Three of the planets, Mercury (shown above in transit of the Sun), Mars and Pluto, are pinheads. Mercury’s pinhead is .03 inches(.08 cm) across. Mars is .04 inches(.10 cm) and Pluto is .01 inches(.03 cm.) We’re including Pluto as a planet even though it has recently been reclassified as a Kuiper Belt Object. Two planets, Venus and Earth, are peppercorns at .08 inches(.20 cm) across. Jupiter is a chestnut at .90 inches(2.40 cm.) Saturn is a hazelnut of .75 inches(2.00 cm) diameter. Both Uranus and Neptune are peanuts of .30 inches(.80 cm.)
Now we have one ball, one chestnut, one hazelnut, two peanuts, two peppercorns and three pinheads. Next we need a place to lay them out to show how far apart they are. We can’t do it indoors because there simply wouldn’t be enough room, so we have to go out. The back yard is too small, too. None of the playing fields is big enough, either. The airport just might do it. We’ll put the Sun, the twenty centimeter ball, down at one end of the airport, in the grass near the fence because the paved runway isn’t long enough by itself. Then we’ll pace off the distances to the planets, assuming one yard (a little less than one meter) per pace.
The first planet out from the Sun is Mercury, at ten paces. We’ll put the first pinhead there, and we’d better mark it with a flag so it doesn’t get lost in all that space. Take another nine paces and put down a peppercorn for Venus. Another seven paces and we can mark Earth with the second peppercorn. Then there’s a jump of fourteen paces to get to Mars and its pinhead. We’re forty paces out now and we’ve placed the four inner planets already.
Now we start to cover ground. Jupiter, the largest planet in the Solar System, bigger than the rest combined and almost one tenth of one percent as massive as the Sun, is 95 paces beyond Mars. Put down the chestnut. Saturn the hazelnut is next at 112 more paces. Uranus is another 249 paces and Neptune 281 paces beyond that. Down go the peanuts. All we have left is one pinhead for Pluto, which is another 242 paces. At a total of 1,019 paces, we might still be inside the fence at the far end of the airport. If the Sun wasn’t shining at the other end, we’d never see it.
To put it in perspective, the Moon, which is the farthest any human has gone, is 2.4 inches(6 cm) away from Earth’s peppercorn. Mars, one of the closer planets, is over 200 times as far as that. This model shows how big and empty space really is.
Here’s a bonus photo from NOAO, showing, from top to bottom, the Moon, Venus, the star Spica and Jupiter, in conjunction above an observatory in Chile.rjb