THE ORIGIN OF THE IDEA 5 
because of the tendency of matter to fall together to a 
common centre, and it was proved to be a sphere by the 
fact that no other form could always throw a circular 
shadow on the moon during an eclipse, and because on no 
other form of Earth could the shifting of the horizon be 
explained as one travelled from north to south and saw 
the old familiar stars drop out of sight, while new con- 
stellations rose into view. 
This stupendous discovery of the true form of the 
Earth brought in its train others of equal magnitude. A 
spherical Earth in the centre of a spherical heaven 
studded with constellations of invariable form was no 
longer an unknown trackless waste. The stars were 
landmarks everywhere, for by noting the height of the 
celestial pole above the horizon the traveller could tell 
how far he was north or south of the middle line or 
equator which could be imagined as dividing the globe 
into halves. And it followed that a distance on the sur- 
face that corresponded to an increase or decrease of the 
elevation of the celestial pole by one degree must be 
exactly one three-hundred-and-sixtieth of the whole cir- 
cumference of the Earth. It would suffice then to measure 
the length in miles or yards of that distance and multiply 
by 360 to know the size of the globe and so to calculate 
the exact dimensions and position of the Habitable World 
surrounding the Mediterranean and the distance it would 
be necessary to sail over the ocean to come from its west 
coast to its east or from its north coast to its south. All 
this was clear to the minds of Aristotle and his followers, 
but there were innumerable difficulties to be overcome 
before the established theory could be applied to elicit 
new facts. Though latitude could easily be reckoned 
from the altitude of the pole by night, or that of the sun 
