This document discusses the history of determining longitude at sea. Early explorers could measure latitude using an astrolabe or quadrant to observe the North Star, but longitude was nearly impossible to calculate on pitching ships. The lunar distance method used moon observations but wasn't very accurate. An accurate clock was needed since longitude relates to time difference between locations. John Harrison invented an accurate marine chronometer, solving the longitude problem and enabling safe ocean navigation. Now GPS uses precise atomic clocks, functioning similarly to Harrison's invention.

1. THE SEARCH FOR
LONGITUDE

2. AT THE BEGINNING OF THE AGE OF EXPLORATION:
• People knew about lines of latitude and lines of longitude. But the only one
that could be measured at sea was LATITUDE
• (AND THAT WAS VERY DIFFICULT)

4. TO FIND LATITUDE AT SEA, NAVIGATORS USED AN
ASTROLABE ADAPTED TO MARINE USE
The angle between the North Star and the Horizon provided Latitude

5. LATER EXPLORERS USED A QUADRANT WHICH WAS EASIER
TO USE ON A PITCHING AND ROLLING SHIP ON THE OCEAN

6. BUT TO FIND LONGITUDE, ONE HAD TO DO THE
FOLLOWING:
• Make several star sights or sun sights to accurately determine the time
• Compare that with a place of known time(difficult to do when clocks weren’t
accurate)
• Complete about 5 hours of math
• This was impossible on a ship b/c it kept moving during the 5 hours you were
doing calculations.

7. ENTER THE LUNAR DISTANCE METHOD

8. ALL THAT WAS NEEDED WAS A CONVERSION TABLE TO
INTERPRET THE MOON’S POSITION INTO LONGITUDE
However they weren’t very accurate
As it took 18 years of observations of
The moon to accurately predict it’s orbit

9. EVERYONE KNEW THAT IF YOU HAD AN ACCURATE
CLOCK, FIGURING LONGITUDE WOULD BE EASY:
• The Earth rotates 360 degrees every 24 hours
• Every hour the earth rotates 15 degrees
• If you had a clock set to the time at the port you had just left and local time
on the ocean was 1 hour later than that time you were 15 degrees west of the
home port
• However no clocks at that time were accurate enough

10. REASONS CLOCKS WERE NOT ACCURATE ON THE
OCEAN:
• Temperature changes
• Humidity changes
• The motion of the ship

11. ENTER CLOCKMAKER JOHN HARRISON
• He made a clock that was accurate to use at sea. It wasn’t super accurate but it was better than what
they had. Plus it was huge.

12. THE NEXT CLOCK WAS SMALLER, BUT STILL NOT ACCURATE
ENOUGH

13. FINALLY HE MAKES ONE SIMILAR IN SIZE TO A POCKET
WATCH

14. This watch was accurate enough to handle a rough
Ocean voyage and was very accurate. With this watch
Navigators always knew what time it was in their home
Port and used the sun or stars to determine local time.
The difference in time easily told what the longitude was

15. SO WHAT DOES THIS HAVE TO DO WITH US?
The GPS in our phones, cars, boats, and planes determine
Our position by comparing what time it is in the receiver in our
Device and what time it is in the satellite
We are basically using an atomic clock version of John Harrison’s
H4 Watch

16. WORKS CITED:
• "Ancient Map with Longitude Only - Google Search." Ancient Map with Longitude Only - Google Search. Web. 12 Nov. 2014.
• Baudin, Michael. "John Harrison H4 Clock." Michel Baudins Blog. Web. 12 Nov. 2014.
• "The Collection." H1. Royal Museum Greenwhich Collection. Web. 12 Nov. 2014.
• "The Earth's Grid System." -- Kids Encyclopedia. Web. 12 Nov. 2014.
• "The Lunar Distance Method." National Museum of Australia -. Web. 12 Nov. 2014.
• "Map with Lines of Latitude Labeled - Google Search." Map with Lines of Latitude Labeled - Google Search. Web. 12 Nov.
2014.
• "Mariner's Astrolabe." Academic Dictionaries and Encyclopedias. Web. 12 Nov. 2014.
• Pickering, Keith A. "Columbus and Celestial Navigation." Columbus and Celestial Navigation. Web. 12 Nov. 2014. copyright
1997-2010
• Sobel, Dava. Longitude. Print.