Solar system model

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 objects in such 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.

One scale model, designed to be easily replicated, is called "The Thousand-Yard Model" [http://www.noao.edu/education/peppercorn/pcmain.html] and spans about a kilometre. In it, the Earth is represented by a peppercorn. A school class building this model might tape the peppercorn to an index card to make it more visible. Another scale model is the 1:10 000 000 000 model, in which 100,000 km is represented by 1 cm. In this model, the Sun is 600m from the Kuiper belt and dwarf planet Pluto. The largest scale model in the world is the Sweden Solar System.

In July 2005 the Austrian art group monochrom placed the planets true to scale (sun, 4 meters in diameter at Machine Gallery, Alvarado Street, near Echo Park) throughout the Los Angeles cityscape. Then they conducted an [http://www.monochrom.at/experiences/race.htm 'illegal space car race'] through the solar system.

In regional Australia, in the area surrounding Coonabarabran (Australia's Astronomy Capital) there is a model solar system that reaches from the Observatory (The Sun) to surrounding towns and localities.

Scale models in various locations

Several towns and institutions have built outdoor scale models of the solar system. Here is a table comparing these models.

A model based on a classroom globe

Relating the size of the Solar system to familiar objects can make it easier for students to grasp the relative distances. Most classroom globes are 41 cm (16 inches) in diameter. If the Earth were reduced to this size, the Moon would be a 10 cm (4 in) baseball floating 12 metres (40 feet) away. The Sun would be a beach ball 14 stories tall (somewhat smaller than the Spaceship Earth ride at Epcot) floating 5 kilometres (3 miles) away. While a complete model to this scale has never been built, here is what a solar system built to that scale would look like. The scale is approximately 1:31,000,000.

Body	Diameter	Semi-major axis
Sun	44.6 m "(146 ft)"	zero
Mercury	15 cm "(6 in)"	1.9 km "(1.2 mi)"
Venus	38 cm "(15 in)"	3.5 km "(2.2 mi)"
Earth	41 cm "(16 in)"	4.8 km "(3.0 mi)"
Moon	10 cm "(4 in)"	12 m "(40 ft)" from Earth
Mars	23 cm "(9 in)"	7.2 km "(4.5 mi)"
Ceres	3 cm "(1 in)"	13.3 km "(8.3 mi)"
Jupiter	4.55 m "(15 ft)"	24.9 km "(15.5 mi)"
Saturn	3.81 m "(12 ft 6 in)"	45.5 km "(28.3 mi)"
Uranus	1.63 m "(5 ft 4 in)"	92.2 km "(57.3 mi)"
Neptune	1.55 m "(5 ft 1 in)"	144.4 km "(89.7 mi)"
Pluto	7 cm "(3 in)"	190 km "(118 mi)"
Eris	8 cm "(3 in)"	325 km "(202 mi)"
α Centauri A	49.5 m "(162 ft)"	1,323,500 km "(822,400 mi)"

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 α Centauri would be 1/3 of the way to the Moon.

A model based on a sports field

Relating the size of the Solar System to familiar objects can make it easier for students to grasp the relative distances. Most American schools have a football field associated with the high school (100 yards or 92 meters long). Other schools may have a soccer field nearby (90 to 120 m long). If the Sun was reduced to slightly less than one inch (21 mm), Pluto would be a 0.002 inch (0.05 mm) speck floating 100 yards (91.4 meters) away. Jupiter would be less than three-thirty-secondths of an inch (2.38 mm) in diameter and would sit on the 13 yd (11.88 m) line. Uranus would be less than one-thirty-secondth of an inch (0.79 mm) sitting nearly on the 50 yd (45.72 m) line. At that scale, the speed of light would be about 1 inch every 5 seconds (5 mm per second). Light takes about 5.5 hours to go from the Sun to Pluto.Here is what the Solar System built to that scale would look like. This complete model would be simple to make with scale planets taped to wood stakes or metal rods. The scale is approximately 1:64,700,000,000.

Body	Diameter	Semi-major axis
Sun	0.85 in "(21.5 mm)"	zero
Mercury	0.003 in "(0.08 mm)"	1.0 yd "(0.9 m)"
Venus	0.007 in "(0.19 mm)"	1.9 yd "(1.7 m)"
Earth	0.008 in "(0.20 mm)"	2.5 yd "(2.3 m)"
Mars	0.004 in "(0.10 mm)"	3.8 yd "(3.5 m)"
Ceres	0.001 in "(0.02 mm)"	7.0 yd "(6.4 m)"
Jupiter	0.085 in "(2.16 mm)"	13.1 yd "(12.0 m)"
Saturn	0.071 in "(1.8 mm)"	24.2 yd "(22.1 m)"
Uranus	0.029 in "(0.73 mm)"	48.6 yd "(44.4 m)"
Neptune	0.028 in "(0.7 mm)"	76.1 yd "(69.6 m)"
Pluto	0.002 in "(0.05 mm)"	100.0 yd "(91.4 m)"
Eris	0.002 in "(0.05 mm)"	171.4 yd "(156.7 m)"
α Centauri A	0.94 in "(23.9 mm)"	396 mi "(637 km)"

Misleading models

The models sketched here are an eye opener to many people interested in, but not knowing much about astronomy. They are far cry from the drawings of the solar system one usually sees in books or on the internet, such as the one to the right. Even if it is stated in the text that the layout is not to scale, it is often difficult for the reader to fully comprehend how discrepant the scale of the distances in the image is compared to the sizes of the objects depicted.

ee also

*Numerical model of solar system
*Orrery
*The Parable of the Solar System Model

External links

* [http://www.vendian.org/mncharity/dir3/solarsystem/ A list of websites related to solar system models]
* [http://www.troybrophy.com/projects/solarsystem/ An online scale model] (does not work in some browsers)
* [http://www.theage.com.au/news/arts/the-planets-line-up-its-st-kilda-in-summer/2005/12/02/1133422111080.html An article about a temporary exhibit in Melbourne, Australia]
* [http://kletzenbauerzeitlichtung.homepage.t-online.de/Texte/Planetenwanderwege/planetenwanderwege.html A map with solar system models in Germany]