The Sunday Nation
Nairobi,
31 January 2010
“We are told that the Earth is spinning at a very high speed. My question
is: how come we don’t feel this movement?”, asks a reader who prefers to
remain anonymous.
In case you didn’t know, the Earth measures about 12,800km in diameter.
Using simple geometry it is easy to calculate the circumference of the
planet; thus, “pi-D”, gives us approximately 40,000km.
Now, the Earth actually rotates very slowly, completing one rotation each
day; that is, 24 hours. This means that if you are standing at one
location, you will be taken on a 40,000km journey in 24h.
Speed, is the distance travelled in unit time. Most often, we state it in
kilometres per hour. Thus if you have completed a journey of 40,000km in
24h, your speed is about 1,667km/h [40,000 divided by 24].
Now that’s very fast by normal standards – they are only a few aeroplanes
that can reach that speed. So, we can now appreciate the anonymous
reader’s question.
The reason we don’t feel this fast motion is that the Earth carries
everything on it along as it spins. This includes the trees,
buildings….and even the air in the atmosphere.
Think about it: when you are inside a car, how do you know that it is
moving? Is it because you feel the wind blowing in through the window?
Is it because you notice that you are leaving the roadside trees behind?
Or is it because you hear the sound of the wheels running on the road?
All these observations combined make you realise that you are moving.
Unfortunately, in the case of the spinning of the Earth, the trees and
the atmosphere are moving with the planet (they are stuck to it!)…and
there is no road so it does not make any noise. Therefore we cannot feel
the spinning motion.
The only way we detect our motion is by looking “outside”; that is up in
the sky. There we see the Sun rising in the East, traversing across the
sky and setting in the West… then the stars rise and follow (almost) the
same movement.
Contrary to popular belief, this observation does NOT tell us that the
Earth is moving around the Sun. NO! It tells that our planet is spinning
(from West to East).
The revolution of the Earth around the Sun requires more careful (not
necessarily more accurate) observation of the stars. It’s easy: go out
to night at a particular time, say 9pm; look up in the sky and observe
carefully the pattern of stars that will be directly above you; then
wait one month (end of February) and go outside again at the same time
and search for “you stars”.
You will not find them directly overhead. They will have shifted about 30
degrees eastwards. This is evidence that either the Earth has moved
relative to the stars or the stars have moved in relation to the Earth.
The debate over that question cost Galileo
Galilei house arrest for close to 400 years! Nonetheless, I shall
discuss it in a future article. Hopefully, I shall not have to pay the
prise that Galileo paid!
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