Angles: measuring turn

Skaters, builders, even pizza cutters all care about one thing: angles! But what actually is an angle?

An angle is an amount of turn. Two arms meet at a point called the vertex, and we measure how far one arm has turned in degrees.

Watch this arm turn. Less than ninety degrees is acute. Exactly ninety is a right angle. Between ninety and one hundred and eighty is obtuse. One hundred and eighty is a straight angle. Past that, up to three hundred and sixty, is reflex. And all the way round is a full revolution!

Angles are everywhere. A skate ramp might rise at thirty degrees: acute. At three o'clock, the clock hands make a right angle. And cut a pizza into eight equal slices? Each slice is forty five degrees.

To measure an angle, grab a protractor. Put the centre on the vertex. Line up zero with one arm. Then read the scale that starts at zero, not the other one!

Here's a handy trick. Angles on a straight line add up to one hundred and eighty degrees. So if one angle is one hundred and ten, the other must be seventy.

And angles around a point add up to three hundred and sixty. One hundred and fifty, plus ninety, leaves one hundred and twenty for the missing angle.

So: angles measure turn in degrees. Centre and zero on your protractor. A straight line makes one eighty, and a full turn makes three sixty.