If the natural phenomena that we routinely experience every day, namely the sun rises from the east sinks to the west. Why phenomenon? Because is it true that the sun moves around the earth or vice versa the earth around the sun? This is exactly what created Kepler Law. Further consider the explanation of the law of kepler I, II and III in this article.

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**A Brief History of Kepler’s Law**

Kepler’s Law is a law discovered by German mathematicians and astronomers, namely Johannes Kepler (1571 – 1630). The discovery of the law is based on data observed by a famous Danish astronomer, Tycho Brahe (1546-1601).

Before Kepler’s law there were ancient people who embraced geocentricism, which is the understanding that justifies the earth as the center of the universe. This assumption is based on the experience of the limited human senses. Where humans observe the sun, moon and stars every day, while the earth is still.

This assumption was later developed by Greek astronomer, Claudius Ptolemy (100-170 AD) and this understanding lasted up to 1400 years. According to Claudius, the earth is at the center of **the solar system** while the sun and planets surround the earth in a circular path.

After that in 1543, a Polish astronomer named Nicolaus Copernicus (1473-1543) invented the heliocentric model. Heliocentric which means that the earth and other planets surround the sun in a circular path. However, in this opinion there is still a lack that is still using the circle as a form of the trajectory of planetary motion.

In 1596 Kepler published a book on astronomy, which was his first book. The book is titled * The Mysteri of the Universe* . In the book Kepler gives a description of the shortcomings of the two previous models, namely there is no harmony between the trajectories of the planet’s orbit with data from Tycho Brahe’s observations. Therefore Kepler left the Copernican model and Ptolemy then sought a new model.

In 1609 an orbital shape was found to fit the Brahe observation data, which was elliptical. Then the discovery was published in a book with the title **Astronomia Nova** , which is also accompanied by Kepler’s second law.

After the first law and the second law were publicized, Kepler also published the third law. Kepler’s third law is written in **Harmonices Mundi** . The law was published after ten years of publication of the second law.

**Kepler’s Law I**

Kepler’s Law I explains that all planets move in an elliptical orbit and the sun is the focal point. This shows that the distance of the planet from the sun is not always fixed. At one point the closest point ( *perihelion* ) and at another point is at the furthest point ( *aphelion* ).

Look at figure 2.25, if M and N are the elliptical focal points, the distance of each point on the ellipse to M and N remains so that the PM + Pn = Dn + Dm = BN + BM and so on.

CM / CA = eccentricity (written e)

AC = BC = half a large axis (written a)

DC = EC = small half axis (written b)

**Formula:**

b² = a² (1 – e¹) ellipse area = πab

MA = a (1 – e) if e = 0, the ellipse is a circle

MB = a (1 + e)

So it can be concluded that the distance of the planet to the sun is generally not fixed, sometimes near and sometimes farthest. When seen in Figure 2.25, the sun is at position M, the closest point of the planet (A) is called *perihelion* while the farthest point (B) is called *aphelion* .

**II Kepler’s Law**

Kepler II’s Law is also known as plot law. That is because according to the law of Kepler II the line connecting the sun with the planet at the same time sweeps away the same plot area.

The solar – planet connecting lines in the same time interval sweep away the same area. For example, a planet at a certain point is at point A. Within a certain time interval, for example ∆t, the planet takes AA ‘. A few moments later the planet in B. In ∆t time, the planet traveled BB ‘. So based on the kepler II law the MAA area ‘= MBB area’ if ∆t is the same in both. As a result of this, the planet moves quickly when the distance of the planet is close to the sun and slow when far from the sun.

**Kepler’s Law III**

Kepler III’s Law explains that T ^{2} to the square of time each planet in its orbit around the sun is directly proportional to the square of the average distance between the planet and the sun d ^{3} .

The explanation of Kepler III’s law is related to the planetary revolution. For more details about this please refer to article 3 Types of Earth Movements: Earth Rotation, Earth Revolution and Earth Precession.

Thus the article on the law of kepler I, II and III hopefully can help you in learning the law of kepler. If there are things that need to be added in this article please write in the comments column. Have a good study.