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The steradian (symbol: sr) or square radian is the SI unit of solid angle. It is used in three-dimensional geometry, and is analogous to the radian which quantifies planar angles. The name is derived from the Greek stereos for "solid" and the Latin radius for "ray, beam". The steradian, like the radian, is a dimensionless unit, essentially because a solid angle is the ratio between the area subtended and the square of its distance from the vertex: both the numerator and denominator of this ratio have dimens...

When we compare things, the best way to do so is to measure their attributes. Which is more expensive? Which is taller? Which takes longer? From a very young age children naturally begin to ask these questions when they are given choices. Today, 95% of the world uses the metric system, but this was not always so. A colorful variety of measurements have been used in the past. Many of these are easily recognized, however, there are some which are old-fashioned, foreign, or downright bizarre. Here are just ten...

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