The sky lights serve as a compass and a clock… when you can see them in good weather and when you are on steady land.
On the rolling seas, you could have a decent estimate of latitude by observing things like length of day etc., but not longitudes because they are only connected to the concept of time.
Making accurate sea-worthy clocks was an important requirement of longitude based navigation. Pendulum clocks used to fail at sea.. making longitude calculations all wrong… thus leading to being lost at sea. Also distance b/w longitudes in miles/degree varies with the latitude.
Merchant ships used to stick to latitudes alone for sailing, so the pirates knew where they could attack, the latitude based sea routes had lots of traffic.
Galileo said Jupiter’s moons’ eclipses could be used for timing, but his instrument could only be used on land. It went on to be used in map making.
When measurements became more exact… they realised that jupiter-moon eclipses changed slightly with jupiter’s distance from earth… that was when they first estimated … nearly correctly, the velocity of light…
Astronomers looked down at “mekaniks” making watches.. saying that only the skies told real time. This book also talks about the competition between the astronomical approach to time vs the mechanical approach to time.
The moon travels about a nakshatram (13.33) degrees a day and is half a degree wide, so they figured approx that the moon travels its own size in an hour… and tried using that too 🙂
When John Harrison made his clock, he made a table so that the mean time of his clock could be corrected to the true time of a sundial!!
And he was a poor self educated carpenter so he made his clocks out of wood that greases itself – lignum vitae and used a little brass when he had to..they still work 3 centuries later
He used different kinds of oak for each part.. including gears… he wanted an oiless clock that would work at sea… so he could win the prize for accurate longitude measurement at sea.
And his very first clock worked on the sea… but he kept refusing the prize and refining the clock. Everybody else thought it was perfect … he liked the third version.
He just kept taking 500# advances instead of the 20000# prize for many years.
I liked the first part of the book.. it’s about human ingenuity and spirit.
The last part of the book is like the book ’empire of the stars’…stupid politics… giving Harrison a hard time… instead of the prize. I didn’t like it.
Harrison should have taken the prize in the beginning. Later his competitors and enemies came to control the longitude board and did everything they could against him.
He luckily lived to 79 and the king found out and intervened.. and gave him compensation… but it was all sad mostly.