Saturday, February 9, 2008


Finding your Location

Step 1
You can express your position by Latitude and Longitude

Step 2
Mapmakers or geographers use an imaginary grid to locate a place on a map. This grid is made up of intersecting lines of latitude and longitude.

Step 3
Lines of latitude run east and west around the globe. The latitude is measured in degrees north and south from the Equator.
Lines of longitude that run north and south from North pole to the South pole are measured in degrees east and west from the Prime Meridian.

Step 4
Prime Meridian runs through Greenwich, England. It is the planet’s Home Base.

Step 5
You can find the longitude of your location with an accurate sense of time. Before you can do any longitude calculations, you must convert your local zone time, as shown on your watch, to GMT (Greenwich Mean Time, the clock time back at Greenwich).

Step 6
The local zone time is the local clock time which is the mean solar time of central meridian of your local time zone.
In several places in the world, hundreds of different times were adopted, each one corresponding to its own meridian. To simplify this situation, the Earth surface was divided into 24 time zones, each one delimited by two meridian forming a hour angle of 1 hour at the poles. The mean solar time of the central meridian of each time zone was assigned by convention to all places belonging to the time zone.

Three Basic Ideas to find Longitude

Step 7
Basic Idea1: The first of these ideas is the relationship between time and the rotation of the Earth. It takes an average time of 24 hours for the Earth to rotate 360 degrees. If you divide the number degrees in a circle by the number of hours in a day, we find that the Earth turns 15 degrees each hour.
360° / 24 hours = 15° per hour
We can take this a step further and state that the Earth turns one degree in four minutes.
1 hour = 60 minutes / 15° = 4 minutes per degree.

Step 8
Basic Idea 2: The second idea is that we have to be careful about the difference between the events and time. Events like sunrise in the east always happen before the same event in the west. But time as shown on eastern clocks is later than on western clocks at the same instant. We can summarize this concept:
Local time earlier, position is westward.
Local time later, position is eastward.

Step 9
Basic Idea 3: The third and last idea needed for longitude is the applying of Equation of Time.
Whenever I've mentioned clock time, I've called it average time. Clock time and Sun time are different by as much as 16.5 minutes. The important thing is that if you're going to compare Sun time to the chronometer's clock time, you have to change the chronometer's clock time to Sun time so that you're comparing like terms. And that's what the Equation of Time does. You can find the Equation of Time from the Nautical Almanac.
By applying the Equation of Time to the chronometer's clock time, we convert Greenwich Mean Time (GMT:Clock time.) to Greenwich Apparent Time (GAT:Sun time). GAT is simply the Sun time back at Greenwich, England.

Step 10
Now we can observe Local Apparent Noon and do our simple subtraction of GAT to find our longitude.
It is noon at the very instant that the sun were right over your head.
Local Apparent Noon is simply noon for your exact location, and sets your watch to 12:00 based on Sun Time. The time of Local Apparent Noon, recorded as 12:00 local time, is compared to the time back in Greenwich.