Revised Definition of “Kepler’s Laws of Planetary”

Kepler’s Law of Planetary

 

Introduction

 

The objectives of this assignment is to introduce a parenthetical definition, sentence definition and expanded definition of a relatively complex term. The objectives are:

 

1. Appreciate the importance and role of definitions in technical writing
2. Understand how audience and purpose indicate the need for definition
3. Differentiate between the levels of details in the definition
4. Select the right level of detail according to the situation

 

Reading Situation

 

A professor is explaining about the foundation of astronomy to students in a lecture.

 

Parenthetical Definition

 

Kepler’s laws of planetary (basic rules of planets movement) the motion describes the orbits of planets around the sun.

 

Sentence Definition

 

Kepler’s laws of planetary motion is composed of three laws that make reference to elliptical trajectories and explain how planetary velocities vary.

 

Expanded Definition

 

History

 

Kepler announced his first two laws in 1609, having found them by analysing the astronomical observations of Tycho Brahe. Kepler’s third law was published in 1619. In 1621, he noted that his third law applied to the four brightest moons of Jupiter. After his death, Newton was credited with understanding that the second law is not special to the inverse square law of gravitation and still Kepler’s laws are regarded as a masterpiece of astronomy since this is the perfect theory to explain the movement of planets in human history. (Guillemin and Sternberg 5).

 

Analysis of Parts & Visuals

 

To understand his laws completely, be sure to following three separated law:

 

1. The first law: The orbit of every planet is an ellipse with the sun at one of the two foci (focuses).

The first law assumes that the solar system is a space approximately under the central force which is called gravity. In the above figure, O is the centre of ellipse orbit, “a” implies a longer and “b” means shorter diameter of orbit. F’ and Star are a virtual focus of ellipse, r-min is a distance from Star to Perihelion and r-max is to Aphelion. In short, this law shows that planets move around stars with eccentricity (a constant, used to define shape of circles).  

 

      2. The second law: A line joining a planet and the sun sweeps out equal areas during equal intervals of time.

The second law tells us planet orbits are not a perfect circle. This is proved by:

• The revolution velocity is different by the position of the planet
• When a planet is close to the sun the revolution velocity is high, otherwise slow.

So, the above figure shows that the areas of S1, S2, and S3 are the same.

 

     3. The third law: The ratio of the square of an object’s orbital period with the cube of the semi-major axis of its orbit is the same for all objects orbiting the same primary.

Above formula assumes that M is a mass of Star and m is mass of the planet that is revolving around Star. a is longer diameter and T is sidereal period (required time for one cycle of the movement of a planet around the sun), and G Is gravity from Newton’s gravitational theory. The meaning of this formula proves the third theory is true.

 

Comparison and Contrast

 

Kepler’s laws corrected the model of Copernicus in these three sections:

 

       1. The planetary orbit is not a circle with epicycles, but an ellipse.

• Copernicus explained planetary orbit is epicycles (a perfect circle), but that’s not true.

       2. The sun is not near the centre but at a focal point (the two points the ellipse has) of the elliptical orbit.

• Copernicus said the sun is approximately at the centre of the orbit.

       3. Neither the linear speed nor the angular speed of the planet in the orbit is constant, but the area speed is constant (“Kepler’s laws of planetary motion”)

• Copernicus argued the speed of the planet in the main orbit is constant. 

 

References

Guillemin, Victor, and Shlomo Sternberg. Variations on a Theme by Kepler. American Mathematical Society, 2006. Accessed 30 January 2022.

Wikipedia contributors. “Kepler’s laws of planetary motion.” Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 25 Dec. 2021. Web. 31 Jan. 2022.