Spring tides and neap tides are caused by the gravitational interaction between the Earth, the Moon, and the Sun.
During a spring tide, the gravitational forces of the Moon and the Sun align, causing the highest high tides and the lowest low tides. This occurs during the Full Moon and New Moon phases. In these phases, the Sun, Earth, and Moon form a straight line, creating a greater gravitational pull. The combined effect of the Moon's and the Sun's gravity results in an increased tidal range, leading to spring tides.
On the other hand, during a neap tide, the gravitational forces of the Moon and the Sun counteract each other. This occurs during the First Quarter and Third Quarter phases of the Moon. In these phases, the Moon is at a right angle to the line between the Earth and the Sun. As a result, the gravitational pull is weaker, and the tidal range is smaller, leading to neap tides.