Of the 7 planets in our solar system besides Earth, 5 were known to humans for much of our history: Mercury, Venus, Mars, Jupiter and Saturn. We call these the classical planets as they are visible to the naked eye, meaning we don’t need advanced telescopes to detect them. The word “planet” comes from the Greek “planētēs”, which means “wanderer”. This is because planets did not move with the rest of the night sky when observed over multiple nights. Instead, they were not always visible and they would change directions.
In order to explain the motion of planets in the sky, many theories were made. Some people thought planets were celestial beings (i.e. gods), others made complicated models of the Solar System which had Earth being the center. Eventually, a German astronomer named Johannes Kepler devised 3 laws to describe the motion of planets. As astronomy and physics advanced, we have learned that these laws are only approximations, but they are still useful to understand our Solar System. We will outline them below.
1. Planets Orbit the Sun in an Ellipse
To include all necessary detail, planets orbit the Sun in an ellipse (oval), with the Sun at one of the foci. This improves on previous models of the Solar System where it was assumed that planets orbited in circles. This is not the case, as even Earth’s orbit is an ellipse!
By Original: Arpad HorvathNew version: Rubber Duck (☮ • ✍) - The original PNG version: Kepler-first-law.png, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=646515
2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
This law is quite wordy, but it means that the area of a section of an ellipse that a planet passes through in it’s orbit will always be the same size as long as it takes place over the same amount of time. For example, in the image below if it takes the planet the same amount of time to go from position 1 to position 2 as to go from position 3 to position 4, then the shaded regions A1 and A2 will be the same size.
*Note that the image is not drawn to scale, so the areas here aren’t actually equal.
3. The Square of a Planet’s Period is Proportional of the Cube of the Length of the Semi-Major Axis of its Orbit
To understand this law, we need to go over some terminology:
- Period: How long planets take to complete their orbit. For Earth, this is one year.
- Semi-Major Axis: A line that runs from one end of an ellipse to the other that goes through both foci.
Mathematically, this law is represented as:
Where T is the period of orbit and a is the length of the semi-major axis. Note that this is not an equation (there is no equals sign). It just means that the actual equation relating period and length of the semi-major axis will be on opposite sides of the equation, potentially with other important details involved.
Conclusion
Kepler’s 3 laws provide further building blocks for our understanding of planetary motion that are fundamental to our understanding of our Solar System, gravity, space travel, and more! While modern astronomers and physicists use more complicated and advanced mathematics to describe our Solar System, Kepler helped pave the way for a rational understanding of the motion of planets around the Sun.
This has been an overview Kepler’s laws of planetary motion. For more information on astronomy or other science topics as well as assistance with homework and test preparation, feel free to reach out to an Academic Director toll-free at 1 (877) 545-7737 or via our Contact Us page.
