One week later, and this is the situation:
- 3dgeek couldn't resist explaining how tides work in a RE (I knew you couldn't! );
The tides happen (roughly) twice per day - one high tide when the moon is overhead and (*low tide*) when it's overhead on the opposite side of the world.
Because the moon's motion around the Earth is combined with the Earth's rotation, the tides are actually about 12.5 hours apart, not exactly 12. This fact of tide times really brings home the fact that tides are definitely related most strongly to the cycles of the moon. However, the sun actually adds (or subtracts) it's own tides on a precise 12 hour cycle. But because the sun is SO far away (at least in RET) it's tidal effects are rather small.
So if you look at water level charts, you see two sine-waves added together - one with that 12 hour cycle and another with a roughly 12.5 hour cycle
Explaining all of this subtlety is FAR beyond what FET can manage.
In RET, the explanation is really very simple.
3DGeek makes a valid point, in spite of admonition from Ga_x2. The equation for gravitational force is (and has been proven to be, with or without a flat Earth) (G(m1m2))/(r^2).
But there's one thing. The proportion of gravitational force between Sun&Earth and Moon&Earth are (in SI units) ((Msun)/((distance[Earth--Sun])^2))/(Mmoon/((distance[Earth--moon])^2)), which would be ((1.989 × (10^30)[kg])/((1.496 x (10^8)[km])^2))/((7.34767309 × (10^22)[kg])/((3.844 x (10^5)[km])^2)) = 178.726326. The sun has 178 times the gravitational pull that the moon has upon the Earth. The sun has a higher pull upon the Earth than the moon does. So the tides should, according to this logic, rise and fall with the sun. But it doesn't. What do you people make of that?
Two things you're misunderstanding about tides:
FIRSTLY:
The tides caused by the sun DO exist - if you look at the graphs, they are the sum of TWO sinewaves, a large amplitude swing with a period of about 12.5 hours due to the moon and a smaller wave with a 12 hour period due to the sun.
We see the total of those two superimposed waves.
Sailors and other people who care about tides talk about "neap tides" and "spring tides" (badly named!) - where the tide is less than or greater than "normal". Why? You'd think the moon was always at the same distance and always has the same gravity - so why are there these special tides?
The neap tide happens when the sun and moon are about 90 degrees apart in the sky and the sun is trying to raise the tidal level while the moon is depressing it (or vice-versa at low tide).
The spring tide happens when the sun and moon are either close together in the sky or on opposite sides of the earth and the two tidal effects reinforce each other - so you get a higher "high tide" and a lower "low tide".
Hence we most certainly
do see an effect from solar tides - it's just a lot smaller than lunar tides.
SECONDLY:
Tides don't depend only on the AMOUNT of gravity from a remote body - they depend only on the fact that the gravitational pull from sun/moon on one side of the earth is more than on the other.
So the key factor is the DIFFERENCE between the sun/moon's gravity on one side of the planet versus the other. Since the sun is about 400 times further away than the moon, the difference in distance between the two sides of the planet from the sun is a smaller percentage than that for the moon.
So even though the moon's gravity is FAR less than the sun's in absolute terms, it's proximity to us makes the tides much higher.
Sadly, although all of these effects are easily demonstrated in RET. NONE of these effects would be present in a flat earth...so...the two-tides-per-day thing is hand-waved away - along with issues of spring and neap tides.