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Q. What is an astronomical unit? — Randy H.
A. The AU is a unit of measurement.
Astronomers use the astronomical unit (AU) to measure distances across the Solar System. One AU equals the mean distance from the Sun to Earth. That is approximately 93 million miles or 150 million kilometers.

Navigating spacecraft around the Solar System requires on accurate measurement of an AU. NASA's Jet Propulsion Lab (JPL) Deep Space Network bounced radar echoes off Venus in the 1960s to refine the value of an AU.

Light time. An alternative way to measure distances across the Solar System is in light time — lightseconds, lightminutes, lightdays, lightyears — the distance light travels in a unit of time.

Distances across the Solar System are vast when compared with travel across the surface of Earth. On the other hand, distances across the Solar System when compared with the breadth of the galaxy. For instance, the star nearest us — Proxima Centauri — is four lightyears away. That is about 253,000 AU from the Sun.

Examples of Some Solar System Distances
Kilometers or AULight TimeTrip Example
900,000 km3 secondsEarth-Moon-Earth Round Trip
54,000,000 km3 minutesSun to Mercury
149,600,000 km8.3 minutesSun to Earth – 1 AU
1,000,000,000 km1 hour1.5 x Sun-Jupiter Distance
80 AU11 hoursVoyager 1 – back in 2001
63,000 AU1 yearLightyear
253,000 AU4 yearsProxima Centauri – closest star
Source: NASA JPL

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