Here's why:

* Kepler's Laws of Planetary Motion: Johannes Kepler's laws describe the motion of planets around the sun. The second law states that a planet sweeps out equal areas in equal times. This means that a planet moves faster when it's closer to the sun and slower when it's farther away.

* Earth's Elliptical Orbit: Earth's orbit around the sun is not perfectly circular, it's slightly elliptical. This means that Earth is closer to the sun at certain points in its orbit than others.

* Perihelion and Aphelion: The point in Earth's orbit where it is closest to the sun is called perihelion. The point where it's farthest from the sun is called aphelion. Earth reaches perihelion around January 3rd and aphelion around July 4th.

Therefore, since Earth is furthest from the sun on July 4th/5th, it's moving the slowest at that point in its orbit. It moves fastest around January 3rd when it's closest to the sun.

It's a common misconception to think that Earth is moving slowest in the summer months because it's hot. However, the seasons are caused by the tilt of Earth's axis, not the distance from the sun.