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Lesson 2: Measuring angles

# Measuring angles using a protractor

Learn to use a virtual protractor to measure angles. Created by Sal Khan.

## Want to join the conversation?

• Who invented the protractor
• Thomas Blundeville invented the protractor.
• Does 360° = 0°?
• Good question. 360º and 0º will be on the same place on a diagram, but they mean different things. Think of it this way. You walk onto a basketball court and step onto one of the painted circles. You have just stepped on the circle, so you have traveled 0º around the circle. Now you walk along the circle. When you have walked halfway around it you have traveled 180º. When you have walked all the way around the circle you have traveled 360º. You are in exactly the same physical place that you were when you started at 0º, but saying that you are at 360º tells people that you have traveled around the circle.
• does the angle have to be 100% accurate?
• For example, imagine that you are taking a point in the map. A simple 1 degree error, in 1 mile, that represents easily more than 10m in topographic (depending of the scale of the map). If you go to the astronomy area in NASA, 1 degree is an unacceptable mistake for the engineers.
• Is a angle used in everyday life?
• By humans? Of course. Maybe by nature as well... A leading theory on why moths follow light is based on the concept that the moth keeps a constant angle to the Moon (theta) to navigate at night. Introduce a closer light and the angle is compromised; hence the Moth circling in towards a light bulb. Since the moon is so far away the angle to the moth is relatively the same.
• are there negative angles, and if so is there a protractor that will measure them?
• no, there are no negative angles. once they go all the way around and pass the point, it goes back to 0 and starts all over again.
• Do you really need a protractor to see the degree? Is it really hard to tell by a human eye?
• You can guess with the human eye, but using a protractor will give you a more precise answer.
• At ,why do all of the angles have to be at 0 degrees?
• They don't. You can start at any angle but it would be harder because you would have to subtract the small number from the larger one; so that's why most people start at the 0 degree mark: you don't have to any extra math.
• what would the angle be called if it were smaller than 90 degrees?