draw equator

from mpl_toolkits.mplot3d import Axes3D 
import mpl_toolkits.mplot3d.art3d as art3d


fig = plt.figure()
ax=fig.gca(projection='3d')

for i in ["x","y","z"]:
  circle = Circle((0, 0), 1)
  circle.set_clip_box(ax.bbox)
  circle.set_edgecolor('b')
  circle.set_facecolor('none')  # "none" not None
  ax.add_patch(circle)
  art3d.pathpatch_2d_to_3d(circle, z=0, zdir=i)

plt.scatter(0, 0, s=800, facecolors='none', edgecolors='r') # alternative, looks not bad

plt.show()