I want every one who reads this book to make a little drawing of the
sun and the planets. The apparatus that you will need is a pair of
compasses; any sort of compasses that will carry a bit of pencil will
do. You must also get a little scale that has inches and parts of
inches divided upon it; any carpenter’s rule will answer. The drawing
is intended to give a notion of the true sizes and positions of the
fine family of which the earth is one member. The figure I have given
(Fig. 46) is not on so large a scale as that which I ask you to use,
and which I shall here mention. Try and do the work neatly, and then
pin up your little drawings where you will be able to see them every
day until you are quite familiar with the notion of what we mean by our
solar system.
[Illustration: FIG. 46.--The Orbits of the Four Inner Planets.]
First open the compasses one inch, and then describe a circle, and mark
a dot on this as “MERCURY,” in neat letters, and also write on the
circle “88 days.” At the centre you are to show the “SUN.” This circle
gives the track followed by Mercury in its journey round the sun in the
period of 88 days. Next open your compasses to 1¾ in., which you must
do accurately by the scale. The circle drawn with this radius shows the
relative size of the path of Venus, and to indicate the periodic time,
you should mark it, “225 days.” The next circle you have to draw is a
very interesting one. The compass is to be opened 2½ in. this time,
and the path that it makes is to be marked “365 days.” This shows the
high road along which we ourselves journey every year, along which we
are, indeed, journeying at this moment. If you wanted to obtain from
your figure any notions of the true dimensions of the system, the path
of the earth will be the most convenient means of doing so. The earth
is 93,000,000 miles from the sun, and our drawing shows its orbit as a
circle of 2½ in. radius. It follows that each inch on our little scale
will correspond to about 37,000,000 miles. As, therefore, the radius of
the orbit of Mercury has been taken to be one inch, it follows that the
distance of Mercury from the sun is about 37,000,000 miles.
We have, however, still one more circle to draw before we complete
this little sketch. The compass must now open to four inches, and a
circle which represents the orbit of Mars is then to be drawn. We mark
on this “687 days,” and the inner part of the solar system is then
fully represented. You see, this diagram shows how our earth is in
every sense a planet. It happens that one of the four planets revolves
outside the earth’s path, while there are two inside. By marking the
days on the circles which show the periods of the planets, you perceive
that the further a planet is from the sun, the longer is the time that
it takes to go round. Perhaps you will not be surprised at this, for
the length of the journey is, of course, greater in the greater orbits;
but this consideration will not entirely explain the augmentation of
the time of revolution. The further a planet is from the sun, the more
slowly does it actually move, and therefore, for a double reason, the
larger orbit will take a longer time. From London to Brighton is a
much longer journey than from London to Greenwich, and, therefore, the
journey by rail to Brighton will, of course, be a longer one than by
rail to Greenwich. But suppose that you compared the railway journey
to Greenwich with the journey, not by rail, but by coach, to Brighton,
here the comparative slowness of the coach would form another reason
besides the greater length of the journey for making the Brighton trip
a much more tedious one than that to Greenwich. Mars may be likened to
the coach which has to go all the way to Brighton, while Mercury may be
likened to the train which flies along over the very short journey to
Greenwich.
[Illustration: FIG. 47.--Comparative Sizes of the Planets.]
We can easily show from our little sketch that Mercury must be
moving more quickly than Mars, for the radii of the two circles are
respectively one inch and four inches, and therefore the path of Mars
must be four times as long as the orbit of Mercury. If Mars moved as
fast as Mercury, he would, of course, require only four times as many
days to complete his large path as Mercury takes for his small path;
but four times 88 is 352, and, consequently, Mars ought to get round
in 352 days if he moved as fast as Mercury does. As a matter of fact,
Mars requires nearly twice that number of days; indeed, no less than
687, and hence we infer that the average speed of Mars cannot be much
more than half that of Mercury.
[Illustration: FIG. 48.--Phases of an Inferior Planet.]
To appreciate duly the position of the earth with regard to its
brothers and sisters in the sun’s family it will be necessary to use
your compasses in drawing another little sketch, by which the sizes of
the four bodies themselves shall be fairly represented. Remember that
the last drawing showed nothing whatever about the sizes of the bodies;
it merely exhibited the dimensions of the paths in which they moved. As
Mercury is the smallest globe of the four, we shall open the compasses
half an inch and describe a circle to represent it. The earth and Venus
are so nearly the same size (though the earth is a trifle the larger)
that it is not necessary to attempt to exhibit the difference between
them, so we shall represent both bodies by circles, each 1¼ inches
in radius. Mars, like Mercury, is one of the globes smaller than the
earth, and the circle that represents it will have a radius of ¾ of an
inch. You should draw these figures neatly, and by a little shading
make them look like globes. It would be better still if you were to
make actual models, taking care, of course, to give each of them the
exact size. A comparative view of the principal planets is shown in
Fig. 47.