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?


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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It has to do with timing. Much like a month is 30 days but it takes the moon 28 to complete a revolution. Which is why some months have 2 full moms and some months only 1. Same with tides, which occur every 12 hours and 25 minutes. So some see 2 high and low tide. And others not

High tide is about every 12 hours and 25 minutes. This means that most days you will have two of them.

Some days you will have a high tide around 6 in the morning and another at half past six in the evening.

However the high tide happens 25 minutes later every day.

Every so often you end up with high tide happening around noon. The high tide before that will have happened shortly before midnight the day before and the next one shortly after midnight when it is already the next day.

However when you have only one high tide you will still have two low tides in that day.

There are as many low tides as high tides.

So some days you will also have two high tide and one low tide.

As other stated: the moon is circling the earth, so it is slowly moving the start of the next tide a bit further everyday, having moved them around the earth for a whole day after one (lunar) month.

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.

The moon causes the tide by “pulling the water” towards it in a bulge with a second bulge on the opposite side of the earth. The earth then spins in this uneven bulging mass of water with one full rotation every 24hrs. The moon’s orbit 24hrs and 50mins means the bulges are moving at a slightly slower speed then the earth’s spin. Local tides can be slowed down if the bulge has to squeeze through narrow areas like the Gibraltar straits or the English channel. Buy like others have said they are regular but sometimes just the wrong side of midnight to be counted.