Comparing tidal forces by eye

[steuard]

You've probably heard at some point that tides on Earth are mostly caused by the moon, along with some smaller but still noticeable effects from the sun. In other words, the two objects' tidal forces are comparably strong (rather than being many orders of magnitude different: Uranus doesn't appreciably affect our tides!). You've probably also heard

Comments

[jon_leonard]

That's pretty cool. (And fairly accessible, for that matter.)

[spoonless]

Neat!

[anonymous]

Cool. How much more intense would the tidal effect be if the Sun had the same density as the Moon? Would we have islands flooded twice a day?

[steuard]

We can get a first sense of how this would play out from the existing tidal interactions between moon and sun. Near a full moon or new moon, the tides tend to be somewhat higher (what's called a "spring tide", because the water springs up more than normal): the tidal effects of the moon and sun reinforce each other in those positions. Near a half moon, the two effects are perpendicular and tend to cancel out a bit, leading to lower tides (called a "neap tide", for some reason).

So if the sun's effect were as big as the moon's, I might guess that full and new moons would lead to tides twice as high as the real-life average (while neap tides might come close to having no tide at all, or maybe four small swells a day: I'm not sure). There are probably some islands out there that would be under water during a double-high tide (just as there are a few places today where land is only exposed at low tide), but those places are presumably already routinely submerged during big storms.