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Elementary Science Education

# Determining Latitude and Longitude

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Bernard Nebel
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Review with your students how our ancestors created a grid system of latitude and longitude for mapping the world. Hence, any place on the Earth can be pinpointed by giving its latitude and longitude.

Consider early explorers sailing/trekking into the unknown. They had to determine their latitude and longitude to describe where they were and the location of what they found. How did they do this? The following video describes the concept of determining latitude by measuring the angle from between the horizon and the North Star.

The same process may be used with reference to other stars or the sun albeit adjustments need to made. Our text lesson describes how to do it with reference to the sun on the solstice and at solar noon when the sun is exactly 90 degrees over the equator. (I hope that one of you will post a video of doing this.) You can check your accuracy by comparing it with the most accurately determined latitude of your location, which you can find at:

Given the crudeness of our method described, I consider coming within +/- 3-4 degrees as doing very well. Early sailors/explorers had a more accurate instrument for measuring the angle of the sun above the horizon–the sextant. Study the diagram below to understand what a sextant is and how it works.

How a Sextant Works

There’s nothing mystical or complicated about a sextant. All it is is a device that measures the angle between two objects.

The sextant makes use of two mirrors. With this sextant, one of the mirrors ( mirror A in the diagram) is half-silvered, which allows some light to pass through. In navigating, you look at the horizon through this mirror.

The other mirror (mirror B in the diagram) is attached to a movable arm. Light from an object, let’s say the sun, reflects off this mirror. The arm can be moved to a position where the sun’s reflection off the mirror also reflects off mirror A and through the eyepiece. What you see when this happens is one object (the sun) superimposed on the other (the horizon). The angle between the two objects is then read off the scale.

What makes a sextant so useful in navigation is its accuracy. It can measure an angle with precision to the nearest ten seconds. (A degree is divided into 60 minutes; a minute is divided into 60 seconds.)

You can determine your longitude by contrasting Geenwich Mean Time (time at the prime meridian) with your local time. However, your local time needs to be adjusted forward or back such that such it will correspond to 12:00 noon occurring exactly at solar noon. To do this, see:

http://www.spot-on-sundials.co.uk/calculator.html

Greenwich Mean time can be found at: http://time.is/GMT

Calculate the time difference between Geenwich Mean Time and your local time. Then, on the bases of one hour equals 15 degrees (one minute equals 0.25 degrees) you can calculate your longitude. Check it at: