During one rotation of Jupiter round the sun the earth circles 12 times round it. This we are wont to express by saying that Jupiter needs 12 earth-years for one rotation; in symbols:
t_J / t_E = 12 / 1
To find the analogous expression for the reciprocal ratio:
t_E / t_J = 1 / 12
we must obviously form the concept ‘Jupiter-year’, which covers one rotation of Jupiter, just as the concept ‘earth-year’ covers one rotation of the earth (always round the sun). Measured in this time-scale, the earth needs for one of her rotations 1 / 12 of a Jupiter-year.