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other words the Earth requires twelve months to pass across the faces 
of the twelve zodiacal constellations as seen from the Sun. If now from 
our heliocentric position we watch the journey of Jupiter around the 
same course, we should have lo wait n.86 years to see the course com- 
pleted, or while the Earth passes from one constellation to the next in 
a month, Jupiter takes about a year to travel over the same arc. This 
is partly due to the fact that his orbital velocity in miles per unit of 
time is to that of the Earth as 4 to 9, and partly to the much greater 
length of his orbit, which is a circle having a radius about five times 
that of the Earth's orbital radius. But, although the rates of motion 
would be very different, the directions would be the same ; from Libra 
to Scopio ; from Scorpio to Sagittarius, and so on eastward, just as in 
the case of the Moon. If now, still occupying the Sun's place, we were 
to watch the progress of both planets, it is evident that we should see 
the Earth make the complete circuit of the zodiac nearly twelve times 
while Jupiter made the journey once. And further, we should see the 
Earth pass Jupiter's place, say in Aries, in a particular year, while in 
the following year our planet would pass Jupiter in Taurus, and the next 
year in Gemini, and so on. Now Jupiter, as actually seen from the 
earth, does not journey regularly eastward, but appears, during a part of 
each year to retrograde, going from the east towards the west in the 
zodiac. The olden astronomy figured his path, as well as the paths of 
the other planets, as circles with loops on the circumference. These 
loops -or Epicycles are twelve in the case of Jupiter ; and it is by no 
means difficult to understand how they explain the planet's peculiar and 
complicated motion, as seen from the earth. When, however, we 
adopt the Copernican theory which looks on the Sun as the centre of 
the planetry system, the explanation becomes still simpler. The best 
way to do this is to draw on a large sheet of paper two concentric 
circles of radii 1 and 5 respectively. These circles represent the orbits 
of our Earth and Jupiter, and their common centre is the Sun's place. 
Divide each circle into twelve equal arcs. The points of division will 
represent on the smaller circle the Earth's place at intervals of one 
month, and on the larger circle, Jupiter's place at intervals of one year. 
Subdivide any one of the arcs (of 30) of the larger circle into 12 divis- 
