Calculating the size of an inferior planet's orbit is fairly easy – we need only one observation. Recall that if an inferior planet is at greatest elongation then the sun, the inferior planet, and the Earth (or other superior planet) are at a right angle. See Figure 1 at right. So measuring the angle of greatest elongation allows us to find the size of the inferior planet's orbit in terms of the size of the Earth's orbit. If θ is the greatest elongation, D is the radius of the Earth's orbit, and d is the radius of the inferior planet's orbit, thenFor convenience, astronomers use a unit of distance called an Astronomical Unit, or AU. One AU is the size of the Earth's orbit. Thus we can simplify the above formula to the following: