There were many proposed solutions to the Longitude problem, as discussed in Ships, Clocks & Stars. One of the main contenders was using Jupiter's Moons as a celestial clock - though particular problems arose when attempting this at sea. 

Last week, I explored how a celestial ‘clock’, the moon, could be used to find longitude at sea. This week I want to look at another ‘celestial clock’: the four moons of Jupiter. In the early 17th century the Italian astronomer, Galileo Galilei, made the startling discovery that several moons orbited Jupiter. Noting that the motion of these satellites was easy to predict, Galileo suggested devices for using Jupiter’s moons for finding longitude at sea.

Galileo - National Maritime Museum This is the earliest surviving portrait of Galileo (1564–1642)

The method worked well on land and was being used for surveying and mapping by the late 17th century. However, observing such small and distant objects with a telescope was very difficult from a moving ship.

Jupiter This map compares France’s coastline mapped using Jupiter’s moons with those previously drawn. It is said the French king complained that the astronomers had taken more territory from him than his enemies.

One longitude projector, Christopher Irwin, attempted to create a marine chair as a way of counteracting this difficulty.  However, when Nevil Maskelyne tested it on his voyage to St Helena in 1761, he wrote back to the Board, 'Mr Irwin's marine chair affords no convenience or advantage to an observer in using a telescope for observing the celestial phenomena but sea, but rather the contrary'.

By the early 1800s longitude-finding by timekeeper and lunar distances was proving successful and commercially viable. However, the search for other methods went on and the Board considered proposals for finding longitude from Jupiter's satellites right through to its demise in 1828.

One such proposal came from Samuel Parlour, who wrote to the Board in 1824 from the East India Military Seminary in Surrey, enclosing his sketch and description of a device incorporating a telescope with a magnifying power of 80 times.


Having tested it at sea between London and Lyme Regis, Parlour believed that it would be steady enough for observing Jupiter’s satellites or the conjunction of the Moon with other stars, ‘even in a rough sea, and heavy swell’. These were grand claims, but the Board took Parlour's ideas seriously enough to organise a sea trial the following year. Sadly, the resulting report found that the apparatus was too difficult to manage.

Dockrey Celatone by Matthew Dockrey

Despite the ultimate failure of Parlour’s proposal, his design has continued to inspire. It was the inspiration for Longitude Punk’d competition winner, Celatone by Matthew Dockrey

Longitude Punk Putting the finishing touches to the display in the Royal Observatory










This is what Dockery had to say about his creation,

"I have contrived an interpretation of Samuel Parlour’s “apparatus to render a telescope manageable on shipboard” (RGO 14/30: 502-505), which was itself a reinvention of Galileo’s celatone.

As the design was already quite fantastical, most of the changes made were in order to fit within the size restriction of the competition. The spyglass is an antique, but all other parts were custom made. As per Parlour’s original design, the finder can be moved side to side, allowing the wearer to position it to match their interocular distance. However, in what I flatter myself is a slight improvement, this is accomplished by means of an adjustment screw which can easily be operated and carefully set while the device is being worn. The candle height is also adjustable, which I found to be particularly useful in properly illuminating the finder when using it at night. Overall, I have endeavored to create an item which feels both functional and fantastic, while honouring Parlour’s design and the technology of the era."

You can see Matthew Dockery’s Celatone  in Longitude Punk’d daily at the Royal Observatory until 4 January 2015.