Kepler and the Laws of Motion of the Planets
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Kepler and the Laws of Motion of the Planets
Renee Karikari
Intro to Astronomy
October 4, 2023
Kepler and the Laws of Motion of the Planets
Introduction
Johannes Kepler was a German astronomer who made significant contributions to our
understanding of the motion of planets in our solar system during the late 16th and early 17th
centuries. Kepler's work laid the foundation for our modern understanding of planetary motion
and was instrumental in the development of Isaac Newton's laws of motion and universal
gravitation.
Kepler's Laws of Planetary Motion:
Kepler's First Law (Law of Ellipses):
Kepler's first law states that the orbits of planets around the Sun are elliptical (oval) in shape,
with the Sun at one of the two foci of the ellipse.
This law replaced the previous belief that planetary orbits were perfectly circular.
Kepler's Second Law (Law of Equal Areas):
Kepler's second law, also known as the Law of Equal Areas, states that a line segment joining a
planet and the Sun sweeps out equal areas in equal intervals of time.
This means that a planet moves faster in its orbit when it is closer to the Sun (perihelion) and
slower when it is farther from the Sun (aphelion).
Kepler's Third Law (Law of Harmonies):
Kepler's third law relates the orbital periods (the time it takes a planet to complete one orbit
around the Sun) and the average distances from the Sun for different planets.
Mathematically, it can be expressed as: T^2 = k * R^3, where T is the orbital period, R is the
average distance from the Sun (semi-major axis), and k is a constant that depends on the mas
Kepler's laws provided a precise description of how planets move in their orbits, but they did not
explain why planets followed these laws. It was Isaac Newton who later developed the laws of
motion and universal gravitation, which provided the underlying physics to explain Kepler's
laws.
Isaac Newton's Contributions:
Newton's laws of motion, particularly his laws of inertia and the relationship between force and
acceleration, explained why objects, including planets, move the way they do.
Newton's law of universal gravitation provided a gravitational force equation that explained the
attractive force between all objects with mass, including planets and the Sun.
Using these principles, Newton showed that Kepler's laws could be derived from his own laws of
motion and gravitation, thus unifying celestial and terrestrial mechanics.
Hypothesis
We can derive Kepler's third law by starting with Newton's laws of motion and the universal law
of gravitation. We can therefore demonstrate that the force of gravity is the cause of Kepler's
laws. Consider a circular orbit of a small mass m around a large mass M. Gravity supplies the
centripetal force to mass m
Objective
The first is to observe the period of revolution of the planets. The second is to measure the
distance to each planet from the point of view of the Sun. The third and final objective is to use
Kepler’s 3
rd
law, and prove or disprove the existence of a constant resultant.
Procedure
TABLE 1
Planet
Start
Date(MM/DD/YR)
End
Date(MM/DD/YR)
Orbital
Period ,
p(days)
Orbital Period ,
P (years)
p/365.25
Mercury
11/23/2009
11/23/2012
(60,190
Earth days
165 Earth years Venus
11/23/2008
11/23/2012
2087
0.62 year,
Earth
11/23/2009
11/23/2012
365.25636
365.25636 solar
days
Mars
11/23/2010
11/23/2012
2000
3.53
Jupiter
11/23/2011
11/23/2014
12000
12 Earth years
Saturn
11/23/2015
11/23/2017
40,000
29.4475 yr
Uranus
11/23/2014
11/23/2016
840000
84 years.
Neptune
11/11/2016
11/23/2016
45746234
4
165 Earth years
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