Astrological Tides

On Portuguese TV a well-known Portuguese astrologer recently referred the tidal effect as scientific evidence for Astrology. The reasoning was that the Moon causes tides in the oceans, and since a large quantity of the human body is water, this scientifically confirms the Moon’s effect on people.

Unfortunately, the reasoning is wrong. As anyone can see from the absence of tides in soup bowls, bathtubs or swimming pools, the tide effect is a bit more complex that astrologers apparently believe.

What are Tides?

It is true that tides are partially caused by the gravitational pull of the Moon (and the Sun too). But it is wrong to think that the Moon is pulling the water up the coastline. The Moon’s gravity affects not only the water in the oceans but also the rock in the continents, and is unable to distinguish one material from another. Besides it is so weak that (the force exerted on any object is hundreds of thousands of times smaller than its weight) that its effect is negligible concerning Earth’s tides.

The main point is that the Moon is not really orbiting around the Earth. Though this is a good approximation, in reality they both move around a point between the Earth and the Moon. This point is inside the Earth, but about 4.700 Km from the Earth’s centre. The circular movement around this point will exert a centrifugal force on everything on Earth (like when swinging a stone tied to a string), but since it doesn’t coincide with the Earth’s centre, in most places this centrifugal force won’t be pointing straight up but slightly sideways too.

And this is the most important part in understanding tides: it isn’t about a great force which pulls water upwards, but a tiny force which pulls water sideways. The effect is so small that on 60Kg person the force is about half a gram (click here if you wish to see the calculations), the weight of a small piece of paper. But when applied to an ocean, it can drag a large amount of water.

Near the coastline, where the oceans are shallow, water rise several meters due to the way the coastline and the ocean bottom can funnel the water, causing it to pile up. But in smaller, more isolated, seas such as the Mediterranean the water displaced isn’t enough to cause significant tides.

Lakes, swimming pools or people are far too small for this effect.

What effect does the Moon have on people?

First of all, if people think the Moon affects them, then it probably will. But in this case the people are affecting themselves, and we should not blame it on the Moon.

The gravitational attraction of the Moon causes slight changes in our weight as it pulls us in different directions with different intensities. But this change is so tiny that clipping your fingernails could produce a bigger effect. Besides, neither the Moon nor its gravitational pull is aware of our birthdays, so the Moon can hardly affect us differently according to our Astrological sign, ascendant or whatever.

Science demands above all the honesty to take the facts as they are, and is thus an inadequate justification for myth and fantasy.


We would like to thank the support of the members of the Astroalert mailing list at Whitewater University.

Ludwig Krippahl


All the material of the authorship of the Associação Cépticos de Portugal is public domain and can be used freely for non-profit purposes. Please refer CEPO as a source if using this material.