When thinking about astronomy it is sometimes very difficult
to visualise just how big distances are between objects, and also how they
compare in size.
For example, we know that the Sun is vastly bigger than
Planet Earth, but just how much bigger? The answer is 109 times, in terms of diameter.
Or, to put it another way, if the Sun was the size of a tennis ball, Earth
would be the size of a piece of grit half a millimetre across!
OK – so if we stick with the idea of the Sun as a tennis
ball, which you can easily visualise being held by a tennis player who is about
to serve at the start of a game, where would the piece of grit that we live on
have to be placed to be the same scale? The answer is 12.5 metres or 41 feet
away, which would get the ball over the net. If, when playing tennis, you can
spot an object of that size in the service court on the other side of the net,
you must have excellent eyesight!
The distance from the Sun to the Earth is known as the
Astronomical Unit, because it is a useful measure for making comparisons. The
outermost planet is Neptune (now that Pluto has been “demoted”). This would be
about the size of a small cake-decoration silver ball on our scale, but at 30
astronomical units from the Sun it is unlikely to be visible by even the
keenest-eyed tennis player standing on the baseline!
If you were to place 16 tennis courts end to end, our player
would serve a fault on the 16th court if his serve managed to reach
silver ball Neptune!
© John Welford
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