A scale model of the Universe would, by definition, include ... everything.

In order to help us to visualize the relative sizes of objects and the immense distances between them, it is helpful to create scaled down models. In such a case, the length, width, and breadth of every object are reduced by a set fraction, and the distances between objects are treated similarly.

Let us consider a simple example. We wish to recreate the famous elephant and camel races of California from the late 1890s. In a particularly rosy example, the elephant was eight feet tall, the camel was five feet tall, the racetrack was a quarter of a mile long, and the camel won the race by a nose. We decide to create a one-twelfth model of the race. We must thus divide the length of each spatial measurement (each distance) by a factor of twelve.

• The Elephant
  The elephant was eight feet high, so how high will our model elephant be?
  
  We can take advantage of the fact that there are twelve inches in every foot. For a scale model to one-twelfth size, every measurement of a foot is replaced by an inch.
  

• The Camel
  We can treat the camel accordingly.
  

• The Track
  When recreating the quarter-mile race-track, we begin with the true units (miles). As our model track will be quite a bit smaller, we shall find it helpful to restate the length of the track in feet.
  

• The Camel's Nose
  It is easy to see that a length of one nose will be reduced to one-twelfth of a nose. But clearly we need to be able to restate this fanciful unit of length in terms of units that we know. In this case, we'll assume that the camel's nose was six inches long.
  
  On our recreation of the track, we'll need a space 110 feet long on which to lay down the course. We will build eight inch and five inch models of the two glorious steeds, and set them down with the camel a mere half-inch ahead at the end of the race.

When building a model of astronomical objects, our length scales are much larger than a race track. We thus find it helpful to reduce objects in size by factors of billions (one billion = 1,000,000,000).

We will list here the lengths of various objects in the local Universe, to get you started. Here is an image of the Sun and the nine major planets (courtesy Calvin J. Hamilton), showing their relative sizes. The pictures were taken by the Voyager and Mariner 10 spacecraft, and by the Hubble Space Telescope. Compare our fragile Earth, the third planet out from the Sun, with the giant Jupiter in fifth position, and with the ninth planet (tiny Pluto).

• Our star, the Sun
  The Sun has a radius of 695,000 kilometers.

• Our home, planet Earth
  Earth has a radius of 6,380 kilometers, one-hundredth that of the Sun, and is located one astronomical unit (1 A.U.), or 1.5 × 10⁸ kilometers (150,000,000, or 150 million, kilometers) away from the Sun.

• Planet Jupiter, the Jovian Giant
  Jupiter has a radius of 71,500 kilometers, one-tenth that of the Sun, and is located 5.2 A.U., or 8 × 10⁸ kilometers (800,000,000, or 800 hundred million, kilometers) away from the Sun.

• Planet Pluto, farthest from the Sun
  Pluto has a radius of 1,140 kilometers, just over one-half that of the Earth's Moon, and is located 39.5 A.U., or 6 × 10⁹ kilometers (6,000,000,000, or 6 billion, kilometers) away from the Sun. It was discovered by Las Cruces resident Clyde Tombaugh.

• The nearest star, in the Alpha Centauri System
  The Alpha Centauri solar system is located four light-years from our solar system, meaning that it takes even light four entire years to travel from one to the other. We know that light travels at 300,000 kilometers per second, so a light-year is a unit of length equal to 300,000 × 60 × 60 × 24 × 365.5 = 9.5 × 10¹² kilometers.

• The center of our Milky Way galaxy
  The center of the galaxy is located in the direction of the constellation Sagittarius, some 2.5 × 10¹⁷ kilometers away (it takes light 27,000 years to travel from the center of the Galaxy to Earth).

• The nearest galaxy, Andromeda (M31)
  Our near neighbor Andromeda lies three million light-years away, or 3 × 10¹⁹ kilometers away from us. At some point in the distant future, the Milky Way and Andromeda will be drawn together by the force of gravity and merge into a single galaxy.