The Earth — Define

What equation gives the average orbital speed of a planet or satellite, and what does each symbol represent?

v = 2*pi*r / T, where v = average orbital speed, r = average radius of the (assumed circular) orbit, and T = orbital period (the time taken to complete one full orbit). The term 2*pi*r is the circumference of the orbit, so this equation states that average orbital speed = distance travelled in one orbit / time taken. It can be rearranged to find any one quantity given the other two, e.g. r = vT / (2*pi) or T = 2*pi*r / v.