Astronomical units, light-years, angles, and scale models

Astronomers use special units for enormous distances. An astronomical unit (AU) is the average Earth-Sun distance, about 150 million kilometers, handy within the solar system: Neptune orbits about 30 AU from the Sun. A light-year is the distance light travels in a year, about 9.46 trillion kilometers, used for stars and galaxies. Sizes and separations on the sky are measured as angles, in degrees, arcminutes (1/60 of a degree), and arcseconds (1/60 of an arcminute); the full Moon is about half a degree, or 30 arcminutes, across. Scale models reveal how empty space is: the Sun is about 109 times Earth's diameter, and the Moon, about one quarter of Earth's diameter, orbits about 384,000 km away, roughly 30 Earth diameters. If Earth were a 2-millimeter peppercorn, the Sun would be a ball about 22 centimeters across roughly 23 meters away, and Neptune would be about 700 meters away. That is the main limitation of physical models: a model that fits on a page or desk cannot show sizes and distances at the same scale.

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