Eli5:How come some days will have two high tides and two low tides whereas other days have one high tide and two low tides?
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On a simple level, Tides are driven by the Moon, and the Moon’s position in the sky at one time – say, sunset, shifts backwards a bit each day. So every day the tide comes a little later. So at some time the last tide of a day will be just before midnight, and the ‘4th tide’ of that day will shift until it is after midnight the next day.
But tides are not really that simple. The Moon sets up regular tides on a global scale, but on a local scale, those tides are shifted by the local underwater geography in complex ways. This can cause tides to be shifted in strange ways. Especially around ‘neap tides’, when the tidal variance is small, a high tide can catch up with the previous low tide and overwhelm it, leading to a day with only 2 tides. Or a tide can be sped up by a bit, arriving a little before midnight and leading to 5 tides in a day. Or in straits that connect to ocean basins, you get ties from one side and tides from the other, and you can get up to 8 tides in a day. While a simple concept, when broken down to measurements in any one place, it becomes very complex.
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