If we imagine the length of the period to be expressed in terms of days and fractions of a day, that is to say, in terms of the rotations of the earth around its axis, then the difficulty encountered is, that the moon now requires for each of its revolutions around the earth rather a smaller number of rotations of the earth around its axis than used formerly to be the case.
(we; imagine; the length of the period) (the period; to be expressed; ) (that; is; to say) (the difficulty; encountered; )
we the period that the difficulty
imagine to be expressed is encountered
the length of the period
to say