
The Commissioners for the Discovery of the Longitude at Sea, also known as the Board of Longitude, was formed by the British government in 1714 "to administer a scheme of prizes intended to encourage innovators to solve the problem of finding longitude at sea" in the Age of Discovery, where ships exploring the world were increasingly lost at sea.
The Board offered large prizes: £10,000 pounds for a method that could determine longitude within 60 nautical miles, £15,000 pounds for a method within 40 nautical miles, and £20,000 for a method that could determine longitude within 30 nautical miles.

I'm not going to delve into the details of the history of longitude here, but Neville Maskelyne, the Fifth Astronomer Royal (appointed by King George III in 1765) -- who was a member of the Board of Longitude -- was able to figure out a lunar method for determining longitude that culminated in the publication of the Nautical Almanac in 1764.

This method was far more versatile than the maritime chronometer method credited to clockmaker John Harrison, which only had limited use for a short period of time. (John Harrison was supposed to receive the $20,000 prize, and he did receive most of the award, but due to a variety of issues, mostly political, never got the full amount. Maskelyne never sought payment of the award.)
Maskeylyne's apprentices learned to make astronomical observations at Greenwich Observatory, and his human computers prepared the complicated tables for the Nautical Almanac. Joshua John Moore worked under Maskelyne, first as an astronomer's apprentice and later as a computer for the Nautical Almanac, starting in about 1787 and ending in1793, when he came to America with his surveyor friend, Nicholas King. While I don't know the year of Joshua John's birth, I'm estimating that it was around 1770, making him a teenager when he began working under Maskelyne, as it was mostly young boys who did the arduous work of apprenticing.
Apparently Joshua John was so good at computing that Maskelyne promoted him to that job quickly, in 1788. I have already written about Joshua John's experience as an astronomer's apprentice and as a computer for the Nautical Almanac, so will not delve further into that here. If I find more information later, I will definitely add it.
In 1793, the Board of Longitude decided that the computers were computing too quickly, and were calculating ten years in advance. They demanded that the computers cut back or possibly suspend their calculations for a few years, and thus Joshua John no longer had a job. This was the impetus that led Joshua John Moore to sail to America with his great friend, surveyor Nicholas King.
I found the letter Neville Maskelyne wrote to Joshua John Moore dated June of 1793, informing him that his job was going to be severely curtailed going forward, and have transcribed it below. Obviously, it was a seminal moment in the life of Joshua John Moore, and in the Moore family as a whole, no pun intended!
Greenwich June 12 1793
Sir,
I rec’d yours of May 20th. Be pleased to compute Dec. 1804 [symbol shaped like crescent moon] place at Noon. There was a meeting of the Board of Longitude last Saturday tonight. They did not approve of the computations of the ephemera being carried on so fast and so far in advance, as it precludes us from the use of new improvements that may arrive in the Tables. I represented the inconvenience to the computers, if the computations were to be suspended altogether for a few years; I shall therefore give the computers fewer months in a year in future, that we may rather fall a little back then get forward beyond what we are at present in advance.
I am, Sir, Your humble Servant
N. Maskelyne
Sent to Joshua John Moore at Chesterton near Cambridge [Chesterton is/was a suburb two miles northeast of Cambridge [this is an address I had not known before; I also found him living at the Angel Pub in Cambridge prior to his arrival in America].
This is the reason J.J. Moore came to America!

Oddly enough, this letter exists in the archives of the Historical Society of Pennsylvania. It's hard to know how and why it ended up there. Much of Joshua John's correspondence -- 80+ letters -- is with his collection at the Library of Congress, which was donated by his son, John, and which I have not yet obtained.
In the course of his career, Joshua John worked with Mathew Carey, Philadelphia printer and publisher, when he and a colleague wrote the "Traveller's Directory"in the early 1800s, which was the first "Map Quest" in the world and holds an important place in the history of transportation. The Historical Society of Pennsylvania holds the archives from the Mathew Carey Publishers, among other things. I hired a researcher to look for letters pertaining to that, and this letter was discovered in that process. I have more letters to share in the next few posts, and will go forward in the chronological order of the letters.
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