The further a planet is from the Sun, the slower it moves and the longer it takes to complete an orbit. Mercury – the closest planet to the Sun – takes 88 Earth days to complete an orbit. The furthest planet – Neptune – takes 164 Earth years to complete an orbit.
To demonstrate the orbital speed of planets using a gravity well, place a very heavy ball in the centre of the well to represent the Sun. Roll one marble midway on the sheet to represent the orbit of a planet such as Mars. Roll a second marble onto the sheet to represent a planet with a larger orbit such as Neptune. The period (length of time for one revolution) is much longer for the marble that is further from the Sun.