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# (a) As we discussed in class, in Ptolemy's geocentric model of the Solar System the centers

of the epicycles for the inferior planets Mercury and Venus are tied to the motion of the

Sun, in order to keep these two planets from 'wandering' too far away from the Sun in

the sky. The maximum elongations (i.e., angular distance from the Sun) observed for

Mercury and Venus are max = 22.8° and 46.3°, respectively. Demonstrate that, in the

Ptolemaic model, max can be used to estimate the ratio of the radius of the epicycle, E

for each planet to that of its deferent, D, but not their absolute values nor their ratios to

the Earth-Sun distance.

(b) In the Copernican model, on the other hand, show that the orbital radius of an inferior

planet is given by

T = R sin max,

where R is the Earth's orbital radius, or 1 Astronomical Unit (AU). Use the values of max

given above to evaluate r for Mercury and Venus. Give your answers to 3 decimal places.

(c) With the help of a diagram, calculate the minimum and maximum distances from the

Earth for an inferior planet in the Ptolemaic model in terms of D and the parameter max.

(d) Do the same for the Copernican model, giving your results in terms of R and max.

Show that the ratio of minimum to maximum distance is the same for both models. (Do

this analytically, rather than numerically.) In this sense, the two models are equivalent,

but Copernicus considered the inability to relate the orbital radii of Mercury & Venus to

that of the Earth to be a major failing of the old model. Do you agree?

Fig: 1