Why are the planets' orbits shown as perfect circles?

For simplicity and clarity, this model often depicts orbits as circles. In reality, planetary orbits are ellipses, but their eccentricity (deviation from a circle) is very small for most planets in our solar system. The circular approximation effectively demonstrates the core principles of orbital radius and period without the added complexity of varying orbital speed at different points in an ellipse.

Why doesn't the simulator show the planets pulling on each other?

This is a key simplification. The model calculates the gravitational force between each planet and the Sun individually (a two-body problem), which is the dominant force. Including the gravitational interactions between all planets (an N-body problem) would make the orbits more chaotic and the calculations far more complex, obscuring the fundamental laws we aim to illustrate. Real-world solar system models used by space agencies do include these perturbations.

What does Kepler's Third Law (T² ∝ r³) mean in practical terms?

This law describes a precise mathematical relationship: a planet's orbital period (T, the 'year') increases dramatically with its distance from the Sun (r). For example, Mercury (closest) orbits in 88 Earth days, while Neptune (far out) takes about 165 Earth years. If you double the average orbital radius, the orbital period increases by a factor of about 2.8 (the square root of 2 cubed). You can test this in the simulator by comparing the orbital data.

Is the force of gravity stronger on faster-moving inner planets?

Yes, but not because they are moving faster. The gravitational force is stronger because they are much closer to the Sun (force depends on 1/r²). This stronger force provides the larger centripetal acceleration required to keep them in orbit at their higher speeds. Their high speed is a *consequence* of the stronger gravity at a smaller orbital radius, not the cause of it.

Solar System

Interactive scaled model with time controls and orbital data.

About this model

At its core, this interactive model visualizes the gravitational dance of our solar system. It is governed by Newton's law of universal gravitation, which states that every particle attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of the distance between them. This is expressed mathematically as F = G * (m1 * m2) / r^2, where G is the gravitational constant. Combined with Newton's second law of motion (F = ma), this gravitational force dictates the acceleration, velocity, and ultimately the elliptical paths of the planets around the Sun. The simulator integrates these equations over time to calculate orbital trajectories. Key simplifications are made for clarity and performance: it models a two-body system for each planet (planet-Sun), ignoring the gravitational pull of other planets. It also assumes perfectly circular or elliptical orbits in a single plane, simplifying the true three-dimensional inclinations of planetary orbits. By interacting with the controls, students can directly observe the relationship between orbital radius, speed, and period, verifying Kepler's third law (T² ∝ r³) which states that the square of a planet's orbital period is proportional to the cube of its average distance from the Sun. They can manipulate time to see how orbital motion scales with distance, learning why inner planets orbit so quickly while outer worlds take centuries.

Who it's for: Middle and high school students in introductory physics or astronomy courses learning about gravity, Newton's laws, and Kepler's laws of planetary motion.

Key terms

Newton's Law of Universal Gravitation
Kepler's Laws
Orbital Period
Semi-major Axis
Elliptical Orbit
Gravitational Constant
Centripetal Force
Orbital Velocity

How it works

A stylized top-down view of the Sun and the eight major planets. Each body moves on a circular orbit with period equal to its real sidereal period (Kepler’s third law in years). Distances use a logarithmic mapping in astronomical units (AU) so both Mercury and Neptune stay on screen. This is a teaching model — not a scale-accurate N-body integrator.

Key equations

θ = (2π / T) · t + φ

r_orbit ∝ log(a) display mapping (AU)

Frequently asked questions

Why are the planets' orbits shown as perfect circles?: For simplicity and clarity, this model often depicts orbits as circles. In reality, planetary orbits are ellipses, but their eccentricity (deviation from a circle) is very small for most planets in our solar system. The circular approximation effectively demonstrates the core principles of orbital radius and period without the added complexity of varying orbital speed at different points in an ellipse.
Why doesn't the simulator show the planets pulling on each other?: This is a key simplification. The model calculates the gravitational force between each planet and the Sun individually (a two-body problem), which is the dominant force. Including the gravitational interactions between all planets (an N-body problem) would make the orbits more chaotic and the calculations far more complex, obscuring the fundamental laws we aim to illustrate. Real-world solar system models used by space agencies do include these perturbations.
What does Kepler's Third Law (T² ∝ r³) mean in practical terms?: This law describes a precise mathematical relationship: a planet's orbital period (T, the 'year') increases dramatically with its distance from the Sun (r). For example, Mercury (closest) orbits in 88 Earth days, while Neptune (far out) takes about 165 Earth years. If you double the average orbital radius, the orbital period increases by a factor of about 2.8 (the square root of 2 cubed). You can test this in the simulator by comparing the orbital data.
Is the force of gravity stronger on faster-moving inner planets?: Yes, but not because they are moving faster. The gravitational force is stronger because they are much closer to the Sun (force depends on 1/r²). This stronger force provides the larger centripetal acceleration required to keep them in orbit at their higher speeds. Their high speed is a *consequence* of the stronger gravity at a smaller orbital radius, not the cause of it.

Continue with similar topics in this category — or all 27 in Gravity & Orbits.

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Solar System

About this model

How it works

Key equations

Frequently asked questions

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About this model

How it works

Key equations

Frequently asked questions