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Kepler’s Third Law Calculator

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Kepler's Third Law Calculator is a simple yet powerful tool designed to calculate the orbital period of a planet orbiting a star, specifically how long a planet takes to complete one full orbit around its star. This calculator is based on the harmonious principles laid out by Johannes Kepler in the early 17th century, encapsulated in his third law of planetary motion. With just a few inputs, anyone from students to astronomy enthusiasts can explore the dynamics of celestial bodies in our universe.

Purpose and Functionality

Kepler's Third Law states that the square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. This law applies to all planetary bodies, making it a cornerstone in the field of celestial mechanics. The calculator uses the formula:



  • T is the orbital period of the planet,
  • a is the semi-major axis of the planet's orbit,
  • π is approximately 3.14159,
  • G is the gravitational constant (6.674×10−11 3−1−26.674×10−11m3kg−1s−2),
  • M is the mass of the star (e.g., the Sun).

For planets in our solar system, this can be simplified to 2=3T2=a3, with T in Earth years and a in astronomical units (AU), reflecting the average distance from the Earth to the Sun.

Step-by-Step Examples

Example 1: Calculating Earth's Orbital Period

Given: Earth's semi-major axis =1a=1 AU.

Using the simplified formula 2=3T2=a3, we find:


1T=1 year.

Thus, Earth's orbital period is 1 year.

Example 2: Calculating Mars' Orbital Period

Given: Mars' semi-major axis 1.524a=1.524 AU.

Using the simplified formula 2=3T2=a3, we find:


≈1.88T≈1.88 years.

Mars' orbital period is approximately 1.88 years.

Table with Relevant Information

PlanetSemi-Major Axis (AU)Orbital Period (Years)


Kepler's Third Law Calculator demystifies the complex movements of planets in a way that is accessible and engaging. It bridges the gap between ancient astronomical principles and modern-day digital learning. Whether for educational purposes, research, or just out of curiosity, this calculator offers invaluable insights into the timing of the celestial dance that unfolds in our solar system and beyond. Its simplicity in design and ease of use make it an essential tool for anyone fascinated by the skies above.

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