I didn't know.

For a long time I stood buried in deep thought, when it occurred
to me to try out one of the compasses I had brought and ascertain
if it remained steadily fixed upon an unvarying pole. I reentered
the prospector and fetched a compass without.

Moving a considerable distance from the prospector that the needle
might not be influenced by its great bulk of iron and steel I turned
the delicate instrument about in every direction.

Always and steadily the needle remained rigidly fixed upon a point
straight out to sea, apparently pointing toward a large island some
ten or twenty miles distant. This then should be north.

I drew my note-book from my pocket and made a careful topographical
sketch of the locality within the range of my vision. Due north
lay the island, far out upon the shimmering sea.

The spot I had chosen for my observations was the top of a large,
flat boulder which rose six or eight feet above the turf. This
spot I called Greenwich. The boulder was the "Royal Observatory."

I had made a start! I cannot tell you what a sense of relief was
imparted to me by the simple fact that there was at least one spot
within Pellucidar with a familiar name and a place upon a map.

It was with almost childish joy that I made a little circle in my
note-book and traced the word Greenwich beside it.