This orrery is a fully functional model of the 4 terrestrial planets: mercury, venus, earth, and mars. Ideal for planning your next interplantary voyage on your star-galleon. The planets can be moved to show their orbital placement around the sun. Each planet is sized relative to each other. The size of the sun and diameter of the orbits are not accurate, for display purposes. Working planetary models have existed in various forms since 90 BC. The orrery was given it's name in the 17th century after an Earl who ordered a clockwork copy. Now you can have your own! This piece is nearly 6 inches tall, the working sphere being 4 inches in diameter.