XI. 
ON A DIAGRAM FOR A MODEL OF THE SOLAR SYSTEM TO 
SCALE. 
By Arthur Cottam, F.R.A.S. 
Read at Watford, 2 6th January, and at Hertford, ‘ISMh April, 1886. 
PLATE YI. 
The subject of this paper is not, in the usual acceptation of the 
term, Natural History, which is the history of Nature on this earth 
on which we live. Our Earth is one of the smaller members of 
what is called the Solar System, that is, of the system of planets 
and their satellites which revolve round the Sun as their centre, 
and it is to the influence of the Sun that most of the phenomena 
that we study under the title of Natural History are due. I there¬ 
fore think that a history of nature outside our world ought to 
interest students of Natural History. 
I will begin by asking a question. Have you ever succeeded in 
realising the relative magnitudes and distances of the Sun and 
planets? I believe that very few people have, and my object will 
be to help you to grasp the meaning of the enormous figures with 
which we shall have to deal. The methods usually adopted to 
make these huge distances intelligible, are to describe the time an 
express train or a cannon ball would take to travel over them. Now 
to my mind it does not convey any definite idea of the distance 
between the Earth and the Sun to be told that an express train 
going at the rate of 50 miles an hour would take over 200 years to 
do the journey. It struck me some two or three years ago that 
if I could make a model of the solar system to some small scale, 
I should get a better idea of the matter. I thought I could make 
small models of the Sun and planets with their satellites, and 
that I might set them down in a large field at their proper relative 
distances, and so be enabled to realise what the sizes and distances 
really are. I am aware that there is nothing new in the idea of 
making a model or diagram of the solar system, but I believe the 
idea of making one accurately to scale is a complete novelty. An 
orrery is a model of a Sun with the planets and their satellites on 
long arms, so arranged that by clockwork the motions of the various 
bodies can be imitated, but no attempt has been or could be made 
to construct an orrery to scale. 
The scale that I have adopted for my diagram is the smallest 
that I could use, so as to make the smaller planets and the satellites 
visible at all. I have taken a scale of 10,000 miles to an inch, so 
that every tenth of an inch represents 1000 miles. Our initial 
difficulty is to obtain some unit by which we can compare our 
various measurements, and the only unit we can adopt is our own 
Earth. Our difficulty here is, that being so close to our Earth we 
cannot easily realise what its size is, but we must endeavour to do 
so, and for this purpose we will call in the help of our old friend 
the express train. The earth is 8000 miles (in round numbers) in 
diameter, its circumference being therefore about 24,000 miles. 
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VOL. IV.—PART IV. 
