Why Planets Speed Up Near the Sun

Planets do not move at constant speeds as they orbit the Sun. Instead, they speed up when closer to the Sun and slow down when farther away. This happens because gravity changes in strength depending on distance. When a planet swings near the Sun, gravity pulls harder and accelerates the planet. When the planet travels to the far side of its path, gravity weakens and the planet slows down.

Scientists explain this pattern through understanding elliptical orbits. Most planets follow oval-shaped paths rather than perfect circles. At the closest point, called perihelion, the Sun's gravitational pull is strongest because the distance is smallest. The planet responds by moving faster through space. At the farthest point, called aphelion, gravity's influence decreases because the distance is greatest. The planet then travels more slowly. This cause-and-effect relationship between distance and speed creates a predictable pattern. Think of it like swinging a ball on a string in a circle. When you pull the string tighter, the ball moves faster. When you let it out, the ball slows down.

Evidence shows this effect clearly in Earth's motion. Our planet reaches perihelion around January 3 each year, when it sits about 147 million kilometers from the Sun. At this point, Earth moves at approximately 30.3 kilometers per second. Six months later, around July 4, Earth reaches aphelion at about 152 million kilometers away. Earth's orbital velocity then drops to roughly 29.3 kilometers per second. That difference of one kilometer per second may seem small, but it demonstrates how gravity controls planetary motion across vast distances.

Understanding why planets change speed matters for predicting their positions accurately. Astronomers use this knowledge to calculate where planets will appear in the night sky. Space agencies apply these principles when planning spacecraft missions to other worlds. The relationship between distance and speed, described by Kepler's second law, helps scientists explain how gravity shapes motion throughout our solar system. This law states that planets sweep out equal areas in equal times, meaning they must move faster when closer to the Sun to cover the same area as when they move slower farther away.

Interesting Fact: Mars experiences an even more dramatic speed change than Earth because its orbit is more elliptical. Mars travels about 26.5 kilometers per second at perihelion but slows to only 22 kilometers per second at aphelion.