How to play Tide Clock
- Choose a test and read the times, the period, or the speed on the card.
- Use T = Δt ÷ number of periods, low tide at T ÷ 2, n = 24 h ÷ T, and λ = v·T.
- Type your prediction and press Start. The days go by, sped up.
- Compare the orange lines with the white dots of the high tides on the right, or the orange bar with the blue one.
The period of the tides
Tides are the slowest waves most people ever watch. The Moon's gravity, helped by the Sun's, pulls the oceans into two bulges, one facing the Moon and one on the far side. As the Earth turns, a coast passes through both bulges, so the sea rises to high tide and falls to low tide twice. The Moon also moves along its orbit, so the Earth has to turn a little more than a day, 24 h 50 min, to face it again: the period of the tide is half of that, about 12 h 25 min. That is why high tide comes about 50 minutes later every day, and why some days have only one morning or one evening high tide. Low tide comes half a period after high tide. Out in the deep ocean the tide behaves like a shallow-water wave whose speed depends only on the depth, about 200 m/s over 4000 m of water, so its wavelength is thousands of kilometres. Real tides also change size through the month and depend a lot on the shape of each coast.
A focused learning model
The tide is an ideal sine wave with a period of 12 h 25 min and a height of 2 m above and below mean level. Real tides vary between spring and neap tides and with the shape of the coast. Time is sped up by the factor shown, so each run lasts about 4 s; the clock shows the time at the harbour. The game starts at high tide at 06:10. Answers within 2% are right.