Spring tides occur when the Sun and Moon line up, since the gravities of the two bodies add together to create larger tidal forces. Neap tides occur when the Sun and Moon are at right angles to one another as seen from Earth. In this case, the gravitational forces counteract each other, creating weaker tidal forces. — Neil F. Comins, "A New Slant on Earth", Astronomy, Vol. 20, Iss. 7; pg. 44, 6 pgs; Jul, 1992