ASTRONOMY 
313 
starting-point O Tlicre was apparently a tradition that the 
Groat Year of Plato was 36000 years: this corresponds to 
the minimum estimate of the precession of the equinoxes 
quoted by Ptolemy from Hipparchus’s treatise on the length 
of the year, namely at least one-hundredth of a degree in 
a year, or 1° in 100 years, 1 2 that is to say, 360° in 36000 years. 
The period is connected by Adam with the Geometrical Num 
ber 12960000 because this number of days, at the rate of 360 
days in the year, makes 36000 years. The coincidence may, 
it is true, have struck Ptolemy and made him describe the 
Great Year arrived at on the basis of 1° per 100 years 
as the ‘ Platonic ’ year; but there is nothing to show that 
Plato himself calculated a Great Year with reference to pre 
cession : on the contrary, precession was first discovered by 
Hipparchus. 
As regards the distances of the sun, moon and planets 
Plato has nothing more definite than that the seven circles 
are ‘ in the proportion of the double intervals, three of eao-h ’ 3 : 
the reference is to the Pythagorean rerpaKTvs represented in 
the annexed figure, the numbers after 1 being 
on the one side successive powers of 2, and on 
the other side successive powers of 3. This 
gives 1, 2, 3, 4, 8, 9, 27 in ascending order. 
What precise estimate of relative distances 
Plato based upon these figures is uncertain. 
It is generally supposed (1) that the radii of the successive 
orbits are in the ratio of the numbers; but (2) Chalcidius 
considered that 2, 3, 4 ... are the successive differences 
between these radii, 4 so that the radii themselves are in 
the ratios of 1, 1 + 2=3, 1+2 + 3 = 6, &c.; and again (3), 
according to Macrobius, 5 the Platonists held that the successive 
radii are as 1, 1 . 2 = 2, 1 .2.3 = 6, 6 . 4 = 24, 24.9 = 216, 
216.8 = 1728 and 1728. 27 = 46656. In any case the 
figures have no basis in observation. 
We have said that Plato made the earth occupy the centre 
of the universe and gave it no movement of any kind. Other 
1 Timaeus, 39 b-d, 
2 Ptolemy, Syntaxis, vii. 2, vol. ii, p. 15. 9-17, Heib. 
3 Timaeus, 36 n. 4 Chalcidius on Timaeus, c. 96, p. 167, Wrobel 
B Macrobius, In somn. Scip. ii. 3. 14.