Friday, September 19, 2008

Lesson 5: Latitude and Longitude

Need to Know

Lines on the surface of the earth, running from the true south-pole to the true north-pole are called Meridians of Longitude.

Lines on the surface of the earth which are parallel to the equatorial plane are called Parallels of Latitude.

Both latitude and longitude are needed to specify a single point (nothing more than a grid system).

There are only ninety (90) degrees of either north or south latitude. There are only one hundred eighty (180) degrees of either east or west longitude for a combined total of three hundred and sixty (360) degrees.

On a nautical chart, degrees of latitude are expressed vertically in a north-south direction and likewise, degrees of longitude are expressed horizontally in an east-west direction.

Reference to a recorded position or fix should be labeled either N or S for North or South latitude and E or W for East or West longitude. Directional labels are not interchangeable between latitude and longitude.

An unknown position on a chart is determined or taken from the chart by extending a horizontal line and a vertical line (forming the legs of a right angle) from a position on the chart’s towards its respective scales (either latitude and longitude) printed on the chart’s margins. At the point which each line precisely intersects on each scale, degrees, minutes, and seconds or tenths of a minute (DDDº MM.TT’) of both latitude and longitude can be read and recorded.

Likewise, a recorded or known position (fix) is indicated in degrees, minutes, and tenths of a minute (DDDº MM.TH’) of latitude and longitude and can be taken to the chart or plotted by simply extending a horizontal line from the indicated point on the latitude scale and a line vertically from the indicated point on the longitude scale. The intersection of the resulting right angle indicates a precise visual representation of a real-world position on the chart’s printed surface.

The earth takes 23 hours, 56 minutes, and 4.09054 seconds (side real time) to make one complete rotation (24 hour average or meantime). Therefore 360 degrees divided by 24 means that 1 (one) hour = 15 degrees = 900 nm.

Day Light saving time borrows the next time zone to the east.

1 comment:

esiu said...

I think you got a little confused there in your post. If the rotation of the earth was "24 hour average or meantime" then there would be no need for leap years. It IS ALWAYS 23h56m04s regardless of how you measure the time.

Also, "Therefore 360 degrees divided by 24 means that 1 (one) hour = 15 degrees = 900 nm" is mixing two things together. Assuming you are talking about the latitude, indeed every degree of latitude change will equal to an approximately the same distance of about 60nm (+/-0.4nm). This is an approximation due to earth's "slightly flattened" shape. It has nothing to do with 24 hours though. The earth rotates eastbound, which has nothing to do with the latitude. One degree of longitude cannot be expressed in terms of the distance unless the precise latitude is known. It ranges from about 60 nm on the equator to 0 nm on the poles (e.g. to describe the location of the North Pole you only have to say 90ºN and the longitude is irrelevant).

So technically speaking your statement "Both latitude and longitude are needed to specify a single point" is incorrect as two points exist that are specified by their latitudes alone.