Editing Galilean moons (section)

=== Determination of longitude ===
{{main|History of longitude#Methods of determining longitude|l1=History of longitude}}
[[File:Carte de France corrigée par ordre du Roy.jpg|thumb|Map of France presented in 1684, showing the outline of a previous map (Sanson, light outline) compared to the new survey by Cassini and Picard using the moons of Jupiter as timing reference (heavier, shaded outline). The King of France reportedly quipped that the astronomers had taken more territory from him than his enemies.<ref name =rgm />]]
Galileo's discovery had practical applications. Safe navigation required accurately determining a ship's position at sea. While [[latitude]] could be measured well enough by local astronomical observations, determining [[longitude]] required knowledge of the time of each observation synchronized to the time at a reference longitude. The [[History of longitude|longitude problem]] was so important that large prizes were offered for its solution at various times by Spain, The Netherlands, and The United Kingdom.

Galileo proposed determining longitude based on the timing of the orbits of the Galilean moons.<ref>{{Cite book |last=Howse |first=Derek |url=https://archive.org/details/greenwichtimedis0000hows |title=Greenwich time and the discovery of the longitude |date=1980 |publisher=[[Oxford University Press]] |isbn=978-0-19-215948-9 |location=Oxford; New York |pages=12}}</ref> The times of the eclipses of the moons could be precisely calculated in advance and compared with local observations on land or on ship to determine the local time and hence longitude. Galileo applied in 1616 for the Spanish prize of 6,000 gold [[ducat]]s with a lifetime pension of 2,000 a year, and almost two decades later for the Dutch prize, but by then he was under house arrest for [[Galileo affair|possible heresy]].<ref name="danson-wtw">{{Cite book |last=Danson |first=Edwin |title=Weighing the World: The Quest to Measure the Earth |date=2006 |publisher=[[Oxford University Press]] |isbn=978-0-19-518169-2 |location=Oxford; New York, N.Y}}</ref>{{rp|15–16}}

The main problem with the Jovian moon technique was that it was difficult to observe the Galilean moons through a telescope on a moving ship, a problem that Galileo tried to solve with the invention of the [[celatone]]. Others suggested improvements, but without success.<ref name =rgm>{{cite web|url=https://www.rmg.co.uk/stories/blog/solving-longitude-jupiters-moons|title=Solving Longitude: Jupiter's Moons|date=16 October 2014|publisher=[[Royal Museums Greenwich]]}}</ref>

Land mapping surveys had the same problem determining longitude, though with less severe observational conditions. The method proved practical and was used by [[Giovanni Domenico Cassini]] and [[Jean Picard]] to re-map [[France]].<ref>{{cite book | last=Howse | first=Derek | title=Greenwich Time and the Longitude | publisher=Philip Wilson | year=1997 | pages=26, 31}}</ref>