The planets orbit the Sun at very different distances. The NASA Planetary Fact Sheet gives each distance in millions of kilometers, and the average Earth to Sun distance, about 150 million kilometers, is called one astronomical unit. On a scale where one step stands for 150 million kilometers, Earth is one step from the Sun and Neptune is about 30 steps. The four inner planets crowd close to the Sun and the four outer planets spread far apart. Light takes about eight minutes to cross one astronomical unit, so even sunlight carries old news. This week the crew walks the solar system out across the yard.