Most visitors to the Voyage exhibition are unaware of the level of accuracy of this model Solar System. For those readers with a deeper understanding of apparent motions in Earth’s sky, here is the rest of the Voyage.

The model planets and moons are created through 3-dimensional laser sculpting inside solid crystal. The technique uses a laser to precisely focus energy at a specific location inside a clear slab of glass. The result is an extremely small spherical fracture at the point of focus. Each fracture is a pixel—a ‘picture element’ akin to the pixels that make up a 2-dimensional digital image. But in this case, the computer-guided laser can create 3-dimensional structures of significant complexity, fully inside the glass slab, and constructed one pixel at a time. Visit www.crystalimpressions.com.

We therefore had the ability to create spherical model planets and moons of highly accurate size; place moons in their appropriate orbital planes and at the correct distances from their parent planet; and reproduce the ring systems of the gas giant planets Jupiter, Saturn, Uranus, and Neptune, including accurate ring width and position of ring divisions. In the case of Jupiter and Saturn, their equatorial diameters are noticeably greater than their pole-to-pole diameters and this ‘oblateness’ could also be accurately portrayed through laser sculpting.

For a given planet, its shape, ring system, and orbits of moons, are all structures that are oriented in a well-defined manner relative to the planet’s equatorial plane and spin axis. All these data would be provided to the computer driving the laser. But one still needs to correctly orient the planet’s spin axis in space, relative to the spin axes of the other planets, if you’re truly going to create a model of the Solar System. We wanted the Voyage model Solar System to provide an experience as close as possible to visitor as cosmic giant wandering through the real Solar System.

But how were we going to define the orientation of the planetary spin axes relative to each other?

Earth-bound observers see themselves at the center of a spherical canopy of stars, called the celestial sphere. Astronomers project Earth’s latitude and longitude onto the celestial sphere to create a grid that can be used to precisely define the position of an object in the sky. It is also true that every planet’s spin axis, if infinitely extended, will intersect this grid at two well-defined points. In other words, every planet’s spin axis is oriented relative to the celestial sphere.

To realize the vision for Voyage, we imagined a model celestial sphere centered on the model Earth. Defining the orientation of the model celestial sphere relative to the National Mall is all that’s needed to correctly orient all planetary spin axes. The model planets would then be portrayed with accurate orientations in space, which would be visible to the visitor through the orientation of the model ring systems, planetary oblateness, and moons in the glass.

The solution was to first adopt the horizontal plane of the National Mall as the plane of the Earth’s orbit around the Sun, known as the ecliptic plane. The Earth spins on an axis that is tipped 66.5 degrees to that plane (tipped 23.5 degrees from vertical). The spin axis maintains a fixed orientation in space as Earth orbits the Sun. This is the case for all the planets, though the orientation in space of each spin axis is different.

To fully define the model celestial sphere, given that the plane of the Mall is the ecliptic, one only needs to specify that the model Earth’s location represents a particular time in its orbit around the model Sun. The moment in time that is chosen is the date of the Vernal Equinox, near March 21.

The reason for the choice derives from the orientation of the National Mall. The exhibition is placed along Jefferson Drive, which runs precisely east-west. The model Earth is located 15 meters due west of the model Sun. At the time of the Vernal Equinox near March 21, when standing at the model Earth, the real Sun rises due east—directly behind the model Sun, and both Suns appear to be the same angular size.

The positions of the model Earth and Sun therefore reflect the time of the Vernal Equinox, the orientation of the celestial sphere is fixed, and the planetary ring systems, planetary oblateness, and position of moons can all be accurately portrayed.

The sizes, orientations, and oblateness associated with the Voyage model planets are shown in the illustration. You see crystalline, 3-D versions of these images, together with moons where appropriate, when looking through the glass of the Voyage planetary Units.

The only significant liberty taken in the Voyage exhibition is that the other planets are also placed along the Earth-Sun line, so the visitor can get an accurate understanding of the relative distances of the planets from the Sun. But the planets never line up, which is stated on every storyboard.

A computer model of the Solar System was developed to test the assumption that the planets are never aligned. In the computer model, planets were initially placed in their orbits at random locations, the clock was started, and the planets proceeded to orbit the Sun. Over the course of four billion years, the planets were never seen to line up in any reasonable interpretation of ‘aligned’.