Jupiter is back in my evening skies. Two years ago it was prominent in September. Last year it was October. Each year it slips by a month. It has taken the Earth a year and a month to catch up with Jupiter again. Obviously in the mean time, Jupiter has also been moving along its orbit. One month forward every year. There are 12 months in an Earth-year so after 12 years, Jupiter would be back to the same place. The orbit of Jupiter round the Sun must be roughly 12 Earth-years.