Brass Sun — Ian Edginton

The Orrery is a fully functional, life-size clockwork solar system, a clutch of planets orbiting a vast brass sun via immense metal spars. But the once-unified collection of worlds has regressed into eccentric fiefdoms, and ice is encroaching on the outer planets as the sun is dying. Wren and Eptimus must find the key to restart the sun, but first must escape the world known as The Keep.