The motion of planets around the Sun follows Kepler’s laws of planetary motion, which describe how their speed changes depending on their position in orbit. Two key points in an elliptical orbit are:
- Perihelion – the closest point to the Sun.
- Aphelion – the farthest point from the Sun.
A planet moves fastest at perihelion and slowest at aphelion due to the influence of gravity and angular momentum. Understanding this concept helps explain planetary orbits, space missions, and celestial mechanics.
What Determines Orbital Speed?
Orbital speed is influenced by:
- The Sun’s gravitational pull – stronger when a planet is closer.
- Conservation of angular momentum – objects move faster when closer to the center of attraction.
- Orbital shape – more elongated orbits cause greater speed variations.
The formula for orbital velocity (v) at any point in an elliptical orbit is:
Where:
- G = gravitational constant
- M = mass of the Sun
- r = distance from the Sun at a given point
- a = semi-major axis of the orbit
This equation shows that velocity depends on distance from the Sun.
Orbital Speed at Perihelion
Characteristics of Perihelion
- Closest approach to the Sun
- Strongest gravitational attraction
- Highest orbital speed
According to Kepler’s second law, a planet sweeps out equal areas in equal time. This means that when the planet is near perihelion, it moves quickly to cover the same area as when it is farther away.
For Earth, perihelion occurs in early January, when it is about 147 million km from the Sun. At this point, its speed is approximately 30.29 km/s.
Orbital Speed at Aphelion
Characteristics of Aphelion
- Farthest distance from the Sun
- Weakest gravitational pull
- Slowest orbital speed
Since gravity is weaker at aphelion, the planet moves slower. For Earth, aphelion occurs in early July, at a distance of 152 million km, with a speed of about 29.29 km/s.
Why Do Planets Change Speed?
The variation in speed is explained by:
1. Kepler’s Second Law (Law of Equal Areas)
A planet must move faster at perihelion and slower at aphelion to sweep out equal areas in the same amount of time.
2. Conservation of Angular Momentum
Since angular momentum is conserved, when a planet is closer to the Sun (small radius), it must move faster to maintain balance. When farther away (large radius), it moves slower.
3. Gravitational Influence
Gravity pulls the planet inward, accelerating it at perihelion and decelerating it at aphelion.
Real-Life Applications
Understanding orbital speed variations is crucial for:
- Space missions – spacecraft use gravitational assists to gain or lose speed.
- Satellite positioning – geostationary satellites rely on stable orbits.
- Climate studies – Earth’s distance from the Sun affects solar radiation slightly.
Orbital speed varies based on a planet’s distance from the Sun. At perihelion, speed is highest due to strong gravitational attraction. At aphelion, speed is lowest as gravity weakens. These principles, governed by Kepler’s laws and conservation of angular momentum, help explain planetary motion and guide space exploration.
