The Heat of the Sun, 
[March, 
122 
The distance which separates us from the Sun has been 
determined with very great accuracy, because ot lts ir ^‘ 
portance to astronomers, this distance forming their unit ot 
length. With tolerable accuracy we may take this as 
equal to 92,000,000 of miles. Although this distance is 
infinitely small when compared with the distances of many 
of the stars which are familiar to astronomeis, it is ye so 
great compared with our ideas of distance that we can loim 
no conception, however remote, of it. Fortunately, howevei, 
we are now dealing not with conceptions and imaginings, 
but with bare and unvarnished fadts. Knowing this dis- 
tance, knowing also the apparent magnitude,- t.e., the 
observed diameter of the Sun,— it is easy to calculate his 
real diameter. This has been found to be approximately 
given by 880,000 miles. These distances being determined, 
and the fadt established that the planets revolve round the 
Sun, it follows that there must be some force pulling these 
planets, emanating from the Sun, which causes them o 
revolve around it. This force we know to be that of gravi- 
tation. Again, the period which it takes for the Eart 1 
revolve around the Sun is given by the length of what is 
called a mean solar year. It is also known that the length 
of this mean solar year has not altered during the period of 
recorded astronomical observation. It therefore follows that 
the force pulling the Earth, and keeping it in its orbit is 
just sufficient to do so and no more. For if the Sun s foice 
adting upon the Earth were more than just sufficient to do 
this, the year would be gradually diminishing in length by 
an amount which would be proportionate to this excess of 
force. In short, the Earth would be gradually appioaching 
the Sun. If, on the contrary, the force were less than just 
sufficient to do this, the year would be gradually mci easing 
in length : in short, we should be receding from the Sun. 
Observations extending over a great number ot years piove 
that neither of these things is occurring. Now if we know 
the mass of a body which is moving in a certain orbit, it we 
know the radius of that orbit, and also the velocity with 
which the body is moving, we can find, by employing a 
formula well known to scientific men, the force which is 
emanating from the centre, and which is just able to keep 
the body from moving out of that orbit. Calculating in this 
manner, it has been deduced that the attraction which the 
Sun exerts upon our globe is equal to that which would b 
exerted supposing it to consist of 360,000 earths rolled into 
one. Now we have just seen that the diameter of the Sun 
is 880,000 miles ; that of the Earth is 8000 ; thus the Sun s 
