Solar System models, especially mechanical models, called orreries, that illustrate the relative positions and motions of the planets and moons in the Solar System have been built for centuries. While they often showed relative sizes, these models were usually not built to scale. The enormous ratio of interplanetary distances to planetary diameters makes constructing a scale model of the Solar System a challenging task. As one example of the difficulty, the distance between the Earth and the Sun is almost 12,000 times the diameter of the Earth.
If the smaller planets are to be easily visible to the naked eye, large outdoor spaces are generally necessary, as is some means for highlighting objects that might otherwise not be noticed from a distance. The Boston Museum of Science placed models of the planets in major public buildings, all on similar stands with interpretive material.[1] For example, the model of Jupiter, shown, is located in the cavernous South Station waiting area. The properly scaled, basket-ball sized model is 1.3 miles (2.14 km) from the model Sun which is located at the museum, graphically illustrating the immense empty space in the Solar System.
The objects in such large models do not move. Traditional orreries often did move and some used clockworks to make the relative speeds of objects accurate. These can be thought of as being correctly scaled in time instead of distance.
Permanent true scale models
Many towns and institutions have built outdoor scale models of the Solar System. Here is a table comparing these models with the actual system.
Other Models of the Solar System: historic, temporary, virtual, or dual-scale
Several sets of geocaching caches have been laid out as solar system models.
A model based on a classroom globe
If the Earth were reduced to the size of a typical classroom globe, 41 cm (16 inches) in diameter, the Moon would be a 10 cm (4 in) baseball floating 12 metres (40 feet) away. The Sun would be 14 stories tall (somewhat smaller than the Spaceship Earth ride at Epcot) floating 5 kilometres (3.1 mi) away. While a complete model to this scale has never been built, a Solar System built centered in Washington DC, London, or Sydney, to that scale (approximately 1:31 000 000) would look like this:
If the scale of the above model is increased to 1:310 000 000, i.e. all distances and sizes reduced by a factor of 10, then the Earth and Venus can be modeled by ping pong balls, the Moon and smaller planets by various size marbles or lumps of modeling clay, the gas giants by balloons or larger playing balls, and a circle the diameter of the Sun can be drawn on the floor of most classrooms. The scale distance to Alpha Centauri would be 1/3 of the way to the Moon.
Some planetaria and related museums often use a scale model of the Solar System featuring a planetarium dome representing the Sun. Examples of this can be seen in planetaria like the Adler Planetarium, the Hayden Planetarium at the American Museum of Natural History, the Clark Planetarium, the Griffith Observatory, the Louisiana Arts and Science Museum, the Adventure Science Center, etc.
A commonly-portrayed scale model of the Solar System would use fruits of varying sizes to represent the planets: The Sun would be represented by an adult human, Mercury would be represented by a pea, Venus by a cherry or a grape, Earth by a strawberry or an apricot, the Moon by a peppercorn, Mars by a blueberry, Jupiter by a watermelon or a medium-sized pumpkin, Saturn by a grapefruit or a large melon like a cantaloupe or a honeydew, Uranus by an apple or an orange, and Neptune by a lime or a plum.
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