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Making astronomical distances personal

by Henry Tjernlund
Jan. 27, 2004

This is one of those “if the Sun were the size of a grapefruit...” comparisons of planetary sizes and distances. However, what if we made it more personal? Let’s imagine a typical person as representing the size of Earth. Now this is a little tricky as people are not even close to spherical shaped. Since we think of distances in the context of walking around, we will use the projection or “foot print” of the space a person occupies on the ground. Shoulder to shoulder distance makes a good rule, and for simplicity a radius of 10 inches will be used for the radius of the Earth. So, using the shoulder to shoulder distance to be the approximate size of the Earth, let’s see what we come up with for the other objects in our solar system. Distance between the shoulders is approximate to the length of a person’s step. In this way, we can get a sense of planetary distances in the context of walking, running, biking, or driving, the type of distances that are most intuitive to us.

So, if Earth were a person standing in a spot (occupying a twenty-inch circle on the ground), then our Moon would be a newborn infant some 50 feet away. The closest planet, Venus, would be another person standing 1 mile away. Mars, the next distant planet but the opposite direction away from the Sun, would be a toddler standing 2 miles away. These distances, of course, are Venus and Mars at their closest approach to us, to Earth. The Sun would be 3.7 miles away and be the size of a 180-foot, 15-story office building.

Let’s change our reference and do the typical Sun-outward listing of planets. Again, using the same scale, the Earth is the size of a person, thus the Sun begins as a 15-story office building. Mercury is then an infant about 1.4 miles away. Venus is a person 2.7 miles away. As we stated before, the Earth is a person 3.7 miles away from the Sun. Mars is a toddler 5.6 miles away. Ceres, largest of the minor planets, is a golf ball at 10.2 miles away. Jupiter is a large room area about 20 feet on a side, and is near milepost 19. Saturn is a slightly smaller room area, 16 feet per side, 35 miles out. Uranus is 71 miles distant and a small room 9 feet per side. Neptune would be a similarly sized room, or maybe the size of a van, but some 111 miles distant from our office building of a Sun. Pluto is a trans-Neptunian planet that sometimes crosses just within Neptune’s orbit, but at its most distant Pluto could be as far away as 148 miles on our “personal” scale. Pluto would be about the size of a grapefruit, with its moon Charon as a lemon orbiting only 2 and a half feet away. Some astronomers want to reclassify Pluto and Charon as a binary planet rather than a planet and moon system.

An interesting result of this scale is that the speed of light would be about 25 miles per hour, a fast dash pace for an athlete in good shape. By comparison, space flight speeds would be a slow insect’s crawl. Let’s walk a little farther out for completion’s sake. The Kuiper Belt, a newly discovered super-sized version of the inner Asteroid Belt, starts at Neptune and extends out to about 185 miles on our personal scale. We will include, and finish with, the Oort Cloud of comets and other objects that just barely manage to stay in orbit around our sun. These extend about a light year out from our Sun. On our scale they would be 234,000 miles out, which ironically brings us back to where we started, the real distance from the Earth to the Moon. This last distance to the Oort Cloud is somewhat fuzzy, as the actual limits of our solar system are still being determined.

(This article was published in Sigma, the PARSEC science fiction club newsletter.)