100 
A. COTTAM—-ON - A BIAGE AM FOR 
Our express train, travelling at the rate of 50 miles an hour, would 
take 20 days to complete the circuit. Our model of the Earth is 
-i%-ths of an inch in diameter, and upon the same scale the Moon 
is only i- 0 -ths of an inch. The Sun to the same scale is a globe of 
7ft. 2^-ins. in diameter. If we place our diagram of the Earth and 
Moon (the distance of the Moon from the Earth, 240,000 miles, 
being represented by a space of 2 feet) in front of the diagram of 
the Sun, it will he seen that there is about one-third of the Sun’s 
diameter outside the Moon’s orbit. Perhaps nothing will give you 
a better idea of the immense distance of the Sun than the fact that 
to us on the Earth’s surface the Sun and the Moon appear to be 
the same size. 
I propose to go into the question of magnitudes first, and after¬ 
wards into that of the distances. The members of our system 
naturally divide themselves into three groups—first, a group of 
four small planets comparatively near the sun; second, a group 
of very small planets revolving in orbits close together, and even 
intersecting one another; and third, a group of four large planets 
at great distances from the Sun. 
The Sun, the centre of the system, has a diameter of 865,000 
miles, represented in our diagram by 7ft. 2^-ins. Its density is 
rather less than 1£ times that of water. It revolves on its axis in 
about 25-g- days, and is travelling through space at the rate of six 
miles a second, carrying all the planets with it. As to what the 
actual heat of the solar photosphere is we can say very little. It 
has been calculated that less than one two-thousand-millionth part 
of the heat emitted by the Sun reaches the earth. The four smaller 
planets, in the order of their distance from the Sun, are Mercury, 
Venus, Earth (with one satellite, the Moon), and Mars (with two 
satellites). The four larger planets are Jupiter (with four satellites), 
Saturn (with eight), Uranus (with four), and Neptune (with one 
satellite). Their diameters and distances, with those of their satel¬ 
lites, are given in the accompanying tables (pp. 98, 99), which 
also show how they are represented in our diagram. 
Until 1877 Mars was supposed to be without moons, but in 
August in that year Professor Asaph Hall discovered, with the great 
refracting telescope of the Washington Naval Observatory, two 
minute satellites. These moons are remarkable in many ways. 
They are the smallest by far of all the known members of the 
solar system, neither of them exceeding 10 miles in diameter. It 
is impossible therefore to show them to scale on our diagram, no 
dot visible to the eye being small enough to represent them. I have 
indicated their positions on the diagram of Mars. The smallness 
of these moons is not however their most remarkable feature, their 
. nearness to the planet and the rapidity of their motions being still 
more curious. All the members of the solar system move from 
west to east. Our moon moves so slowly that she only passes over a 
space equal to her diameter in every hour. The moons of Mars, 
however, move much more rapidly; the outer one, although it 
must appear to an observer on the surface of Mars to move from 
