THE HAMILTON ASSOCIATION. 49 
Moon does the Earth, make their trip around their planet from 
12 hours to 16% days. Their service to the astronomer in his 
researches is valuable. In the first place, it is a magnificent 
illustration in diminutive way of the Copernican system. 
Copernicus taught that the Sun was a centre and that the 
planets revolved around it. Here is Jupiter controlling five 
bodies revolving around it, with Jupiter as a centre. 
Then they assist in computing the bulk, the weight of Jupi- 
ter from the attractive power Jupiter exercises over them. 
Then they show that Jupiter’s light is only borrowed. 
[ Lantern. | 
You see a perfect shadow of a satellite on Jupiter. This 
means that the satellite, coming between Jupiter and the Sun, 
obstructs the light of the Sun from falling on Jupiter, and that 
spot remains dark because Jupiter has no light of its own. 
Finally the satellites have enabled the scientist to reckon 
the velocity of light. 
I will attempt to explain. [Lantern] 
Here we have Jupiter and its shadow. There is a satel- 
lite in occultation and here is one in eclipse. Both disappear 
from us, but for different reasons. Occultation put Jupiter 
between us and the satellites. The disappearance is short and 
abrupt. 
The other is out of sight, because, though exposed to our 
view it has no light to reflect. It is eclipsed by the shadow ot 
Jupiter. This disappearance is slow and beautiful and can be 
watched closely. The time of those eclipses has been deter- 
mined, and are regular under ordinary circumstances. 
It was not long, however, until it was remarked that the 
eclipse was sometimes delayed, say 5 minutes. Now this was 
found to be caused by the different nositions which Jupiter oc- 
cupied relative to the Earth. Sometimes Jupiter is 200,000,000 
miles further from the Earth when the eclipse takes place than 
former eclipses. At this great distance the satellite does not 
disappear from our view until several minutes later, and this 
number of minutes is the time which it took the last ray of the 
satellite’s light to travel over those extra 200 million miles, and 
