Meeting # 4 – September 15, 2008
Navigating – Finding the Way There & Back
Oceans permit human movement over surface if safely boated
Topics:
1. Piloting
Coastal navigation
2. Magnetic Compass
3. Dead Reckoning
4. Maps and Charts
Latitude Determination
Longitude Determination
5. Celestial Navigation
North
Star/Polar Star
Sun rise/set/meridian
Cross
staff/quadrant/sextant
6. Sailing a Parallel -- Verrazzano
7. Experienced navigator/captains
1. Piloting:
Short-distance travel the norm for millennia
Only sail for
what can be seen – no winter/nighttime sailing
Point-to-point trips accomplished through various acts of piloting
Use of visual
aids – shoreline configurations; high points
markers/buoys/lighthouses
entering ports by lining up
markers
moving along by checking
depth/bottom content (sand/mud)
Repeated trips build up knowledge
of familiar waters
Communicated from sailor to sailor
Sailing Instructions drawn --
printed –
Medieval sailing routes selected for the brevity of time
spent out of sight of land:
Circling Mediterranean more than
crossing it; island-hopping
Hugging the shores of the Baltic and
North Sea; the English Channel; the French and Iberian coasts
1000 -- Vikings to America à Bergen to Faroes to Iceland to Greenland to Labrador
[no compass]
1420s -- Chinese to East Africa à sailed upper crescent of Indian Ocean [compass]
1460s --Portuguese to West Africa à Lisbon to Maderias to
Canaries to Cape Verde [compass]
2. Magnetic Compass
Introduction of magnetic compass into the West as a navigational aid -- early 14th
century
Of Chinese invention
Magnetized needle floating free above a compass card/rose
Needle pointed to magnetic north –
differed/s from true north by degrees of variation
Card indicating N/S/E/W – the four
cardinal points
32 points – N/NNE/NE/ENE/E/ESE….
360°
Compass operates independent of cloud coverage; is affected
by bad weather/ship rolls à gimbals to mitigate effect
Sailors become more risk-taking with
compass – sail longer, later in year, farther from shore….
3. Navigationally useful maps à
charts/portulani – early 14th century
Sailing directions point-to-point rendered as visualizations/lines on paper
Lines on latitude/longitude grid
Latitude – distance from Equator; Equator = midpoint of sun’s apparent N/S sweep of earth
Longitude – distance from arbitrary meridian line passing through the poles
Home port -- Seville/Paris/London/Washington DC
15th century charts – horizontal lines/parallels/markings
of latitude from 00°at
Equator to 90°at
North Pole
Toscanelli
Map (1473) – Known to Columbus; likely relied upon its gross
underestimation of the distance between
Spain and the Indies
Mercator Projection – Adjusted a projection’s ratio
of latitude to longitude so as to permit a course to be drawn as a straight line
Lat/lon boxes become more pronouncedly portrait as move toward the poles
– distortion exaggerates the size of polar locales
4. Charts of use by navigators in Dead Reckoning:
Genoa to Barcelona –[know Genoa/Barcelona lat/long coordinates]
straight/rhumb line at angle of 230°
(west of southwest) – compass reading
of boat’s heading
Magnetic North
is 130°over
helmsman’s right shoulder
Distance = 500 nautical miles
If able to sail rhumb line course of 230°
Speed – measured by distance boat covers/time
Boat length – 60’
Lapsed time for float in water to go from bow to stern – 10
seconds
60’ in 10 seconds à 360’ in 1 minute à 21,600’/1 hour 21,600/6000’ = 3.6 knots (nautical
miles/hr)
87 nautical miles in 24 hours/ à 5 ½ day transit
Complications:
Can’t sail rhumb line – tacking to either side
Can’t maintain constant speed
Current pushing you (“being set”) sideways even as compass bearing remains
constant
Errors cumulative with gap between dependable “fixes”
Columbus primarily/exclusively a dead-reckoning navigator
Overestimated his speed; under-estimated the girth of the globe by 1/3rd
But did steer a pretty straight course from the Canaries to Bahamas
