Function to define the boundary Earch polygon in longlat projection for a given resolution.
Earth_poly(resol = 300, crs = "+proj=moll +units=km")
a 'st_sfc' object with the Earth polygon.
is the number of subdivisions along the latitude coordinates and half the number of subdivisions along the longitude coordinates.
a string with the projection. Default is the Mollweide projection with units in kilometers.