 1767. later volumes three to four years in advance). The almanac was published by the Board of Longitude under the personal direction of Nevil Maskelyne. It was primarily for lunars. All times in apparent time. The "Tables Requisite..." were published to accompany the NA&AE containing data which did not change from year to year."

That raises two questions I would ask the historians on the list to address:

1. What ephemerial astronomical data was available to aid a sailor with a chronometer pre 1766?

2. How did a navigator check the accuracy of his chronometer before telegraph and time balls--especially at sea?

Bill B
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