## Some Points of Interest

The orbits of the planets lie nearly in the same plane, except for Mercury and Pluto, which deviate from this plane by 7 degrees and 17 degrees, respectively. Between the orbit of Mars and Jupiter is a concentration of asteroids known as the asteroid belt. Most of the solar system's asteroids are here.

The orbits of the planets are not equally spaced, tending (very roughly) to double between adjacent orbits as we move away from the sun.

To say that the distances between planets and the sun are very great is an understatement. Interplanetary distances are so great that it becomes awkward to speak in terms of miles or kilometers. For that reason, astronomers have agreed on something called an astronomical unit (A.U.), which is the average distance between the earth and the sunâ€”that is, 149,603,500 kilometers or 92,754,170 miles.

Let's use these units to gauge the size of the solar system. From the sun to the average distance of the outermost planet, Pluto, is 40 A.U. (3,710,166,800 miles, or almost 6 billion km). At just about a million times the radius of the earth, that's quite a distance. Think of it this way: If the earth were a golfball, Pluto would be a chickpea about 8 miles away, Jupiter would be a basketball about 1 mile away, and the sun would go floor-to-ceiling in a 10-foot room and be less than a quarter-mile away. However, compared to, say, the distance from the earth to the nearest star (after the sun), even Pluto is a near neighbor. Forty A.U. is less than 1/z000 of a light-year, the distance light travels in one year: almost 6 trillion miles. Alpha Centauri, the nearest star system to our sun, is about 4.3 light-years from us (more than 25 trillion miles). On our golf ball scale, Alpha Centauri would be about 55,000 miles away.

## Telescopes Mastery

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