The Moon has a four-week cycle; it is full every four weeks (actually every 29.5 days). But ocean tides exhibit a two-week cycle; they are large one week and then smaller the next.
Specifically, as in Fig. 1 below, ocean tides are stronger (“spring tides”) around New Moon and Full Moon than they are at First Quarter Moon and Last Quarter Moon (“neap tides”). The pattern is seen, roughly at least, all around the world (though the details are not simple, as they depend on the shape of the coastline and other factors.)
![](https://profmattstrassler.com/wp-content/uploads/2024/01/image-2-1024x454.png)
What’s behind this pattern? And what does it tell us about the Sun and Moon that we wouldn’t otherwise easily know? Perhaps surprisingly, it tells us that the average mass density of the Sun — its mass divided by its volume — is only a little smaller than the average mass density of the Moon. Here’s why.
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