Why are tides greatest during a full Moon or new Moon?

Tides are greatest during a full Moon or new Moon because the gravitational forces of the Moon and the Sun align in such a way that they reinforce each other, resulting in what are known as spring tides. During these phases of the Moon, the Earth, Moon, and Sun are in a straight line, leading to a combined gravitational pull that is stronger than when the Moon is in its first or third quarter phases. This alignment enhances the tidal effect, causing higher high tides and lower low tides.

The gravitational pull of the Moon is the primary driver of ocean tides, and the Sun also plays a significant role due to its large mass. When their forces align during these lunar phases, the resulting tidal force is maximized, creating the most extreme tidal conditions.