... One possible application of a solar sail is a spacecraft to warn Earth of approaching interplanetary disturbances and shocks. ... Spacecraft that can provide such a warning are currently stationed at (or near) the Lagrangian L1 point , between the Earth and the Sun--at about 4 times the distance of the moon and 1/100 times that of the Sun. ... Because of the greater distance from Earth, the Earth's pull there is weaker, not enough to make the spacecraft match the Earth's motion around the Sun. ...