The Law of Ellipses
Kepler's first law - sometimes referred to as the law of ellipses - explains that planets are orbiting the sun in a path described as an ellipse. An ellipse can easily be
constructed
using a pencil, two tacks, a string, a sheet of paper and a piece of
cardboard. Tack the sheet of paper to the cardboard using the two tacks.
Then tie the string into a loop and wrap the loop around the two tacks.
Take your pencil and pull the string until the pencil and two tacks
make a triangle (see diagram at the right). Then begin to trace out a
path with the pencil, keeping the string wrapped tightly around the
tacks. The resulting shape will be an ellipse. An ellipse is a special
curve in which the sum of the distances from every point on the curve to
two other points is a constant. The two other points (represented here
by the tack locations) are known as the foci
of the ellipse. The closer together that these points are, the more
closely that the ellipse resembles the shape of a circle. In fact, a
circle is the special case of an ellipse in which the two foci are at
the same location. Kepler's first law is rather simple - all planets
orbit the sun in a path that resembles an ellipse, with the sun being
located at one of the foci of that ellipse.The Law of Equal Areas
Kepler's second law - sometimes referred to as the law of equal areas - describes the speed at which any given planet will move while orbiting the sun. The speed at which any planet moves through space is constantly changing. A planet moves fastest when it is closest to the sun and slowest when it is furthest from the sun. Yet, if an imaginary line were drawn from the center of the planet to the center of the sun, that line would sweep out the same area in equal periods of time. For instance, if an imaginary line were drawn from the earth to the sun, then the area swept out by the line in every 31-day month would be the same. This is depicted in the diagram below. As can be observed in the diagram, the areas formed when the earth is closest to the sun can be approximated as a wide but short triangle; whereas the areas formed when the earth is farthest from the sun can be approximated as a narrow but long triangle. These areas are the same size. Since the base of these triangles are shortest when the earth is farthest from the sun, the earth would have to be moving more slowly in order for this imaginary area to be the same size as when the earth is closest to the sun.
The Law of Harmonies
Kepler's third law - sometimes referred to as the law of harmonies - compares the orbital period and radius of orbit of a planet to those of other planets. Unlike Kepler's first and second laws that describe the motion characteristics of a single planet, the third law makes a comparison between the motion characteristics of different planets. The comparison being made is that the ratio of the squares of the periods to the cubes of their average distances from the sun is the same for every one of the planets. As an illustration, consider the orbital period and average distance from sun (orbital radius) for Earth and mars as given in the table below.
Planet
|
Period
(s) |
Average
Distance (m) |
T2/R3
(s2/m3) |
Earth
|
3.156 x 107 s
|
1.4957 x 1011
|
2.977 x 10-19
|
Mars
|
5.93 x 107 s
|
2.278 x 1011
|
2.975 x 10-19
|
Planet
|
Period
(yr) |
Average
Distance (au) |
T2/R3
(yr2/au3) |
Mercury
|
0.241
|
0.39
|
0.98
|
Venus
|
.615
|
0.72
|
1.01
|
Earth
|
1.00
|
1.00
|
1.00
|
Mars
|
1.88
|
1.52
|
1.01
|
Jupiter
|
11.8
|
5.20
|
0.99
|
Saturn
|
29.5
|
9.54
|
1.00
|
Uranus
|
84.0
|
19.18
|
1.00
|
Neptune
|
165
|
30.06
|
1.00
|
Pluto
|
248
|
39.44
|
1.00
|
(NOTE: The average distance value is given in astronomical units where 1 a.u. is equal to the distance from the earth to the sun - 1.4957 x 1011 m. The orbital period is given in units of earth-years where 1 earth year is the time required for the earth to orbit the sun - 3.156 x 107 seconds. )
Kepler's third law provides an accurate description of the period and distance for a planet's orbits about the sun. Additionally, the same law that describes the T2/R3 ratio for the planets' orbits about the sun also accurately describes the T2/R3 ratio for any satellite (whether a moon or a man-made satellite) about any planet. There is something much deeper to be found in this T2/R3 ratio - something that must relate to basic fundamental principles of motion. these principles will be investigated as we draw a connection between the circular motion principles discussed in Lesson 1 and the motion of a satellite.