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The Question

(Submitted January 22, 1998)

I want to learn how to calculate the astronomical unit. How can I calculate it? I don't know formula and I want to learn.

The Answer

The Astronomical Unit is the average distance between the Sun and Earth. Its value is 149,597,870 km (about 93 million miles).

There are a variety of ways to measure it, but the most accurate is to fly spacecraft to various planets. Kepler's laws relate the period of a planet's orbit (in years) to the average distance of the planet to the Sun in (astronomical units).

Using these laws, you can determine the distance from Earth to a planet in astronomical units. By sending radio signals (which travel at the speed of light) to a spacecraft, and measuring how long it takes for the spacecraft to return those signals, you can determine how far away a spacecraft is in kilometers. Thus a spacecraft in orbit around another planet has its distance known in both kilometers and astronomical units, and you can just divide one by the other to get the number of kilometers in an astronomical unit.

David Palmer
for Ask an Astrophysicist

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