The sun is a very long way off. It is not easy for you to imagine a
distance so great, but if you want to learn astronomy you must make
the attempt. This is the first measurement that we shall have to make
on our way to that far-off country called Star-Land; but long as we
shall find it to be, we shall afterwards have to consider distances
very much longer. When you are out in the street, or taking a walk in
the country, you can see at once that this man is near, or that house
is far, or that mountain is many miles away. This is because you have
other objects between to help you to judge of the distances of these
different objects. You will see, for example, that there are many
houses or farmyards, and you will notice hedges dividing different
fields between you and the mountain. You also see that there are
woods and parks, and perhaps stretches of moorland extending up the
slopes. You have an impression that the farmyards and fields are of
considerable size, and that the woods or moors are wide and extensive;
and putting these things together, you realize that the mountain must
be miles away.
But when we look at the sun we have no aids conveniently placed to
help us in judging his distance. There are no intervening objects, and
merely gazing at the sun helps us but little in obtaining any accurate
knowledge. We must go to the astronomer and ask him to tell us how far
he has found the sun to be, and then we must also beg from him some
explanation of the method he has used in making his measurements.
It has been found that the sun is, on the average, about ninety-three
millions of miles from the earth; but sometimes it is a little further
and sometimes it is a little nearer. Let us first try to count
93,000,000. The easiest way will be to get the clock to do this for
us; and here is a sum that I would suggest for you to work out. How
long will the clock have to tick before it has made as many ticks as
there are miles between the earth and the sun? Every minute the clock,
of course, makes 60 ticks, and in 24 hours the total number will reach
86,400. By dividing this into 93,000,000 you will find that more than
1076 days, or nearly three years, will be required for the clock to
perform the task.
We may consider the subject in another way, and find how long an
express train would take to go all the way from the earth to the sun.
We shall suppose the speed of the train to be 40 miles an hour; and
if the train ran for a whole day and a whole night without stopping,
it would then accomplish 960 miles. In a year the distance travelled
would reach 350,400 miles, and by dividing this into 93,000,000 we
arrive at the conclusion that a train would have to travel at a pace
of 40 miles an hour, not alone for days and for weeks and for years,
but even for centuries. Indeed, not until 265 years had elapsed would
the mighty journey have been ended. Even though King Charles I. had
been present when the train began to move, the destination would not
yet have been reached. No one who started in the train could expect to
reach the end of the trip. That would not occur till the time of his
great-great-grandchildren.