Kepler's laws of planetary motion

By Oleksandr Koliakin

Johannes Kepler discovered 3 laws of planetary motion that had a big impact on astronomy and astrophysics. Here are the three laws, according to "Sciencia":

  1. Planets move in ellipses with the sun at one focus

  2. A line drawn from the sun to the planet sweeps out equal areas in equal times.

  3. The square of a time which a planet takes to orbit the sun divided by the cube of its major semi-axis is a constant throughout the solar system

Let's take a moment to understand those laws. The first law is simple, and does not need explaining. Astronomers have never observed a planet or any other object in space that moves around a certain point in 100% precise circles.

The second law is a bit more complicated. If we have an object with a highly elliptical orbit, it would be easier to observe this effect. When this object is far from the focus around which it orbits, it would move slower. When it is closer to the focus, it would move faster. Here is a diagram to demonstrate this law:

The third law does not really need explaining. If we take the square of the time it takes for the planet to orbit the sun, and divide it by the planet's orbit's major semi-axis (the longest "radius" of the ellipse) we always get the same number for every planet or object orbiting around the sun in the solar system.


  • Sciencia (book)

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