The average speed of Earth as it orbits the Sun is approximately 29.78 kilometers per second (18.5 miles per second).

Here's why it's an average:

* Elliptical Orbit: Earth's orbit isn't perfectly circular, it's slightly elliptical. This means Earth's distance from the Sun varies throughout the year.

* Speed Fluctuation: Due to the elliptical shape, Earth's speed changes as it orbits. It travels faster when closer to the Sun and slower when farther away.

* Average Calculation: The 29.78 km/s figure is a calculated average speed based on the total distance traveled in a year and the time it takes.