328 



DISCOVERY 



The Movements of 

 the Planets 



By H. Spencer Toy, B.Sc, A.Inst.P., 

 F.R.A.S. 



The whole s^'stem of the stars appears to move across 

 the sky from east to west every twentv-four hours. 

 This is due to the rotation of the earth on its axis 

 once a day. Of this motion we need not think in 

 this article, since the sky seems to move as a whole. 



The sun and moon have a second motion that is 

 easily observed, for they move among the stars from 

 west to east. Thus, for instance, if the moon is near 

 the Pleiades one night, it will be some distance from 

 them on the next. 



Both solar and lunar motions are continuous 

 towards the east. But the planets move among the 

 stars in no such simple way, although on the whole 



they also appear to travel towards the east. It will 

 be convenient to consider them in two groups : those 

 that lie beyond the orbit of the earth, and those 

 that are within. 



By those beyond the orbit of the earth we mean 

 Mars, Jupiter, and Saturn, for Uranus and Neptime 

 are not visible to the naked eye. Their apparent 

 motion can roughly be described as a progression in 

 loops. For the greater part of the time they move 

 from west to east, just as do the moon and the sun ; 

 then they stop altogether at a so-called " stationary 

 point," move backwards or " retrograde," become 

 stationary again, and then advance towards the next 

 loop. 



The loop takes several weeks to make, and can be 

 followed in the sky by comparing the positions of the 

 planet night by night with those of the fixed stars. 

 It is easy to follow this changing configuration when it 

 takes place in a conspicuous constellation, such as 

 Taurus, but it is not very difficult even when it occurs 

 in a barren one, like Cancer. This year Jupiter, for 

 example, made its loop in Virgo, and was therefore 

 very easily followed. \Miilst there are many celestial 



phenomena that most people can never hope to see, 

 among which may be included a total eclipse of the 

 sun, from the town in which they live, and- a transit 

 of Venus, this looping the loop occurs so frequently 

 and is so obvious that it may be seen by all. Yet 

 it is improbable that one person in ten, or even one 

 in fifty, ever sees it ! It excites no attention in the 

 Press, it gives rise to no great sensation as might a 

 comet or a shower of meteors, and the loops are made 

 unnoticed. 



Jupiter traces out a loop once in everj' 399 days, 

 a trifle over thirteen months, and eleven loops are 

 made as it revolves completely round the circle of the 

 sky in nearly twelve years. In twelve years' time, 

 therefore, Jupiter will again be looping its loop in 

 or near Virgo. Saturn makes a smaller loop once in 

 378 days, a year and a fortnight, and completes 

 twenty-eight of them in its journey round the heavens. 

 The loops of Mars are much bigger than either of 

 those already mentioned, but occur less frequently, 

 only once in 780 days, nearly two years and 

 two months. This planet retrogrades for 70 of the 

 7S0 days, the corresponding period in the case of 

 Jupiter being 123 days, and in the case of Saturn 

 137 days. 



This loop- like movement was observed from prehis- 

 toric times, and its explanation exercised the ingenuity 

 of all the ancient astronomers. They started with 

 two fundamental assumptions, both of which were 

 incorrect. The first was that the earth was fixed 

 in space, and was the centre of the imiverse ; round 

 it the stars revolved. The second was that the circle 

 was the perfect figure, just as seven was the perfect 

 number, and that the planets, being celestial bodies, 

 could therefore move only in circles. With these 

 ideas as his foundation, Ptolemy elaborated a system 

 to explain the movements of the planets, and a very 

 ingenious one it was. It was constructed at Alexan- 

 dria about A.v. 140, and set out in the famous 

 Almagest, which remained for fourteen hundred years 

 the standard work on astronomy. Reference was 

 made to it whenever a dispute arose, and by it all 

 questions about the stars were settled, just as all 

 questions on religion were settled by reference to the 

 Koran. 



Ptolemy imagined the sun to revolve round the earth 

 in a simple circle once a year. The planets, on the 

 other hand, revolved round a small circle, which 

 itself went round the earth. In the diagram (Fig. i), 

 let E represent the earth, s the sun, and j an outer 

 planet, say Jupiter. Jupiter was supposed to revolve 

 round the small circle, centre o, and o itself to go 

 round the earth in the period of twelve years, which 

 has already been explained to be that which Jupiter 

 takes to make the circle of the skies. The small 



