2. Weight, by definition, is a measure of the force which must be applied to support an object (i.e. hold it at rest) in a gravitational field.The Earth’s gravitational field causes items near the Earth to have weight.
2.1. the Earth’s field is nearly uniform at all locations on the Earth’s surface
2.2. an object’s weight changes only slightly when it is moved from one location to another
3. The concept of amount is very old and predates recorded history, so any description of the early development of this concept is speculative in nature
4. A balance scale balances the force of one object’s weight against the force of another object’s weight
5. Johannes Kepler was the first to give an accurate description of the orbits of the planets, and by doing so; he was the first to describe gravitational mass.
5.1. In 1600 AD, Kepler sought employment with Tycho Brahe and consequently gained access to astronomical data of a higher precision than any previously available.
5.2. In Kepler’s final planetary model, he successfully described planetary orbits as following elliptical paths with the Sun at a focal point of the ellipse.
5.3. Kepler discovered that the square of the orbital period of each planet is directly proportional to the cube of the semi-major axis of its orbit, or equivalently, that the ratio of these two values is constant for all planets in the Solar System.
