The seven disks allow light to shine through giving the same milky blue appearance as the seventh planet. Outside the entrance to the visitor centre, you can see the Kirkhill Pillar which inspired the project to recreate the model solar system in West Lothian. There is a leaflet available from the visitor centre explaining the history of the pillar. The Kirkhill Pillar Project has its own website, which shows where the other planets can be seen.
The 11th Earl of Buchan, David Stewart Erskine, built his model in 1776 to a slightly unusual scale of 12,283 and 23/100ths miles to one inch. This gives the planet Uranus a diameter of 66mm and it is placed 3650metres from the sun (1.83metres or 6 feet in diameter), which is on the roof of Broxburn Academy.
At this scale, the speed of light would be just less than one mile per hour. Watch out for Star Trek style time travel and warps in the space-time continuum!
Don't forget to make a note of the number under the lid to help you find the Halley's Comet bonus cache