5. Reliance upon sky/celestial objects as guides –
Celestial Navigation
Offshore/Off Soundings aids – The visible sky
Sun – rises in east/meridian in
south/sets in the west
Moon –
Planets --
Venus/Mars/Jupiter/Saturn/Mercury
Stars – Many of the brightest, like
the planets, only seasonally useful
Exception à Stars – North/Polar
Star
Consistently in the northern sky – Near
the Big Dipper
Appears higher the sky as viewer proceeds north – directly overhead at North
Pole (90°
above horizon)
not visible in southern hemisphere
When sailing westward, Polar star on
ship’s starboard side; if higher in sky on successive nights, ship moving
northward as well as westward;
if lower in sky on successive
nights, ship moving southward as well as westward
Instruments for measuring angle – variations on a
protractor, with 0°
at horizon – declination angle between line to star and horizon
cross staff/quadrant/sextant
Cross-Staff
Sextant
Sun – Angle with horizon at local noon/when sun’s path
across southern sky reaches its meridian (highest point in sky)
Need to know date and where the sun is overhead relative to the Equator
March 21 – directly over Equator and moving northward
June 21 -- directly over Tropic of
Cancer (23.5°
North Latitude and turning down/southward)
September 21 – directly over Tropic of Capricorn (23.5°South Latitude and turning up/northward)
Local Latitude between March 21 and September 21 (when sun in northern hemisphere): Observed declination minus (-) Sun’s declination on given day
Local Latitude between September 21 and March 21 (when sun
in southern hemisphere): Observed declination plus (+) Sun’s declination on
given day
Sun’s declination on September 15th: +3 degrees above Equator
Sun’s declination on September 28th: -3 degrees below Equator
Battery, NYC – Observed Sun’s declination 9/15: 44° --NYC Latitude = 44-3
= 41° North Latitude
Observed Sun’s
declination 9/28: 38°--
NYC Latitude = 30+3 = 41°North Latitude
1 degree of Latitude = 60 nautical miles
1 minute of Latitude = 1 nautical mile
Good navigators can use sextant to get accurate shootings within 10 minutes
(= 6 nautical miles) or errors
resolvable upon shore sighting by visual corrections
GPS gives Latitude in Degrees/Minutes/and decimal of Minutes -- Battery = 40°42’ .500 – down to
yards
Verrazzano – Got himself a Parallel/Latitude (32°North
Latitude) on northwest corner of Madeira Islands and sailed it westward to
NCarolina coast
regularly checking his N/S position
with fixes against North Star at night and the sun at noontime
To maintain latitude:
North Star should have remained on
the ship’s starboard side at 32° above the horizon
Sun should have remained in southern
sky at noon but 15’ (1/4 degree) higher
in the sky every day (a post December
21/winter passage)
On February 1st – Sun’s reported declination = + 17 degrees
south of Equator
If on course, at 32°
North, Noontime Sun in South, 49°above the horizon/41° below being directly
overhead
Longitude – Much trickier to determine at sea
Solution not until 18th C – a reliable clock at sea that could keep
track of GMT [John Harrison
chronometer]
What was GMT when ship at LANoontime? – Difference in hours/minutes resolvable
into angular difference between prime meridian and ship’s meridian/longitude
Each hour = 15 degrees of Longitude (360° divided by 24 hours)
In Western Atlantic, GMT Noon occurred
earlier than LAN -- If difference = 1
hour, then ship’s longitude = 15°West Longitude; if 2 hours, 30° West Longitude
NYC Longitude = 71°West
Longitude – LAN occurs nearly six hours (5 hours and 45 minutes) later than it
does at Greenwich (uncorrected for time zones)
Navigators pre-18th century reliant upon dead reckoning estimates (speed variable) for east-west location crossing Atlantic – How many days from home? Last shore point??
There when we see it…. Smell it… see the birds… hear the
surf….when I tell you, now get below and keep praying….