Are textbooks wrong about the Earth's elliptical orbit?

We are taught that the Earth's orbit is elliptical, so when the Earth is farther from the Sun, the weather is winter and when it is closer it is summer. Actually, this is not entirely correct.

Most people still believe that in summer, the Earth is closer to the Sun than in winter. This is true for summer in the southern hemisphere, but not for summer in the northern hemisphere.

The southern hemisphere is 5 million km closer to the Sun in summer than in winter, but this is the opposite in the northern hemisphere. The average distance between the Earth and the Sun is 150 million km, and the main reason why Earth's weather has seasons is because the Earth is tilted relative to its orbital plane, so each pole sometimes points closer to the Sun. higher and sometimes fall farther from the Sun.

The Earth's orbit is called an ellipse but is actually only slightly flatter than a perfect circle . Yet it is often illustrated as the shape of an egg. So how do we accurately visualize the shape of Earth's orbit?

Senior lecturer Stephen Hughes at the College of Mathematics and Physics, University of Queensland, Australia, reduced the scale to compare the shape of the Earth's orbit with a 66cm diameter bicycle rim and asked for his opinion. bike shop technician about the actual degree of deformation of a rim. The results were very surprising.

The Earth's orbit is very close to a perfect circle. If this orbit were a 66cm diameter rim, it would only be flattened by 0.1mm compared to a perfect circle. The thickness of 0.1mm is equivalent to a thin layer of paint, it can be said that it is impossible to tell whether it is a perfect circle or not just by looking at it with the naked eye.

If the Earth's orbit is the size of a 66 cm diameter bicycle rim, its distortion compared to an absolute circle is only a thin layer of paint. (Photo: Stephen Hughes).

Mr. Hughes also looked at the orbits of other planets for a broader perspective. The orbits of Venus and Neptune are also almost perfectly circular. The distortion of Venus's orbit is only 14μm and that of Neptune is only 31μm (1μm or 1 micrometer is 1 millionth of a meter).

The two planets with the least circular orbits are Mars and Mercury. If you reduce the scale of Mars' orbit to the size of a 66cm diameter bicycle rim, the deformation is less than 3 mm, a flattening that you almost cannot feel if you ride on this bicycle. Mercury's orbit is the most oblate, with a distortion of 14mm, and thus only 2%.

Tiny little distortion

Thinking mathematically, after reading the above section, you may have a question: If the average distance between the Earth and the Sun is 150 million km and this distance fluctuates 5 million km depending on the time. During the year, the orbit's distortion compared to an absolute circle is just over 3%, right?

The true shape of Earth's orbit: almost a perfect circle. Length a is the major semi-axis and length b is the minor semi-axis of the ellipse. Aphelion (aphelion) is the farthest distance and perihelion (perihelion) is the closest distance from the Earth to the Sun (Photo: Stephen Hughes).

The answer is that the Sun is not in the center of the ellipse but is off to the side at the focal point. If, during formation, a planet moved at the correct speed to counteract the force of gravity, it would move in a circle.

However, in reality, planets rarely move at a steady speed that would allow for a circular orbit . Sometimes they move faster, sometimes slower, so their orbits are elliptical.

Almost reaching the perfect circle

Thousands of years ago, the ancient Greeks believed that all objects in the universe orbit the Earth in absolute circles. People believed this until Polish astronomer Nicolaus Copernicus (1473-1543) noticed that the planets, including Earth, actually orbit the Sun.

Astronomer Copernicus believed that the orbits of the planets were circular. Later, German astronomer and mathematician Johannes Kepler (1571-1630) discovered that Copernicus was wrong and he came up with three laws of planetary motion.

The first law is that the orbits of the planets are elliptical, not circular. The third law relates the size of a planet's orbit to the time it spends in orbit in a way that is very complicated and unnecessary for us to cover here.

The second law is about the speed at which each planet moves in its orbit. If you draw a straight line from the Sun to any planet, along with the motion of that planet, this line will sweep into an area that will take the same amount of time for the planet to go through than the other planet. through the scanning range of another planet. That's because planets move faster when they are closer to the Sun.

The main reason the books draw the orbit into a distinct ellipse is to illustrate Kepler's second law . If the Earth's orbit were drawn to the correct scale, no difference between the scanned areas would be visible to the naked eye.

Illustrations in books often lead to the misunderstanding that the Earth's orbit around the Sun is shaped like this. (Photo: Stephen Hughes).

However, overdrawing the image like this makes it easy for viewers to mistake the Earth's orbit for being very flat.

Although the ancient Greeks were wrong about the Earth being the center of the Solar System, they were not so wrong about the shape of the planets' orbits. Thus, if we ignore the exaggerated drawing to easily illustrate Kepler's second law, we can say that the Earth's orbit is circular.