A model celestial sphere positioned relative to the National Mall. The Voyage model Earth is at the center of the sphere. The model Earth orbits the model Sun in the plane of the Mall, called the ecliptic plane. The model Earth’s spin axis is tipped at 23.5 degrees to the vertical. The Earth’s equator is projected onto the sphere as the celestial equator. The position of the model Sun on the celestial sphere is its position at sunrise at the time of the Vernal Equinox, near March 21. On that date, when standing at the model Earth, the real Sun rises directly behind the model Sun.

Orienting a model celestial sphere relative to the Mall determines the spatial orientation of all the planets relative to one another. The result is a highly accurate model of the Solar System.