I have just returned from a visit to Lisbon, where I had been invited to speak about the Longitude Act and project at the Seminário Nacional de Historia da Matemática. An added bonus of the visit was that an exhibition marking the tercentenary of the first Longitude Act had just opened at the Museu da Marinha.

It was a small display on the first floor that succeeding in getting across many of the key points about the Act, the various contenders for rewards and the Portuguese context. There were three cases of books, tables and charts, two of navigational instruments and a series of wall text and graphic panels.

The first panel was welcome reading, pointing out a long history, several valid areas of research and the development of two of these - the chronometer and lunar distances - as workable solutions in the second half of the 18th century. Although John Harrison was mentioned in the panel dealing with the development of timekeepers, the exhibition did not present either him or chronometers as the most significant part of the story.


Other panels were devoted to the navigational methods used before the Act, magnetic variation, Jupiter's satellites, lunar distances, chronometers and the Portuguese context. The last of these included the role of Jupiter's satellites in settling longitudes on land, especially those that had been contested by Spain and Portugal.

IMG_0147Objects included an altaziumth compass (to measure magnetic variation), a telescope (linking to Jupiter's satellites, but not a type that could have been used for this tricky observation) a box chronometer and several instruments for astronomical observation.

Included among the printed material were books on navigation, ephemerides and almanacs. Among them were an 18th-century edition of the Nautical Almanac, a French account of testing timekeepers at sea, and several Portuguese ephemerides, including those based on the observations of the observatory at Coimbra University.

Two early charts on display also made use of a Portuguese prime meridian, and the IMG_015219th-century almanacs clearly played on a sense of history - a reminder of Portugal's stellar role in maritime navigation in the past - and a claim to a central position in the globe, marking the division of the two hemispheres and the old and new worlds.

I got a good sense of the importance of Portugal's maritime past to the nation during my visit to Lisbon. The conference was held at the Escola Naval where naval history and, by extension, the histories of navigation, mathematics and astronomy, were very evident. While Greenwich and other maritime location in Britain tend to celebrate the 18th-century Navy above all, many sites in Lisbon have (mostly 20th-century) paintings, mosaics and statues to the heroic navigators of the 15th and 16th centuries.

I was fascinated to learn about the myth-making surrounding the Sagres "School of Navigation", supposedly founded by Henry the Navigator, and also the extent to which the regime of the 20th-century dictatorship had consciously developed and celebrated this heroic maritime and imperial history - to the benefit of institutions like the Naval School, observatory, museum and planetarium.


Being a guest at the naval base, and visiting the naval museum (still directed by a uniformed naval officer, who was kind enough to guide me around), was a truly memorable experience. I particularly enjoyed being entertained over lunch in the officers' dining room with stories of the school, and having to teach and learn lunar navigation techniques.

The saying in the Portuguese Navy is that "the moon lies", emphasising all the many things that can go wrong with instruments, observations and calculations, especially when officers are less and less used to performing them. However, the tables and sextants are still there as back up, not least because of concern about the ease with which GPS signals can be blocked or (more dangerously) tampered with.

Best of all, I was told a story of a ship's commander, making for a large island but hampered by very poor weather and the loss of one navigation system after another. Left with just radar and dead reckoning, for a moment the moon appeared and he took his chance to take observations. Making his calculations, he couldn't believe the result: "the moon lies", he said to himself. But, continuing on his course, the island still didn't appear and, eventually, he decided to go back to the lunar observation and try his luck - radar soon picked up the target destination.

It sounded like the tale of an old salty sea dog but, later that day, I met the man himself. He was in youthful middle age. The moon sometimes lies but, it turns out, sometimes, even now, she can still be pretty helpful.