Roy Harvie, age 13, of Peterborough,

Why are there two high tides a day?

The tides, as we all know, are caused by the pull of gravity from the moon, High tides follow the moon as it passes overhead around the globe. Yet, we also know that the moon passes overhead only once every 24 hours, One high tide and one low tide a day seem reasonable, There are two high and two low tides, mainly because the world‑wide ocean is liquid water,

The pull of the moon overhead drags its extra water from half way around. the globe. This forms two troughs of low water on opposite sides of the world, The water on the far side of the low troughs lags behind and piles up into a high tide of its own, There are always two high tides, one under the moon and one on the opposite side of the globe, Between them are the low tides, In 24 hours, high tide and low tide, high tide and low tide, follow each other around the world.