Question 1

What causes tides, and why are there two a day?

Accepted Answer

Tides are caused by the **difference in gravitational pull** across the Earth, primarily from the Moon and secondarily from the Sun. The two-a-day pattern follows from that difference rather than from gravity itself.

**Why there are two bulges.** This is the part that confuses people, since only one side faces the Moon.

Gravity weakens with distance. The Moon therefore pulls:

**Most strongly on the near side** of Earth — the water there is pulled toward the Moon, creating a bulge.

**Moderately on the Earth's centre.**

**Least on the far side** — so the far-side water is pulled **less** than the planet is, and effectively gets left behind, creating a second bulge on the opposite side.

What matters is not the Moon's gravity but the **gradient** of it across Earth's diameter — the **tidal force**, which is the difference between the pull at a point and the pull at the centre.

**Earth rotates through both bulges**, so most coasts experience two high and two low tides roughly every 24 hours 50 minutes — the extra 50 minutes because the Moon has moved along its orbit in the meantime, which is why tide times shift daily.

**The Sun contributes too**, at roughly **45%** of the Moon's tidal effect. Despite being vastly more massive, it is far further away, and tidal force falls with the **cube** of distance rather than the square.

**Spring and neap tides.** When Sun, Earth and Moon align — at full and new moon — the effects add, producing **spring tides** with the largest range. At the quarter moons they partly cancel, producing smaller **neap tides**. Spring has nothing to do with the season.

**Why local tides vary enormously.** The idealised model assumes a uniform ocean. In reality, coastline shape, water depth, basin resonance and Coriolis effects dominate. The Bay of Fundy sees over 16 metres; parts of the Mediterranean see centimetres, and some coasts have only one tide daily.

Question 2

Why does the Moon always show us the same face?

Accepted Answer

Because the Moon is tidally locked to Earth: it rotates on its axis exactly once for every orbit it completes. The two periods are synchronised, so the same hemisphere always faces us. A common

Question 3

What actually causes the seasons?

Accepted Answer

The seasons are caused by the tilt of Earth's rotational axis, about twenty-three and a half degrees relative to its orbital plane. They are not caused by distance from the sun, which is the most

Question 4

Does water really swirl the other way in the southern hemisphere?

Accepted Answer

Not in your sink, no. The Coriolis effect is real and genuinely does act in opposite directions in the two hemispheres — but it is far too weak to determine which way a basin drains. What the

Question 5

Why is the ocean salty?

Accepted Answer

Because rivers have been carrying dissolved minerals into the sea for billions of years, and the salt has nowhere to go. The main source is rock weathering. Rain is slightly acidic — atmospheric

Question 6

Why does the moon look bigger near the horizon?

Accepted Answer

It does not actually get bigger. The moon's angular size when it is low is essentially identical to when it is high, and if anything it is marginally smaller near the horizon, because you are

Question 7

Why does the sea look blue when water is colourless?

Accepted Answer

Two effects combine, and one of them surprises people: water is not actually colourless. Water genuinely absorbs red light. Water molecules absorb light at the red end of the visible spectrum through

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