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Questions and answers provided by Science Line's Dial-a-Scientist on 0345 600444

Q What causes the seasons?

A They are NOT caused by the slight elliptical shape of the Earth's orbit - otherwise how could Australia have summer when Britain has winter? It is because the Earth rotates about an axis that runs from the north to the south pole. This axis is tilted by 68 degrees to the imaginary plane on which all the planets orbit the Sun. As the Earth orbits, this tilt rotates around an imaginary vertical to the plane of orbit. So when we have winter, the tilt of our hemisphere is away from the Sun.

Ironically, the elliptical orbit means we are actually closest to the Sun when we have winter in the northern hemisphere. But the axis is tilted away, so the sunlight strikes the northern atmosphere at a lower angle. Summer is warmer because the rays from the Sun penetrate the atmosphere more directly.

Q What's the big crunch theory in astronomy?

A Physicists hold that if the universe contains enough matter, then its present expansion - the result of the original "singularity", or Big Bang - will one day be halted by the gravity of all its matter, and then begin to contract. This will culminate in a "Big Crunch" in which all matter is crushed out of existence.

The key unanswered question is: does the universe contain enough matter to cause this, or will it just continue expanding forever?

Q Why does an egg that has gone bad float?

A As soon as an egg is laid, it starts to lose moisture through its shell. This moisture is replaced by air which makes the air sac in the egg larger and the egg less dense. At the same time, proteins in the egg white denature, breaking up to produce hydrogen sulphide - giving the egg its pungent "off" smell. This gas also makes the egg lighter. Hence it floats where a fresh one won't.

Q When you exercise, does the blood supply to the brain increase or decrease?

A Neither: it stays the same. The average heart pumps five litres of blood around the resting body every minute. Around 750 millilitres of this go to the brain and 600 millilitres to the muscles being used. When you exercise, the heart works harder - shifting a staggering 17 litres around the body. Of this, 14,000 millilitres go to the muscles, but 750 millilitres still go to the brain.

You can visit the technoquest World Wide Web site at http://www.campus.bt.com/ CampusWorld/pub/ScienceNet. Questions for this column can be submitted by e-mail to chrisr@bss.org

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