At the time of the second trial …
Years: 1765 - 1765
At the time of the second trial of John Harrison's 'Sea watch' (H4), another method for measuring longitude is ready for testing: the Method of Lunar Distances.
The moon moves fast enough, some thirteen degrees a day, to easily measure the movement from day to day.
By comparing the angle between the moon and the sun for the day one left for Britain, the "proper position" (how it would appear in Greenwich, England, at that specific time) of the moon could be calculated.
By comparing this with the angle of the moon over the horizon, the longitude could be calculated.
During Harrison's second trial of his H4, the Reverend Nevil Maskelyne is asked to accompany HMS Tartar and test the Lunar Distances system.
Once again the watch proves extremely accurate, keeping time to within thirty-nine seconds, corresponding to an error in the longitude of Bridgetown of less than ten miles (sixteen kilometers).
Maskelyne's measures are also fairly good, at thirty miles (forty-eight kilometers), but require considerable work and calculation in order to use.
At a meeting of the Board in 1765 the results are presented, but they again attribute the accuracy of the measurements to luck.
Once again the matter reaches Parliament, which offers £10,000 in advance and the other half once he turns over the design to other watchmakers to duplicate.
In the meantime Harrison's watch will have to be turned over to the Astronomer Royal for long-term on-land testing.
Unfortunately, Nevil Maskelyne had been appointed Astronomer Royal on his return from Barbados, and is therefore also placed on the Board of Longitude.
He returns a report of the watch that is negative, claiming that its "going rate" (the amount of time it gains or loses per day) is due to inaccuracies cancelling themselves out, and refuses to allow it to be factored out when measuring longitude.
Consequently, this first Marine Watch of Harrison's fails the needs of the Board despite the fact that it had succeeded in two previous trials.
The moon moves fast enough, some thirteen degrees a day, to easily measure the movement from day to day.
By comparing the angle between the moon and the sun for the day one left for Britain, the "proper position" (how it would appear in Greenwich, England, at that specific time) of the moon could be calculated.
By comparing this with the angle of the moon over the horizon, the longitude could be calculated.
During Harrison's second trial of his H4, the Reverend Nevil Maskelyne is asked to accompany HMS Tartar and test the Lunar Distances system.
Once again the watch proves extremely accurate, keeping time to within thirty-nine seconds, corresponding to an error in the longitude of Bridgetown of less than ten miles (sixteen kilometers).
Maskelyne's measures are also fairly good, at thirty miles (forty-eight kilometers), but require considerable work and calculation in order to use.
At a meeting of the Board in 1765 the results are presented, but they again attribute the accuracy of the measurements to luck.
Once again the matter reaches Parliament, which offers £10,000 in advance and the other half once he turns over the design to other watchmakers to duplicate.
In the meantime Harrison's watch will have to be turned over to the Astronomer Royal for long-term on-land testing.
Unfortunately, Nevil Maskelyne had been appointed Astronomer Royal on his return from Barbados, and is therefore also placed on the Board of Longitude.
He returns a report of the watch that is negative, claiming that its "going rate" (the amount of time it gains or loses per day) is due to inaccuracies cancelling themselves out, and refuses to allow it to be factored out when measuring longitude.
Consequently, this first Marine Watch of Harrison's fails the needs of the Board despite the fact that it had succeeded in two previous trials.
