Orbital speed of Earth!! help! The Earth is 150 million km from the sun. IT completes one orbit in a period of approximately 365.25 days. Calculate the orbital speed of earth in mph? OK guys so they give me the formula which is 2*pI*r /Period r= distance from the planet to the sun. and circumference= 2*pi*r so i set it up 2*3.1416*150^6 /8766 hours = 8.12^9 km/hour? then just convert into mph ?