杏吧原创

Shorelines

Question: Why do waves travel towards a shore no matter which way the wind
blows? Does this mean that there is a place in the centre of the ocean where the
water is still?

Answer: Shorelines are not all concentric, so if there were some still-water
source of waves, it would not be in the centre of the ocean. Anyway, still water
is not the origin of waves.

Waves travel in all directions relative to shorelines but only those that are
travelling towards a shoreline will reach it.

Those that are travelling away from or parallel to a shoreline either strike
another, distant shoreline or peter out as their amplitude shrinks and they
eventually become lost in the random surface disturbance.

Because most waves are caused by a steady wind blowing on an open sea, they
tend to form in a regular and parallel pattern鈥攊n fact it is possible to
steer a vessel using just the bearing of the open ocean swell.

However, on entering shallower water swells are refracted, because a wave
slows as it encounters the shelving sea floor beneath it. This is what
ultimately causes waves to line up with the shore and break as the lower part of
the waves slows in a line parallel to the shoreline and the faster top tumbles
forward.

Jon Richfield

Somerset West, South Africa

Answer: Out at sea, waves tend to travel in the same direction as the wind is
blowing. But the questioner is right鈥攊nshore they often move towards the
shoreline. This is caused by the effects of both diffraction and refraction.

Diffraction causes waves to spread out after they pass through a
gap鈥攕uch as a harbour mouth or entrance to an enclosed bay, as shown in
the diagram (below right)鈥攃ausing the waves to head towards all sides of
the harbour.

Refraction occurs when the waves travel into changed conditions, such as the
shallow water that is found near a shoreline. Have you ever noticed that on a
curved beach, the waves hit the beach in a parallel fashion all the way along?
This is because as the waves enter the shallower water, they change direction
(or refract), slow down and realign as shown in the diagram
(above left).

This means that you get a single long wave breaking along the beach, followed
by other single waves, rather than a number of individual waves that are
breaking randomly.

And, to answer the second part of the question, no, there is no place in the
centre of the ocean where the water is perpetually still.

Chris Bray

Avalon Beach, New South Wales

Lonely planet

Question: Although all the planets of the Solar System travel around the Sun
in the same direction, Venus actually rotates on its axis in the opposite
direction to all the other planets. My son wants to know why this is and I鈥檓
ashamed to admit that I couldn鈥檛 explain it to him. Can anyone help?

Answer: Astronomers assume that Venus started out spinning in the same
direction as the other planets in the Solar System鈥攚hich is the same way
that all the planets rotate about the Sun. So somehow its spin must have been
reversed.

One early idea was that, billions of years ago, Venus was hit by a massive
object that turned it upside down. This theory could still be true, but recent
research suggests that there are less violent ways to give Venus its
backspin.

The Sun raises bulges in Venus鈥檚 dense atmosphere, much like the tides in the
Earth鈥檚 oceans, and the heat of the Sun also creates bulges in the
atmosphere.

As these high-pressure bulges rotate around the planet, they exert a gentle
twisting force which could have gradually tilted Venus over until it was upside
down and spinning in its present direction. But this could only happen if Venus
started off with a high tilt鈥攊t would have had to be nearly on its side to
begin with. That, at least, has been the assumption until recently.

Now, the researchers Alexandre Correia and Jacques Laskar (Nature,
vol 411, p 767) have shown that this needn鈥檛 be so. Again, the atmospheric
bulges come into play, and so do occasional gentle tugs from the gravity of
passing planets such as Earth. The messy combination of tugs from the atmosphere
and other planets makes the whole system chaotic鈥攖he forces acting on
Venus add up in unpredictable ways. Models show that these forces could flip the
axis. Or, if Venus was never a planet that spun quickly, they could have acted
to slow the rotation and start it spinning in the other direction.

Venus鈥檚 unique combination of a thick atmosphere and being close to the Sun
make it much more likely to reverse than any other planet. Because these changes
are chaotic, the researchers can鈥檛 say that it was inevitable, only that there
was always a good chance.

Stephen Battersby

Physical sciences editor

New 杏吧原创

This week鈥檚 questions

Under a spell: We all know there are many words that are spelled differently
in American English than in British English. How did this arise and when?

Hamish Barjonas

London

Water everywhere: People are currently obsessed with drinking 2 litres or
more of water every day. Is there any evidence that this 鈥渄etoxifies鈥� you or is
in general good for you, as many people believe? If so, how does this work? How
much water do we really need?

Emma Dunkley

London