Have you ever wondered why planets don’t move in perfect circles around the sun? Why do they sometimes seem closer or farther away from us? The answer lies in the shape of their orbits, which are not circular but elliptical.
An ellipse is a curve that looks like a stretched circle. It has two points called foci (singular: focus) that determine its shape. The sum of the distances from any point on the ellipse to the two foci is always constant. If you draw an ellipse with a pencil and a string, you can use two pins as the foci and loop the string around them. Then, you can trace the curve by keeping the string taut as you move the pencil.
In an elliptical orbit, one of the foci is the sun, and the other is an empty point in space. The planet moves around the sun in such a way that the sum of its distances to the two foci is always constant. This means that the planet is closer to the sun at some points of its orbit than at others. The closest point is called perihelion, and the farthest point is called aphelion.
The shape of an elliptical orbit can be described by a parameter called eccentricity, which ranges from 0 to 1. A circle has an eccentricity of 0, meaning that both foci are at the same point (the center). An ellipse has an eccentricity between 0 and 1, meaning that the foci are separated by some distance. A parabola has an eccentricity of 1, meaning that one focus is at infinity and the other is at a fixed point. A hyperbola has an eccentricity greater than 1, meaning that both foci are at infinity.
The eccentricity of a planet’s orbit determines how much it deviates from a circular shape. The lower the eccentricity, the more circular the orbit. The higher the eccentricity, the more elongated the orbit. For example, Mercury has the highest orbital eccentricity of all planets in our solar system, with a value of 0.2056. This means that its distance from the sun varies by about 20%. On the other hand, Venus has the lowest orbital eccentricity of all planets in our solar system, with a value of 0.0068. This means that its distance from the sun varies by less than 1%.
The reason why planets have elliptical orbits instead of circular ones is because of gravity and inertia. Gravity is the force that attracts objects with mass to each other. Inertia is the tendency of objects to resist changes in their motion. When a planet is far away from the sun, gravity is weaker and inertia is stronger, so it moves slower. When a planet is closer to the sun, gravity is stronger and inertia is weaker, so it moves faster. This creates a balance between the two forces that keeps the planet in a stable orbit.
Elliptical orbits are not unique to planets. They also apply to other celestial bodies, such as moons, asteroids, comets, and satellites. In fact, any object that orbits another object under gravity follows an elliptical path, unless it has enough speed to escape or enough drag to fall.
Elliptical orbits are one of the most important concepts in astronomy and physics. They help us understand how our solar system works and how we can explore other worlds. They also show us how beautiful and elegant nature can be.
