Spring and neap tides
What is a spring tide?
Spring tides have a larger difference between high and low water. They occur around new and full moons, when the Sun, Earth and Moon line up. The name is unrelated to the season of spring.
Twice a month, every month
Springs come at both new moon and full moon, because the tides do not care which side of Earth the Moon lines up on — a straight line is a straight line. So the rhythm is: springs, then about seven days later neaps, then springs again, riding the phases all month. It has run at that beat for as long as there has been an ocean.
Read this week’s tides off tonight’s moon
This is a genuinely usable trick: look at tonight’s phase. Full or new, and the biggest ranges of the fortnight are landing within a day or two — the ocean lags the alignment slightly. Half-lit, and the week’s tides are at their tamest. Then check the real numbers against your station’s predictions, which solve it properly instead of estimating from the phase.
Why some springs are king tides
Not all springs are equal. When the full or new moon also lands near the Moon’s closest approach to Earth — the supermoon coincidence — the two effects stack, and those perigean spring tides are the ones that flood low streets and make the news as king tides.
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What the angle looks like from above
The whole story is one angle: Sun–Earth–Moon. At 0° and 180° the two tidal stretches align and add; at 90° the Moon’s high water sits on the Sun’s low water. The Sun’s contribution is a bit under half the Moon’s — enormous mass, but tides fall off with the cube of distance, and the Sun is four hundred times farther away. That cube law is also why the nearer, smaller Moon runs the show at all.