Gravity, the spacecraft's own mass, and the angle of incidence of light on the sail. The definition of tacking for solar sails is that your vector is at an angl

Gravity, the spacecraft's own mass, and the angle of incidence of light on the sail. The definition of tacking for solar sails is that your vector is at an angle to that of the incident sunlight. In other words you are able to generate some sideways motion rather than just getting pushed.