018.3 Timing the Tides

To a first approximation, the Earth simply rotates beneath the tidal bulges raised by the Moon's gravity.

If the bulges did not move, the rotating Earth would carry any given point under a bulge every 12 hours. That would make the tides 12 hours apart.

But the bulges do move, to follow the Moon in its (approximately) 30 day orbit of the Earth. In one day, they rotate through 360°/30 or 12°. In 12 hours, they rotate through 6° so, after turning for 12 hours, the Earth must turn another 6° to catch the next bulge.

The Earth turns 360° in 24 hours or 360°/24 = 15° per hour. To turn 6° takes it 24 minutes, so the time from one high tide to the next is about 12 hours and 24 minutes.