air navigation almanac apollo spacecraft astrolabe atomic clock aviator azimuth bomber british merchant navy constellation dead reckoning earth ephemeris ex-meridian fixed-wing aircraft flight simulator history of longitude intercept method john harrison kamal latitude longitude longitude by chronometer marine chronometer measuring instrument meridian altitude moon nautical almanac nautical mile navigation navigational stars planet planetarium polaris position fixing quartz clock radio navigation royal air force sextant spherical geometry spica spirit level star clock sun the new york times thomas hubbard sumner trigonometry weather zenith