Astronomical Unit

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An Astronomical Unit, commonly abbreviated  “AU,” is a standardized unit of measurement for distances on the order of magnitude of the size of our solar system.  1 AU is equal to the mean distance between the sun and the Earth’s orbit, or 92,955,807.273 miles.  Light takes approximately 499 seconds to travel this distance.  This measurement is very useful for putting the relatively vast distances involved with planetary travel into an understandable perspective.  For example, Pluto, toward the extreme outer edge of the solar system, is around 40 AU away from the sun. This means that the diameter of our solar system is at least 80 times the distance between the Earth and the Sun. 

The AU can also be converted into other units of measure for comparison.  For example, one AU is approximately 14,959,787,070.0 meters.  It is, along with the light year and the parsec, one of the main measures of astronomical distances currently in common usage.  An interesting fact to note, for example, is that the sun is around 1,700,000,000 AU from the center of the Milky Way.  That is to say, in order to travel from our solar system to the middle of the galaxy, one would have to travel the 93 million miles from Earth to the Sun and then repeat that journey no less than 1.7 trillion times.  This can, perhaps, begin to give one an inkling for the true nature of the vastness of not only interplanetary, but of interstellar, distances.  Perhaps even more sobering is the revelation that this distance is so large that it takes light well over 25,000 years to reach the Earth from that far away. 

By comparison, the moon, which is the furthest that any human being has ever ventured, is only 0.0026 AU from the Earth.  This shows rather starkly how poorly explored local space really is in the modern era. The furthest the human species has yet traveled is literally less than 1 percent of 1 1.7-billionth of the distance to the center of the galaxy it inhabits.

In concert with other units of measure, both familiar and strange, the AU can help to give a very real impression of the jaw-dropping size of the corner of the universe in which humanity finds itself.  It is a tool that helps to make the solar system, stars, and local galaxies more relatable to the human mind.

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