2. Planets sweep out equal areas in equal times. Every planet moves at varying rates depending upon its distance from the sun. A planet moves fastest at the perihelion and slowest at the aphelion. For example, the Earth’s average velocity in orbit is 18.6 miles per second. At its perihelion, it travels at 18.9 miles per second. At aphelion, it travels 18.3 miles per second. The velocity of the planet is inversely proportional to its radius from the sun at that time. Kepler’s second law says that if the planet in orbit in the above figure took the same amount of time to travel from point A to point B as it did from point C to point D, then the area A-B-sun is equal to the area C-D-sun. 3. Planets whose orbits are closer to the sun travel faster than planets whose orbits are farther from the sun. For example, Mercury’s average orbital speed is 30 miles per second and Pluto’s average orbital speed is 3 miles per second. Kepler knew how long each planet took to travel around the sun (it’s period) and could calculate the planets’ average distance from the sun using the following equation:
P2 = R3
where P is the planet’s period and R is the average orbital radius (average distance from the sun).
Kepler could calculate the average orbital speed for any of the planets and then, once he had found the average orbital radius of one planet, could determine the average orbital radius for any planet in the solar system. The third law can also be used with any satellite orbiting a planet. Because our solar system is orderly, we can predict many things—like where a planet will be at a specific time. However, there are also many unique characteristics of the planets and moons in our solar system. |
