Interplanetary Versus Interstellar Distances

Billions of light-years? Such distances we cannot truly grasp. But in our tour outward in the structure of the universe I have deliberately left out most of the figures about distances. I want us now to go back and try to comprehend first the distances within our own solar system, then the gulfs between the main figures of this book—the stars.

The distances between the planets utterly dwarf those we are used to on our own world. The Moon orbits an average of about 30 times Earth's diameter away from us. That's a little less than 10 times as far as a trip all the way around our world. But the closest that Venus, our closest planetary neighbor, comes is a little more than 100 times as far as the Moon. The closest that Pluto, famed for its remoteness, ever comes to Earth is about 100 times farther than the close approach of Venus. Thus, although our astronauts managed to visit the Moon at the end of a journey that was just a few days long, Pluto never comes closer than 10,000 times as far away.

But these distances are as nothing compared to those between the stars.

Perhaps the best way to convey first the hugeness of the solar system and then the ultra-hugeness of interstellar space is with astronomy writer Guy Ottewell's famous demonstration, the "1,000-Yard Model" of the solar system.

In this activity, teachers and students use a soccer ball or ball of similar size—about 8 inches wide—to represent the Sun. How big is Earth? The size of a peppercorn—a minute object only about .08 inches wide. And the distance one has to walk from the soccer ball to reach the peppercorn-size Earth in its orbit? Twenty-six yards! That sounds impressive, but you have to actually get objects this size and do the walk yourself to see how amazing it really is. Even the most knowledgeable astronomer, who thought he knew how vast the difference was by mental calculations, is surprised when it is there before his or her very eyes.

But the real shockers are yet to come. Mars is another 14 yards farther from the Sun than Earth is. But when the teacher/presenter starts pacing off the distance between Mars and Jupiter, is when the gasps start coming. We've already walked 40 yards to get from the Sun to Mars. Now the walk from Mars to Jupiter is another 95 yards—almost the length of an (American) football field. ButJupiter is the fifth of the traditional nine planets, so we're halfway out to the edge of the solar system, right? No, the gaps between the orbits of the outermost planets are much larger. The halfway point comes at about the orbit of Uranus, with only two traditional planets left. The average distance from a soccer-ball-size Sun to a pinhead Pluto (one-hundredth of an inch wide) is 1,010 yards.

Kuiper Belt Objects and vast numbers of comets are much farther out than Pluto yet still under the gravitational sway of the Sun. But all save a few of these bodies would have to be represented by objects too small to see with the naked eye. So in the 1,000-yard model, how much farther than the orbit of Pluto would you have to go to reach the nearest star system (the Alpha Centauri system)? 5 miles? 10 miles? Maybe 100 miles?

If the soccer ball (our Sun), the peppercorn, and the pinheads (Earth and its fellow planets) were scattered in a large field in New York, we would have to picture all of North America completely deserted—except for a similar field with three balls and perhaps a few more bits of nuts and rubble— somewhere in Alaska. That would be the Alpha Centauri star system, the nearest to our own.

Interestingly, of all the transitions—from on-Earth distances to the Moon, the Moon to the planets, the planets to the stars, the stars to the galaxy, and the galaxy to the universe—by far the greatest ratio is that of stellar distances to planetary. The Milky Way's satellite galaxies are little more than one Milky Way length away from the Milky Way. M31 is only about 25 Milky Way lengths away. The center of the Virgo Galaxy Cluster is only about 600 Milky Way lengths away. Even the farthest galaxies and quasars ever observed are only about 12,000 or 13,000 Milky Way lengths away. By comparison, Earth's distance from Alpha Centauri is about 270,000 times farther than Earth's distance from the Sun.