How do you measure the distance between two latitudes?

It is frequently observed that there is a distinct correlation between latitude and the wealth of nations. The continents along the equator, Africa and South America are the poorest. Even within Africa and South America this can be seen as the nations farthest from the equator are wealthier. In Africa the wealthiest nations are the three on the southern tip of the continent, South Africa, Botswana, and Namibia, and the countries of North Africa. Similarly in Latin America Argentina, Chile and Uruguay have long been the wealthiest. Within Asia, Indonesia, located on the equator, is among the poorest. The wealthiest nations of the world with the highest standard of living tend to be those at the northern extreme of areas open to human habitation, Canada, and the Nordic Countries. Within the wealthy continents, and even within large countries wealth increases with distance from the equator. Southern Europe has long been poorer as has the Southern United States.