Longitude for insomniacs

My sleepless nights are spent looking at the skies.
The waxing and waning moon are a feast for the eye of the insomniac.
In the picture I took a couple of nights ago, on the border of the lit crescent, you can clearly see two big craters. They disturb the clean rim of sunlight/shadow transition. The rim of the cratered moon surface facing the sun is dark, the illuminated crater bed resembling a freckle on a white ivory nose, a little flaw on an impeccable sickle-shaped lit curve.
The bottom crater is called Petavius and is 188 km wide. It is named after the French Jesuit theologian Denis Pétau (not the best looker in the bunch if you glance at his -dare I say- ugly mug).
The other crater is 127 kilometers wide and sports the name Langrenus crater. It is named after Michel Florent van Langren, a selenographer (selenography: the scientific mapping of the moon) and engineer who lived and worked in the 17th century. He is regarded to have made the first map of the moon under the patronage of the Spanish Crown. I was familiar with the crater, but ignorant of where the name of the selenological bowl-shaped cavity actually was derived from.
Mr van Langren is quite the interesting Renaissance man as it turns out. No 'scolarly' education (although his father Arnold was in the same line of work), but skilled in navigation, cartography, cosmology, military, civil and naval engineering,...
I'll delve deeper into his longitude solving, time permitting. He devised a way of determining longitude through the moon at sea through use of the illumination and eclipse (i.e., darkening) of lunar mountains, frequent phenomena like the moons of Jupiter, that could be observed from all points of the earth.
The Brits claim it was a Yorkshire clockmaker who solved the longitude problem, albeit a century later (but I'm just saying this off the top of my head). I suppose it was much more accurate and reliable in absence of celestial objects but I'll definitely have to read up on that before I post anything stupid. I have a little paperback by Dava Sobel on my shelf on John Harrison, so that could probably answer my question.
As soon as I've finished the book on the Byzantinian empress Theodora (by Paolo Cesaretti), I'll get stuck into that one.