This is a summary of Chapter 1 in Bowditch. It is not everything we went over in the 1st three classes. Review your notes to see what is most important! Read Bowditch!
The Earth and its coordinates
The true sphere - All lines from the center of the sphere to the surface are equal, every radius is equal
Oblate Sheroid a sphere flattened at the poles. The earth is flattened due to the weight of the polar ice caps. The degree of flattening/bulging changes over time. For Navigation purposes we assume the Earth to be a True Sphere.
Axis of Rotation the line connecting the North and South Poles. Each day the Earth rotates once around its axis.
Magnetic North Pole The Earth has a magnetic field because the core is made up of metal. Like any magnet, it has 2 poles. The poles are the points of the magnet from which the magnet field flows from and then enters. The Magnetic North and South Poles have nothing to do with the geographic poles. In fact, the Magnetic North Pole is of south polarity when speaking magnets. The Magnetic North Pole is located in Canada and moves a small distance every year. This is important to Navigation because the needle of a Magnetic Compass points toward Magnetic North.
Coordinate System - A grid placed to determine location. This grid can be places over any surface, it is not limited to just the entire Earth. Disney World has a map to find rides. If you want "Space Mountain" you look up the name of the ride and it tells you that its at H-5. The letters are accorss the top of the map & numbers down the side. All you need to do to find "Space Mountain" is find the square where the H coulmn intersects with the 5 row.
The Earth's Coordinate System is made up of Latitude and Longitude. This grid is not made up of lines that are perpendicular to each other because the Earth is a sphere. The Longitude lines converge at the Poles.
Circles of the Earth
Great Circles a line that bisects a sphere. It cuts the Earth in half. It is the shortest distance between two points on the surface of the Earth. The circumference of the Earth. The largest circle created when the Earth is cut in two.
Equator the terrestrial great circle whose plane is perpendicular to the polar axis. It is midway between the poles. It is the line of 0 degrees Latitude and separates the Northern and Southern Hemispheres.
Meridian A Great Circle through the geographic poles of the Earth. All meridians meet at the polar axis. The lines of Longitude.
The lines of Longitude are numbered byt the angle created at the pole, between the prime meridian and the meridian of a point on the Earth, eastward or westward from the prime meridian measured through 180 degrees and labeled E or W to indicate the direction of measurement.
Upper Branch a line that connects the poles through a given point. The side of the meridian that you are are on.
Lower Branch the continuation of the upper branch on the complete other side of the polar axis. The prime meridian is the upper branch for Greenwich, England and the International Date Line is the lower branch.) The other half of the great cicle for the degree of longitude you are on. If the upper branch is East then the lower branch is West. The sum of the degrees of the Upper & Lower Branch is 180.
Prime Meridian a.k.a. the Greenwich Meridian. The meridian used as the origin for measurement of longitude
Small Circles on the Earth these circles do not intersect the Earth.
Parallels the lines of latitude, they are all small circles except the equator. Latitude is the angular distance from the equator measured northward or southward through 90 degrees, and labeled N or S to indicate the direction of measurement. It is the angle drawn from the center of the Earth to the center of the Earth.
Latitude is abbreviated L or Lat.; difference of latitude is abbreviated l.
Longitude is abbreviated Long., difference of longitude is abbreviated DLO.
Distance on the Earth
Distance the spacial separation of two points and is expressed as the length of a line joining them.
Nautical Mile it is equal to 1852 meters exactly or approx. 6076 feet. This is the most commonly used measure of distance for ocean navigators. It is roughly the length of one minute of latitude or of any great circle of the earth, including 1 minute of longitude at the equator.
Statute Mile A mile used on land and for navigation on the Great Lakes. 1 statute mile = 5280 feet
1 nautical mile is equal to 1.15 statute miles
Important Conversions
1 meter = 3.28 feet
1 fathom = 6 feet
1 League = 3 nautical miles
1 nautical mile = 6076 feet
Speed a rate of motion, or distance per unit of time
Knot a unit of speed used by ocean navigators.
1 knot = 1 nm/hour
A knot per hour is acceleration not speed!!!
Speed of Advance (SOA)the speed intended to be made along the track
Speed over Ground (SOG) the speed along the actual path (zigs & zags included!)
Speed through the water the speed of the water moving over the hull of the vessel. This includes the speed of the ship moving plus the speed of the water due to currents.
Speed Made Good (SMG) speed along the course made good - take away the zigs & zags to get how far you went in an hour.
