An interesting question intruded upon yesterday's water staring: How do they calculate sea level when the level of the sea is always in flux? Waves, tides, rainfall, evaporation, the rotation of the planet... They must do some sort of averaging. So I asked the Google god.
Yup, averaging of a lot of data gathered over a long period.
However, a link at the bottom of that answer reminded me of another question: If the tide is caused by the gravitational pull of the moon, why are there high tides on opposite sides of the Earth? In my limited and easily confused mind, the oceans should only bulge toward the moon. A diagram would look like a lengthwise slice of hard boiled egg. The explanation offered here seems goofy.
The [ocean] bulge on the side of the Earth opposite the moon is caused by the moon "pulling the Earth away" from the water on that side.Wait. How can the moon pull the Earth but not the water on the opposite side? After all, Earth's gravitational pull is greater because of its greater mass. That's why the moon orbits the Earth instead of the other way around. The moon can pull the oceans because their mass is less than the moon. And because water is slippery.
Ah, but then I found a better, more complete, answer. With a visual aid. Pictures are good.
Not to scale, of course
Ah-ha! Centrifugal force! And the mind-blowing technicality (remember, my mind is easily blown) that the Earth and moon actually orbit each other. It's just that the center of their orbit is inside the Earth. And they both orbit the sun. In an ellipse. While the solar system orbits in the galaxy that's moving through space. Whoa, I'm getting dizzy. I need to sit down. On the beach. And watch the waves.