In fact, the Moon does not produce this effect on its own. The rise and fall of sea level – the so-called tides – are also influenced by the Sun, depending on the intensity of the attraction force of the Sun and the Moon on our planet. Just as the Earth attracts the Moon, making it revolve around it, the Moon also attracts the Earth, only in a more subtle way. The gravitational pull of our satellite has little effect on the continents, which are solid, but considerably affects the surface of the oceans due to the fluidity, with great freedom of movement, of the water. Each day, the lunar influence causes ocean currents that generate two high tides (when the ocean faces the Moon and in opposition to it) and two low tides (in the intervals between highs). The Sun, even though it is 390 times farther from Earth than the Moon, also influences the behavior of the tides – although the solar attraction corresponds to only 46% of the lunar one.
Summary of the story: depending on the position of the two stars in relation to our planet, the tides behave differently. That’s where the lunar phases come in. When the Earth, the Moon and the Sun are aligned – or, as astronomers say, in opposition or conjunction -, the gravitational attraction of the last two adds up, amplifying its effect on the maritime mass. On the other hand, when the attractive forces of the Moon and the Sun oppose each other, there is almost no difference between high and low tide. But this game of forces is not the same everywhere, because the contour of the coast and the dimensions of the bottom of the sea also change the dimension of the tides. “In certain open regions, the water spreads over a large area and rises only a few centimeters at maximum tides. In others, such as a narrow sea arm, the level can rise several meters”, says oceanographer Joseph Harari, from the University of São Paulo (USP).
Space push-and-pull
NEW MOON
When the Earth, the Moon and the Sun align, the gravitational attraction exerted by the two stars on the oceans adds up, generating sea currents that cause a maximum rise in sea level in the direction of that line. It is the time of the highest high tides, called spring tides or maximums
WASHING MOON
In this lunar phase, the influence of the Sun and Moon on ocean tides decreases. On the night when half of the Moon is visible, the attraction reaches its lowest value. In Santos, on the coast of São Paulo, for example, the difference between high and low tide does not exceed 5 centimeters
FULL MOON
About two weeks after the New Moon, our satellite travels back to a position where it lines up with the Sun and Earth. This combination brings a new wave of maximum tides. On the beaches of Santos, the sea level can rise by around 1 meter during this period
CRESCENT MOON
Now, the Moon and the Sun form a right angle of 90º. In this situation, lunar gravitation opposes solar gravitation – they just don’t cancel each other out because the Moon, closer to Earth, exerts greater power of attraction. Even so, the level differences between high and low tides are much smaller and are called quadrature or minimum tides.
radical changes Sea level could rise 18 meters
There are some places on the planet where the influence of the moon’s phases on the tide is greatest. In the Bay of Fundy, Canada, the difference between high and low tides reaches 18 meters. On Mont Saint-Michel, on the coast of France, 14 meters. In the Derby region of Australia, 11 meters. In Cook Inlet, on the south coast of Alaska, the elevation reaches 9 meters