If we look at the sky, the brightest and apparently largest objects are the sun and the moon. The first important thing that we will cover is the concept of unit of measure. Humans have agreed on some set of units in order to understand each other.
Units are a matter of convenience.
So far so good. One problem arises when we need to write measurements of quantities that are very small or very large. You would have to carry five zeroes all the time, one cell is about 0.000001 feet. 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 .