Unit 5Intro to Oceanography Part 2

Why the Oceans Matter

• National Geographic - Why the Oceans matter

Age of the Earth• The oldest rocks on the surface of the

earth have been dated to 3.8 billion years old

• Moon samples have been dated to 4.2 billion years old

• Meteorites from our solar system have been dated to 4.6 billion years old

• Based on these and other data, the accepted age of the earth is about 4.6 billion years old.

Natural Time Periods• How long does it take the earth to orbit

the sun?• 365 ¼ days which we call a year• How long does it take for the earth to

rotate on its axis?• 23.93 hours which we call a day

The Earth’s tilt• The earth is tilted at 23.5 degrees from the

vertical.• The earth moves along its orbit with its axis tilted

which accounts for the seasons• Here in the northern hemisphere we are tilted

towards the sun during part of the year (summer) and away from it during part of the year (winter)

Tropics• When the Northern hemisphere reaches its

maximum tilt towards the sun, the sun is directly over the 23.5 N latitude. This latitude line is called the Tropic of Cancer.

• When the southern hemisphere reaches its maximum tilt towards the sun, the sun is directly over the 23.5 S latitude. This latitude line is called the Tropic of Capricorn

• The zone of the earth that lies between these two latitudes is known as the tropics.

Solstice• On June 21, the sun is directly over the Tropic of

Cancer. We call it the summer solstice. It is also the longest day of the year in the Northern Hemisphere.

• On December 21, the sun is directly over the Tropic of Capricorn. We call it the winter solstice. It is also the shortest day of the year in the Northern Hemisphere.

Equinox• Halfway between the two solstices the sun is directly

over the equator. This occurs twice a year during the vernal equinox (March 21) and the autumnal equinox (September 23)

• Each season starts on a solstice or equinox

Arctic and Antarctic Circles

• On the summer solstice the sun does not sink below the horizon above 66.5 N latitude. This latitude is called the Arctic Circle.

• During the winter solstice the sun also does not rise above the Arctic Circle.

• The same thing happens in reverse at 66.5 S latitude which is called the Antarctic Circle

Shape of the Earth• Because the earth is spinning on its axis it tends

to bulge at the equator making it slightly pear shaped, just like a spinning water balloon.

Latitude and Longitude

• In order find our way around the earth and locate our position we need a reference system

• We use a grid system of right angle reference lines called latitude and longitude.

Latitude• Latitude lines (also called parallels) begin with

zero latitude at the equator and reach 90 degrees at the north and south poles.

• North of the equator, all latitude lines are designated N and south of the equator they are designated S

Longitude• Longitude lines (or meridians) are formed at

right angles to the latitude grid and run from pole to pole.

• Unlike latitude lines, longitude lines all cross each other (at the north and south poles)

Prime meridian• Longitude begins with zero at an arbitrary

chosen point called the prime meridian.

• The prime meridian is located at the Greenwich Observatory just outside London England.

• On the opposite side of the earth is the 180 degree line which also approximates the international date line.

• To the east of the prime meridian all longitudes are designated E and to the west they are designed W.

Nautical Miles (knots)• Each degree of latitude and longitude is divided

up into 60 minutes.• One minute of latitude is equal to one nautical

mile• Why do we use degrees of latitude and not

longitude to define the nautical mile?• The knot is a unit of speed equal to one nautical

mile per hour and is about 1.15 mph.

Charts and Maps• Charts and maps are used to show the earth’s 3-

dimensional shape on a flat or 2-dimensional surface.

• Maps display the earth’s land features• Charts display the earth’s sea and sky features

Map projections• Every type of map projection introduces some

type of distortion. Either shapes are distorted, or areas, or both.

Topographic maps and bathymetric

charts

• Maps that show lines connecting points of similar elevation are called topographic maps

• Charts that show lines connecting points of similar depth are called bathymetric charts

Finding latitude• Navigators since the ancient days have used the

sun and stars to find latitude.• The North Star provides latitude in the Northern

Hemisphere (easy)• The sun can provide latitude anywhere it is visible

if you know what day it is (more complicated)

Finding Latitude with Sextants

• Sextants are nautical instruments used to find latitude using the elevation of the sun or stars.

• Real sextants have complicated mirror and lens systems to protect your eyes from looking directly at the sun

Finding Longitude• Longitude cannot be found using the sun or

stars because the earth is constantly rotating.• Ancient mariners had to wait for the invention

of the clock in order to find longitude• To find longitude you must know the exact

time of noon at your location and the exact time of noon at the prime meridian.

• The difference between these two noon times will tell you how far around the earth you are from the prime meridian and hence your longitude.

• Each hour of difference equals 15 degrees of longitude.