Economists use per capita gdp to measure and compare nations relative

Economists use per capita gdp to measure and compare nations relative income. Originally, per capita income is the average annual income of every person from particular area. This sup is usually calculated by dividing total income by its total population.
But it also can be defined as a measure of prosperity, which means that it also can be used in order to compare the wealth of one population with some others ones. I think that the second part of the answer is the most suitable for this question.
Hope that helps!

Answerweathering;

Why the planet stays in orbit is because of inertia (object stays in motion until a force changes it’s motion) also the atmosphere and gravity!