Ocean tides vary during the Lunar month according to the combined influences of the Sun and Moon. In this animation the viewpoint is fixed relative to the Earth, and so the Sun and Moon appear to orbit clockwise. The Sun is much more massive than the Moon, but is also much further away. Thus, its gravitational pull (yellow arrow) is only about half that of the Moon (gray arrow).

Note the new moon (top, right) and spring tide (left) at the start of the month. As the month proceeds, the Sun's apparent motion is faster than the Moon's. Thus the Moon's gravitational pull lags behind that of the Sun, leading to a neap tide after 7 days. On day 14, the Moon is opposite the Sun, resulting in a full moon.

by Declan DePaor

>>see: p.414