Longitude: Definition & Examples

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  • 0:02 What Is Longitude?
  • 1:21 Figuring Out Longitude
  • 2:30 Examples
  • 4:05 Using a Map
  • 5:09 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain what longitude is, give some examples of longitudes around the world, and describe approximately how to figure out your longitude. A short quiz will follow.

What Is Longitude?

Coordinates are super useful. Two numbers: an x value and a y value, describe the exact position of any point you want to plot on a graph. It's a way of describing a location, and that's something we often need to do in real life, too.

Navigators, mappers, sailors and scientists need a way to describe an exact position on the Earth's surface. But the Earth's surface isn't flat like the graph paper: the Earth is a sphere. Or to be exact, an oblate spheroid, or flattened sphere.

So how do we describe a place on the Earth's surface? We use latitude and longitude, which are angles measured in degrees. Unfortunately people tend to get the two mixed up. So let's go through the meaning of the two words:

Latitude is the angular distance of a place north or south of the Earth's equator in degrees. It's the number of degrees you are north or south of the equator.

But the subject of today's lesson is longitude. Longitude is the angular distance of a place east or west of the meridian at Greenwich, England.

Zero degrees of longitude means you have to be either north or south of Greenwich. A longitude of 180 degrees west (or 180 degrees east) on the other hand, means you on the opposite side of the world to Greenwich when measured east to west.

Figuring out Longitude

To figure out your longitude, these days you can just do a Google search for the town in which you live. But if you didn't have the internet what would you do?

Most importantly, you would need to be able to contact Greenwich. You would need to figure out how much time has passed between noon in Greenwich and noon where you live. If noon where you live is after noon in Greenwich, you have a west longitude. If noon where you live is before noon in Greenwich, you have an east longitude. For every four minutes between noon and Greenwich, and noon where you live, that's one full degree of longitude. So, just divide the number of minutes by four, and you have your longitude!

The only thing that makes it a bit more complicated is that we use big time zones. So to really figure out when noon is where you live, you would have to put a stick in the ground and watch for when the shadows are as short as possible. This is called noon local time.

To summarize: Figure out the number of minutes between noon in Greenwich, and noon where you live (the time when the shadows are shortest). Divide that number by four. If noon for you came before noon in Greenwich, add the word east, otherwise add the word west. And then you're done.


Let's go through an example of how this might work. You call your friend in Greenwich, and ask them what time it is there. You find that it's around 11 am there, but it's only 6 am where you live. So Greenwich is going to reach noon before you do. You already know that your latitude will be west, not east. You ask your friend to call you when it's exactly noon.

An hour later, they call you, and you start your stopwatch. Then you go outside and put a stick in the ground. You watch the shadow carefully all morning as it gets shorter and shorter. You measure the shadow and note down the time on the stopwatch for each measurement. Using your data, you figure out the stopwatch time when the shadow is at the shortest it ever gets.

It turns out that the shadows were shortest when the stopwatch had a time of 4 hours and 56 minutes. That's 296 minutes in total. You divide that number by 4 to get 74. So, your longitude is 74 degrees west.

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