Observe how the gravitational forces of the Moon and Sun combine to create Earth's tidal bulges.
Occur during New Moon and Full Moon phases. The Earth, Moon, and Sun are aligned (syzygy). Their gravitational pulls combine, creating the highest high tides and lowest low tides (a very pronounced tidal bulge).
Occur during First Quarter and Third Quarter moon phases. The Moon and Sun are at right angles to each other. The Sun's gravitational pull partially cancels out the Moon's pull, resulting in moderate tides (a more circular bulge).