I could not be measured -
there wasn't the time
Yet my knowledge was treasured
to help reach friendly climes
My ignorance - feared,
for it might lead to sinking
Now I'm handily found -
it's a matter of lateral thinking
Who or what am I? And what are the historical period and/or events being referenced?
Edit: Congratulations to Laconic Druid for posting a correct answer. Here are some notes with additional details to complement his/her solution:
(obviously, these notes contain major spoilers)
This riddle is about the story of longitude, a fascinating chapter in the history of sea navigation, science and technology. The historical background is that ships have always needed to know their location (latitude and longitude) to navigate successfully and safely. Measuring latitude is easy by simple reference to celestial bodies, but to measure longitude one needs to also know the time at a specific reference point on Earth, say at the port from which you departed. As silly as it sounds these days, measuring time accurately aboard a ship proved to be a major technological challenge that hindered the goal of measuring longitude, even long after accurate land-based pendulum clocks were developed. This and related navigational difficulties caused major sea disasters such as the Scilly naval disaster of 1707 (1500 fatalities and 4 ships lost). The problem was so vexing that large monetary rewards were declared by the British government and the King of Spain to the first person to design a reliable system of measuring longitude.
The longitude problem was eventually solved in the second half of the 18th century by John Harrison, a clockmaker who (after dedicating much of his life to solving the longitude problem) developed the marine chronometer, a compact but very accurate clock.
Finally, here is a line-by-line explanation of the riddle:
"I could not be measured - there wasn't the time"
Longitude could not be measured, for lack of an accurate time-measuring device.
"Yet my knowledge was treasured to help reach friendly climes"
Knowledge of longitude was highly valued (treasured) to help ensure the safe arrival of a ship at its destination.
"My ignorance - feared, for it might lead to sinking"
Not being able to tell where you are while at sea (through ignorance of your longitude) was feared, and could lead to disaster.
"Now I'm handily found"
These days, if you want to know your longitude, just pull out your smartphone and your GPS/compass/navigation app will tell it to you. Very handy (and makes it easy to forget that we are relying on centuries of scientific and technological innovation, and many billions of dollars worth of infrastructure - the 31 Global Positioning System satellites - to tell us this mundane piece of information that was once considered so valuable and elusive).
"it's a matter of lateral thinking"
Aside from the Puzzling-related notion of lateral thinking ostensibly required to solve the riddle, note that longitude represents one's position on the surface of the Earth along the lateral (east-west) axis or direction. Hence this is (I claim) a true lateral thinking riddle.