# Why Your High-School Textbook Misrepresented the Shape of Earth’s Orbit: Insights from the Sun and a Bike Wheel

The common misconception that Earth’s orbit around the Sun is oval-shaped and brings the planet closer to the Sun in summer has been debunked. While it is true for the southern hemisphere, it does not hold true for the northern hemisphere. The average distance between Earth and the Sun is 150 million kilometers, and the main reason for the seasons is Earth’s tilt.

To better understand the circularity of Earth’s orbit, the author compared it to a 26-inch bike wheel and found that the deviation from a perfect circle is less than 0.1mm. Other planets, such as Venus and Neptune, have even closer to perfect circular orbits, with deviations of 14?m and 31?m respectively. Mars and Mercury have slightly more elliptical orbits, with deviations of less than 3mm and 14mm respectively.

Mathematically, one might expect a deviation of over 3% in Earth’s orbit due to the variation in distance from the Sun. However, this is not the case because the Sun is offset to one side of the ellipse as a point called the focus. Planets rarely travel at just the right speed for a perfect circle, resulting in elliptical orbits.

The ancient Greeks believed in perfect circular orbits around Earth until Copernicus discovered that planets, including Earth, orbit around the Sun. Kepler later realized that orbits are elliptical and came up with the three laws of planetary motion. The second law states that equal areas are swept out in equal amounts of time as a planet moves. This is why orbits are often depicted as ellipses in textbooks, although it can give the impression that they are more elliptical than they actually are.

In conclusion, while the ancient Greeks were wrong about Earth being at the center of the solar system, they were not far off about the shape of planetary orbits.