If transported back to the 18th century could you solve the Longitude Problem without an accurate clock? Seeing an interesting BBC article today at http://www.bbc.co.uk/news/science-environment-23514521 about the Longitude Problem, I wondered if it could have been solved, in a way practical at the time (the 18th century), by any means other than the solution eventually found.
So if fanciful, but possibly entertaining and instructive, questions are allowed here, suppose you were transported back to the early 18th century and forbidden from disclosing any fundamental physics unknown at the time, explicitly or otherwise. Would you, knowing what we do today about classical physics and astronomy etc, have any ideas for bagging that prize money, aside from building a sufficiently accurate clock?
 A: John Harrison's marine chronometer H4 did in fact solve the longitude problem with 18th century technology although the OP is I think wondering if it could be done without constructing a "sufficiently accurate" marine chronometer. H4 was first tested at sea, on HMS Deptford, from November 18 1761 to January 21 1762 and lost only five seconds, which is better rate stability than a lot of modern quartz wristwatches.  (The crew is said to have been relieved to find the clock accurate when the Deptford found Madeira at the position predited by the chronometer as missing Madeira would have been ". . . inconvenient, as they were in want of Beer," as a contemporary account reports.)  
It was also incredibly complex and difficult to build and the design was a bit of a dead end but it was a phenomenal proof of concept.  The non-chronometer based methods for finding longitude relied on astronomical observations of (IIRC) eclipses of the Galilean moons of Jupiter, which are vulnerable to weather conditions and the difficulty of keeping a telescope stable on the deck of a ship.  Offhand the only non-chronometer contemporary tech I can think of that you could work on would be an improved telescope mount, but then again, if you could make one that answered, you would probably have something on which you could put a pendulum clock and you wouldn't need to invent the balance-and-spring dependent marine chronometer!
