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credits: AURA/NOAO/NSF

If we look at the sky, the brightest and apparently largest objects are the sun and the moon. As we all well know, it would be incorrect to conclude that they are equally distant from the earth! How do astronomers then measure distances?

## Units of Measure

The first important thing that we will cover is the concept of unit of measure. I am sure that you are familiar with some of them: pounds, feet, years. Humans have agreed on some set of units in order to understand each other. I stress the fact that a unit of measurement is an agreement: length doesn't "prefer" to be measured in meters rather than feet, although people might.

Units are a matter of convenience. This is why most of the units you use in everyday life are easy to use: you never have to deal with numbers too big or to small. We can say that humans measure nature using themselves as "units"... one foot, one inch.

So far so good. One problem arises when we need to write measurements of quantities that are very small or very large. Say you wanted to write down the size of a human cell... would you use feet? You would have to carry five zeroes all the time, one cell is about 0.000001 feet. For the same reason, if you wanted to express the distance from here to the closest star (beyond the Sun) you would need about sixteen (!) zeroes. Can you imagine how much paper astronomers would waste?

This is the reason you need to spend some time in order to familiarize yourself with the kind of units that are used in astronomy. The very first step is to adopt the metric system. After all we were not born with six finger per hand.