Why is the first ionisation energy of Potassium less than Sodium?

The first ionisation energy is the energy required to move one mole of electrons from one mole of atoms in its gaseous state.
The outer electron in Potassium is in the 4s orbital, which is further away from the nucleus than the 3s orbital of the Sodium. The greater distance means that the attraction between the electron is weaker.
The electron in Potassium is also more affected by shielding due to more shells, further weakening this attraction. This means that less energy is needed to remove the outermost electron and therefore the ionisation energy is lower.