# How is orbital period calculated if perihelion and aphelion are known? For example, the orbit of a spacecraft about the sun has a perihelion distance of 0.5 AU and an aphelion of 3.5 AU., what is its orbital period?

##### 1 Answer

Using Kepler's 3rd law, we get an orbital period of about

#### Explanation:

The orbital period can be calculated using Kepler's 3rd law, which states that the square of the orbital period is proportional to the cube of the average distance from the sun. In other words;

Where

We know that the orbital period for the Earth is

Remember that

Now we can calculate the orbital period.

So the spacecraft has an orbital period of about

***Note:** Kepler's 3rd law works for things orbiting bodies other than the sun, but the constant,