How does SceneKit apply a texture to a sphere primitive?

I've got some simple SceneKit code to display a representation of the Earth:

  let earthNode = SCNNode()
  earthNode.geometry = SCNSphere(radius: CGFloat(Double.earthRadius))
  scene.rootNode.addChildNode(earthNode)
  var mat = SCNMaterial()
  mat.diffuse.contents = NSImage(named: NSImage.Name(rawValue: "world.topo.bathy.200408.3x1600x800"))
  earthNode.geometry?.materials = [mat]

This seems to work well enough; I get a nice sphere with a decent texture map (not sure if the projection ends up being accurate; not worried about that right now).

My question is this: How is this image mapped onto the sphere? I assume the left edge of my image probably starts where one of the basis planes intersects the sphere, but which plane would that be? The x-y plane? The x-z plane?


Thanks.