Ptolemy and the ancient observation

The ancient observation explains that we are at the center of the universe and any other star revolves around us. The observation is true for the Moon and Sun but wired for the other planets like Mercury, Venus, Saturn, and so on.

They saw that these planets also moves back and forth around us. In order to explain this, they added the concept of epicycle motion. Overtime, they realized it is not true and trying to add more epicycle and making things more complicated. They started to think that the Universe cannot this be complex understanding and this idea is possibly right.

Copernicus and the modern observation

Copernicus, he believed that there is another simpler explanation and completely changed the idea of observation. He stated that the Sun is actually at the center and the Earth off-center along with other planets moving around the Sun in complete circles.

Let’s take Saturn as an example, because Earth and Saturn move at different speed where the Earth is faster, we would see the Saturn is a little bit falling behind. And when the Earth comes back from the other side, we see the Saturn is moving ahead.

Kepler and the Elliptical orbits

Everything still does not agree with the observations as the data collected are so precise. Kepler came up with another single concept that the orbits are elliptical and this very well explains the observations that meet all the criteria for a good physical theory.

1. Agree with observations and experiments
2. Has a unified description
3. And simple

Newton’s second law of motion and Universal law of gravitation in combination well explain the elliptical motion of a planet.

• F = ma means net force is equal to mass times acceleration.
• F = G(Mm)/square(R) means the gravitational force (F) of two masses M (the Earth) and m (satellite/planet) is inversely proportional to the square of distance (R) between them.

Newton’s second law of motion describes how difficult it is to influence the course of motion. An example of this is when a car and a bicycle in stationary motion are provided the same push, it is more likely for the bicycle to move. The Universal Law of Gravitation shows how strongly it is pulled by the force of gravity. If m (a satellite/planet) gets closer to M (the Earth), the force (F) increases, and so does the acceleration (a).

Kepler’s three laws:

1. The orbit of every planet is an ellipse with the Sun at one of the two foci.
2. A line running from the sun to the planet sweeps out equal areas of the ellipse in equal times.
3. The square of the orbital period of a planet is directly proportional to the cube of the radius of its orbit.