(a–f) illustrate both the spatial and temporal frequency of city-level population points for different time periods. (a–d) represent the pre-modern period, from 3700 BC–AD 1800, and use the same scale to measure frequency of data points per city. (e,f) represent the modern period and shorter time frame per period and therefore the frequency scale is shorter and separated into thirds. (g) illustrates global mean centers (GMCs) for the same time periods. Each GMC is weighted by city population for each data point and was calculated and is pictured in the Goode Homolosine projection.