# Why does everyone get tides wrong!?

We were having a beautiful evening back home in Antwerp, at the harbor. At one point Simon started jotting some calculations on the reverse side of the menu sheet (he wanted to use a napkin at first). Turned out he was working out the Moon’s gravitational pull:

The number he eventually got was really tiny (0.00003433 m/s^2). So the Moon’s gravitational pull alone, he concluded, cannot be responsible for the ocean tides! Simon later concluded that the number should actually be even smaller because he should have taken the difference beween the Moon’s gravitational pull on the center of the Earth and on the side of the Earth that is closer to the Moon.

This is exactly what he did for calculating the gravitational pull of the Sun: he calculated that the difference between the Sun’s gravitational pull on the center of the Earth and its gravitational pull on the side closer to the Sun is negligible in terms of directly effecting the bulges phenomenon:

Why are most explanations of tides so focused on saying it’s the Moon’s gravity “pulling” on the Earth’s water? Why aren’t smaller water reservoirs like lakes or even swimming pools experiencing tides then, if the Moon is pulling on everything? Simon told me his favorite astrophysics channel PBS Spacetime has made a lecture on this. We have also found this wonderful interview with Gabe Perez-Giz, an astronomer and astrophysicist at NYU, interviewed by PBS Space Time, saying that “Every YouTube video I’ve ever seen about tides, including ones made by smart people, explains tides incorrectly.”

“Our tidal bulges are actually the product of a complex dance of gravity between the Moon, Earth, and Sun. And the total effect is more of a ‘push’ than a ‘pull’ on Earth’s water”, Gabe Perez-Giz says in the interview. Since the Earth’s own gravity exerted on the water molecules is so much stronger than that of the Moon, the ocean bulges are not strictly a result of which side of the Earth is facing the Moon, but of the cumulative effect that the myriads of water molecules nudging one an other and changing the pressure. This is why we don’t see that effect in smaller water basins.

The molecules on the poles experience more gravitational acceleration towards the Earth than the ones on the equator. Simon got hooked on that idea and continued his calculations the next day, this time trying to work out the difference between the Earth’s gravitational pull on the equator as opposed to that on the poles: what he came up with was a difference of 1%, a significant factor in creating the tidal bulges.