The Moon and the Tides

Tides are the slow, regular rise and fall of the sea, and they are caused mainly by the gravitational pull of the Moon on the Earth's oceans.

Water is free to move, so the Moon's pull draws it into a bulge on the side of Earth facing the Moon. A second bulge forms on the opposite side, because there the Moon's pull on the water is weakest and the water is effectively left behind. So there are always two bulges of high water on the planet at once.

The Earth then rotates underneath those bulges. As a beach at Umhlanga or Kalk Bay is carried through each bulge and each dip, it experiences roughly two high tides and two low tides in every 24 hours and 50 minutes — so successive high tides come about 12 hours 25 minutes apart.

Why 24 h 50 min rather than 24 h? Because while the Earth spins, the Moon has moved a little further along its orbit — and that is also why the Moon rises about 50 minutes later each day.

Worked example. A tide table for a beach shows high tide at 06:00. When is the next high tide?

Successive high tides are about 12 hours 25 minutes apart:

So the next high tide is at about 18:25, and the low tide in between falls roughly halfway, at about 12:12.

This is not just a curiosity. Fishers, ski-boat crews, harbour pilots at Durban and Richards Bay, and anybody exploring the rock pools along the Cape coast plan around a tide table. Being caught on rocks by a rising spring tide is one of the most common coastal emergencies in South Africa — always check the table before you climb down.