Lessons Index:
6. CHART MAKING FOR NAVIGATORS
Osher Map Library
University of Southern Maine
Charting Neptune's Realm:
From Classical Mythology to Satellite Imagery
An exhibition at the Osher Map Library and Smith Center for Cartographic Education, University of Southern Maine, Portland, 4 April 2000 to 11 January 2001
Donald S. Johnson, guest curator
LONGITUDE
Osher Map Library Lesson
Charting Neptune's Realm
Peter Rice, Andy Alley
E. Presentation of the Lesson
(Item) (Activity) (Time/Elapsed Time)
1. Opening Statement (1 min/1 min)
To establish the location of an object on the surface of the
earth has been one of the chief concerns of mariners. Latitude
was determined by the position of the sun and the north star,
and sailors learned very quickly how this was done, but longitude
remained the problem. An error in longitude could mean that a
ship's captain would not reach the island where he could take
on water, or he could drive his ship ashore in the darkness because
he thought he was miles away from the land.
In this lesson we will examine the difficulty of establishing longitude and then you will be able to determine the longitude of this school.
2. History (6 min/7 min)
a. Eratosthenes
(276-194 BCE) had calculated the circumference of the earth as
twenty-five thousand miles.
b. Hipparchus
of Nicaea (160-125 BCE) invented trigonometry and first designated
latitude and longitude.
c. Review
latitude (See 'Latitude, Osher Map Library
Lesson 15')
3. Determining Longitude (5 min/12 min)
a. Local
Apparent Noon
b. Instant
Event; eclipse of the moon
c. Calculating
the time difference
4. Building the timepiece (5 min/17 min)
a. Portable;
sandglass
b. Accuracy;
A second every half hour leads to disaster.
c. The
Chronometer; John Harrison
5. Calculating Longitude and GMT (10 min/27 min)
a. Determine
current location longitude
b. Calculate
difference between current time and EST
c. Calculate
difference between sun time and GMT
6. Calculate longitude using LAN (15 min/ 42 min)
a. Mark
sun shadow
b. Determine
LAN
c. Compare
with calculated LAN
7. Review main points and clarify student questions (2 min/44 min)
8. Closing Statement (1 min/45 min)
With an accurate way to determine longitude, mariners could navigate
across the vast oceans to distant locations without fear of missing
an island or finding the rocky coast too soon. Boston harbor
became a place where a ship could be sent without fear of having
the skipper strike hard on some reef or shoal because he thought
he was one hundred miles away with plenty of sea room. John Harrison
was the man who invented the chronometer that was accurate enough
to place the world into the hands of the mariners.
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