# How Are Distances in Space Measured?

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If you could catch an express cab to the moon, traveling at 128.7 kilometers (80 miles) per hour, your ride would last a little over 124 days. Attempt to drive to the nearest star, and you'd never make it in your lifetime. The moon may look closer than the stars, but distances can deceive when you measure them using brightness and size. Space is so vast that astronomers use several units of measure to help them make sense of it all.

## The Solar System: Much Bigger Than a Breadbox

Space is mostly "space," and many objects in it are incredibly far apart by human standards. The moon, Earth's closest neighbor, has an average distance of 384,400 km (238,855 miles), while Pluto -- now known as a dwarf planet -- lives near the solar system's edge, averaging 5.8 billion kilometers (3.6 billion miles) from the sun. As of March 2013, the Voyager 1 spacecraft launched in 1977 was 11 billion miles from earth.

## Fun With Astronomical Units

To make it easier to refer to vast distances within the solar system, astronomers invented the astronomical unit, or AU. Its value is the average distance from Earth to the sun, or 149,600,000 kilometers (92,584,307 miles). Instead of expressing Pluto's distance in kilometers, you can simply say that it's 39.54 AU away.

## Why Measuring Units Help

Measuring units such as AU can help you visualize vast distances by comparing them to a known standard. Knowing that Pluto is about 39 AU from the sun, imagine a Styrofoam model of the solar system with one ball that represents the sun sitting about a foot from another ball that represents earth. You can then visualize placing another ball about 39 times farther from the sun ball to represent Pluto.

## Bring the Milky Way Down to Size

The solar system sits near the edge of the Milky Way, a galaxy composed of billions of stars. The closest star, Proxima Centauri, is about 271,000 AU from Earth. The entire galaxy, however, is 1,000,000,000,000,000,000 km (621,371,000,000,000,000,000 miles) wide. Instead of trying to measure these vast distances in kilometers or AU units, you can specify them in light years. A light year -- 9,334,200,000,000 km (5,800,000,000,000 miles) -- is the distance light travels in one year. That makes Proxima Centauri's distance in light years 4.22.

## Measure Everything

The universe, which consists of all matter and energy, may boggle the mind when you comprehend galactic distances. Andromeda, a galaxy similar to the Milky Way, is 2 million light years away. Traveling at the speed of light, you'd make the journey in 2 million years. It would take a staggering 30 billion light years to reach z8_GND_5296, the farthest known galaxy as of September 2014. When dealing with these vast expanses of space, use the parsec measuring unit, which equals 3.26 light years. A kiloparsec equals 1000 parsecs and a megaparsec represents one million parsecs.

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