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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).
http://imagine.gsfc.nasa.gov/docs/features/movies/kepler.html
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970609f.html
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
Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html
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