Longitude, Harrison Clocks, Lunars and Captain Cook (Part 2)

(Continued from Part 1)


During these same years, another competing method had been developed for finding longitude using the moon and stars themselves as a clock to determine the time back in Greenwich, England. This was the method of lunar distances or lunars. The moon moves the distance of its own diameter in about one hour, a distance called a ‘lunar’.

In a sense, the moon’s movement is similar to that of a clock handle amidst the stars, and calculations based on the moon’s movements across the background of the stars at any location in the world (providing conditions were right to view the moon) could be used to determine the time back in good old Greenwich, England.

Nevil Maskelyne

Nevil Maskelyne

Through painstaking observations and calculations, the angular distances between certain bright stars and the moon were determined and interpreted to create this celestial clock. This work was primarily integrated by England’s Royal Astronomer Nevil Maskelyne who served in that position from 1765-1811. In 1766—the Longitude ‘contest’ had been going on for 52 years by this time— Maskelyne published a book, the Nautical Almanac, listing all the data necessary to interpret the information and determine one’s longitude using lunars.

The method itself was complicated and was dependent on the moon itself being amenable to observation. A navigator measured the angular distance between the edge of the moon and a given bright star with a sextant, and the elevation of both the moon and the star above the horizon. Calculations were then made to adjust for such things as the distance between the edge of the moon and its center and for what is called the index error of the sextant. Then corrections were made to determine what the lunar distances would be if measured from the center of the Earth and corrections were made for refraction, the bending of light due to the Earth’s atmosphere. Then the numbers were compared to numbers listed in the Nautical Almanac to determine one’s longitude. All and all, a complicated series of observations and calculations, easily prone to error if not done properly, and which is said to have taken upwards of four hours. But despite its complexity, the lunar method worked with good success when done properly.

Lunar method from Wikipedia

Lunar method from Wikipedia


As it turned out, Nevil Maskelyne, the major proponent of the lunars method and Royal Astronomer, had considerable power and control over the proceedings to evaluate Harrison’s clocks. He essentially thwarted Harrison’s work and sabotaged testing of Harrison’s clocks.

Harrison’s clocks (including H-4) and his drawings and plans were ordered to be turned over to the Admiralty (confiscated ). H-1 was broken in the process. Harrison was then ordered to build a copy of H-4 without the benefit of these resources. Harrison built H-5 over three years time and it was tested over an additional two years time.

Portrait of King George III

Portrait of King George III

Harrison was now 79 years old, and despite his clocks being sufficiently accurate to warrant his receiving the monetary awards set out in the Longitude Act, he was not awarded the prize.

After years of frustration, Harrison’s son, William, pleaded the case for his father and his clocks in front of King George III. “By God, Harrison, I will see you righted!” King George was said to have exclaimed.

King George himself tested H-5 at the palace and after ten weeks of daily observations in 1772 found it to be accurate to within one third of one second per day.

Captain Cook

Captain Cook on his first voyage, in which he mapped New Zealand, used the lunar method to determine longitude.

James Cook

James Cook

On Cook’s second voyage, along with lunars, he used of copy of Harrison’s H-4 watch termed K-1. H-4 itself had been secreted away by the Admiralty and was unavailable; another master clockmaker was ordered to build a copy. The clock, K-1, worked admirably. In his journals, Cook referred to K-1 as “our trusty friend, the Watch” and “our never failing guide, the Watch.”



Cook carried K-1 on his third and final voyage and the story goes that at the exact instant Cook was killed, K-1 stopped ticking.

The clocks that Harrison built are best defined as ‘marine chronometers’ or machines for measuring time at sea, although the word ‘chronometer’ didn’t catch on until 1779. Up until the 1790s, chronometers were too expensive to be commonly used. Instead the lunar method continued to be used into the nineteenth century. In the 1790s due to mass production, chronometers did become less expensive to produce and gained acceptance and use, particularly since they provided more precise determinations of longitude than the lunar method and were less prone to the errors of observations and calculations associated with the latter method.

Currently, Harrison’s clocks are exhibited at the Royals Museums Greenwich.



Sobel, D. & Andrews, W. J. H. (1995). The Illustrated Longitude. London: Fourth Estate.


One thought on “Longitude, Harrison Clocks, Lunars and Captain Cook (Part 2)

  1. OK I’m not going to write about the Harrison clock but rather Paul’s new Movin2NewZealand picture. What you all have to know is that Paul isn’t head-over-heels about animals but these horses in the pictures were in LOVE with Paul. They chased him around that whole church, even if he ran. Finally he had to jump into the car to get away from them and even then, they had their faces plastered to the window and were licking it. Matthew and I were laughing so hard…..I guess this proves Paul has “animal magnetism!”

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