By Dean Regas
Enquirer contributor

If the Earth was the size of a golf ball, how big would the universe be?

Space is really, really big. In fact, the distances to planets, stars and other galaxies are so incredible that our minds cannot grasp the magnitude. This is where scale models can help us try to make sense of our place in the universe.

First of all, space is mostly empty. Astronomical bodies are very spread out. Where we live is an exception to the vast emptiness of the universe.

CLASSES

What: Cincinnati Observatory classes are for budding astronomers who want to learn more about astronomy. Instructors discuss what causes moon phases, identify distinctive moon features, planets, stars and constellations, and the use of binoculars and telescopes to view the heavens. Students have access to a 1843 Mitchel telescope, weather permitting. When: Sept. 1, 8, and 15 (7:30-9:30 p.m.) Cost: $50 for all three classes To register: Call Jim Neumeister, 321-5186.

First stop: the moon

The moon is 2,100 miles in diameter (roughly 1/4 that of the Earth) and orbits approximately 240,000 miles from Earth.

If the Earth were the size of a regulation 12-inch globe, the moon would be a 3-inch sphere resting 30 feet away. Twenty-seven people have made the three-day journey to the moon. Those Apollo missions from the '60s and '70s represent some of the most difficult and amazing feats in human history. In comparison, the vast majority of our satellites (the Hubble Telescope, International Space Station and Space shuttles) only travel 150-300 miles above the Earth.

Using the scale above, they would rise a mere 1/6 of an inch above the globe - or 1/800 of the distance to the moon.

Next stop: Mars

Mars is 4,200 miles in diameter and gets within 35,000,000 miles away at its closest approach to the Earth.

Mars is roughly half the size of Earth (6 inches using the model above). While the moon would be 30 feet away from the Earth, Mars would be 4,350 feet away. Could you see a six-inch orb from almost a mile away?

Traveling to Mars makes a trip to the moon look like a drive to grandma's house. To fly from the Earth to Mars takes seven months. Most recently, NASA sent two rovers to Mars which are still in operation. Imagine driving a remote control car from 35,000,000 miles!

Next stop: the Sun

The sun is 864,000 miles in diameter and 93,000,000 miles away.

Although the sun might not necessarily be "on our way" to the outer solar system, let's swing by to compare it to the Earth.

If the Earth were the size of a globe, the sun would be 108 feet in diameter resting more than two miles away.

Next stop: Saturn

Saturn is a gas giant planet 75,000 miles in diameter, a ring span of 230,000 miles and is 800 million miles from Earth.

For this model, and the remaining stages, every inch will represent 1 million miles. At this scale, the sun would be almost one inch in diameter, while the Earth would be a speck of dust 93 inches away. Saturn, from ring tip to ring tip, would be about 1/4 of an inch and orbit 800 inches (66 2/3 feet) from Earth.

Recently the Cassini spacecraft completed its seven-year journey to the ringed planet. Visit the Cassini Web site at: saturn.jpl.nasa.gov/home/index.cfm for more info and great pictures.

Next stop: Pluto

Our most distant planet is also the smallest. Pluto is 1,400 miles in diameter and is about 3 billion miles away. On our scale, it would be an infinitesimal speck 250 feet from a 1-inch sun. Traveling to Pluto would take between 10-15 years, depending on the trajectory. There are plans to send a mission to Pluto - it is, after all, the only planet never visited by spacecraft.

Next stop: Proxima Centauri

The nearest star to us other than the sun is a small white dwarf called Proxima Centauri. Proxima orbits its parent star, Alpha Centauri, which is visible to southern skywatchers as one of the brightest stars in the sky. Both stars are about 4.2 light years away, which is about 25 trillion miles. That is 694 times farther than Pluto. That means using our scale, Proxima Centauri would be out at St. Louis - 400 miles away!

With our current technology, it would take a spacecraft 77,000 years to reach the Alpha Centauri system!

Next stop: Andromeda Galaxy

The nearest spiral galaxy similar to the Milky Way is the Andromeda Galaxy. On a very clear night, away from the city lights, the Andromeda Galaxy is the farthest thing you can see with the naked eye. The nearly 300 billion stars in this galaxy give off a tremendous amount of light, but we are so far away from them that they barely appear as a gray smudge even in telescopes. Current estimates place the Andromeda Galaxy at 2.9 million light years from Earth. Even scaled down, this galaxy would still be 270,000,000 miles away.

End of the Line? The edge of the Universe

Recently the Hubble Telescope captured an amazing image of thousands of galaxies at the edge of the universe. Astronomers estimate that some of these galaxies could be as far away as 14 billion light years.

Link: To learn how to make a 1,000-yard model of the solar system, visit: www.noao.edu/education/peppercorn/pcmain.html.

Dean Regas is the outreach astronomer at the Cincinnati Observatory Center. He can be reached at dean observatory@zoomtown.com.