The determination of the Astronomical Unit is easy since we know the scale of the Solar System. Given Kepler's Third Law of Planetary Motion, that is, the orbital period squared of a planet is equal to semi-major axis of its orbit cubed if the orbital period is measured in years and the semi-major axis of the orbit is measured in astronomical units (A.U.), we use the orbital periods of the planets to imply the sizes of the orbits of the planets. For Venus and Earth

• Venus: a ~ 0.7 A. U.
• Earth: a = 1.0 A. U.

The Earth, at closest approach, is 0.3 A. U. from Venus. This information, although useful, is not what we want. We want to figure out how many kilometers or miles or meters, or ... are contained in 1 A. U. We accomplish this task as follows.

Because radiation travels at the speed of light (c = 300,000 km per second), we can figure out the distance to Venus in km. The astronomical unit is found to be roughly 149,000,000 kilometers.

The A.U. is then used to determine the radius of the Sun and the luminosity of the Sun. Knowing the distances to other stars is important because it leads to estimates of their luminosities. The exercise of finding distances to objects in the Universe turns out to be a crucial exercise for many subfields of astronomy.

Other than this direct method of determining distances, the most reliable method is known as Annual Trigonometric Parallax.