I have a shapefile for world countries and I want to reproject it from GCS_WGS_1984 to Sphere_Eckert IV. However, it asks for Geographic Transformation; it says this is optional but it is required in order to proceed. I saw that the datum is D_Sphere but I cannot find a combination between this and WGS in the provided list.
Can please someone tell me what the right choice is? I use ArcGIS 10.1.