The Equator, the Tropics of Cancer & Capricorn: Association with Earth-Sun Geometry

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  • 0:02 Latitude and Longitude
  • 1:32 The Equator
  • 2:35 The Tropics
  • 3:52 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 watching this video, you will be able to explain what the Equator, the Tropic of Cancer, and the Tropic of Capricorn are in terms of latitude and longitude. A short quiz will follow, allowing for further review of the material.

Latitude and Longitude

Every place on the surface of the Earth has a latitude and longitude. These are the coordinates we use to locate each point on the Earth's surface. Just like how you plot points on a graph with an x and y value, you can plot points on the Earth using a latitude and longitude.

Latitude is the angular distance of a place north or south of the Earth's Equator in degrees. A latitude of zero degrees is on the Equator of the Earth (we'll talk about that later). 90 degrees north is the North Pole, and 90 degrees south is the South Pole.

Longitude is the angular distance of a place east or west of the meridian at Greenwich, England. A longitude of zero degrees means it's directly north or south of Greenwich, England, and a longitude of 180 degrees west means it's half way around the world from Greenwich, when measured east to west (ignoring north to south). With these two numbers, any point on the Earth's surface can be located.

One way to remember which is which is to remember that latitude is like a ladder, because you use them to move up and down. 'Lat' and 'lad' sound the same. And longitude is...the other one - left to right.

There are two types of key lines we draw on our maps of the Earth. Longitude lines that go north to south are called meridians, and latitude lines that go east to west are called parallels. In today's lesson, we're going to talk about a few of the most important parallels we draw on the Earth: the Equator, the Tropic of Cancer and the Tropic of Capricorn.

The Equator

The Equator is one of the Earth's parallels - it's a line where everywhere on that line has a single latitude. In the case of the Equator, this is a latitude of zero degrees. Every latitude on the Earth is measured north or south from the Equator. But what is the Equator? Why is it positioned where it is?

Well, it's all based on the rotation of the Earth. The Earth rotates around an axis that goes through the North and South Pole. If you stood at the exact North Pole, you would literally be spinning on the spot (though you wouldn't be able to tell). The Equator is a line that marks the exact halfway point between the North Pole and the South Pole. The plane of the Equator is at 90 degrees to the Earth's rotational axis.

But did you know that the Equator changes during the year? That's because the rotation axis of the Earth isn't always exactly the same. It moves by about 50 feet over the course of the year, and so does the Equator.

The Tropics

The tropics are two other important parallels. They're individually called the Tropic of Cancer and the Tropic of Capricorn. The Tropic of Cancer is the most northern latitude on the Earth where the sun can appear directly overhead. The Tropic of Capricorn is the most southern latitude on the Earth where the sun can appear directly overhead. This case of the sun appearing directly overhead happens once a year in the very height of summer for each of the tropics. Remember that seasons depend on whether you're north or south of the Equator; when it's summer south of the Equator, it's winter north of the Equator.

The Tropic of Cancer is currently positioned at 23.4 degrees north of the Equator. And the Tropic of Capricorn is at approximately 23.4 degrees south of the Equator. This is because the Earth is tilted at 23.4 degrees.

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