Page 491 - 89 history of astronomy_opt

while she always turns the same face to us,
there is periodically a slight twist to let us
see a little round the eastern or western
edge. This was called libration , and the
explanation was clear when it was under-
stood that in showing always the same face
to us she makes one revolution a month on
her axis uniformly , and that her revolu-
tion round the earth is not uniform.
491