Week 6 - Kepler's Laws and Retrograde Motion
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Dec 6, 2023
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Kepler’s Laws and Retrograde Motion
1
st
Law
1.
Consider the 4 ellipses above.
Just by looking at them, which of the ellipses has the lowest eccentricity?
Explain how
you know.
Ellipse A because it is the least squashed (closest to a circle).
Just by looking at them, which of the ellipses has the highest eccentricity?
Explain how
you know.
Ellipse D because it is the most squashed/stretched out.
2.
Now we will calculate the eccentricity of the 4 ellipses above.
For each ellipse, use a ruler
to measure the length of the major axis and the minor axis.
The major axis is the longer direction
and the minor axis is the shorter direction.
Make your measurements in centimeters and record
in the table on the next page.
Once you have measured the length of the major and minor axis, calculate the eccentricity for
each ellipse using the following equation:
Eccentricity
=
√
1
−
Minor Axis
2
Major Axis
2
A
B
D
C
Ellipse
Major Axis (in cm)
Minor Axis (in cm)
Eccentricity
A
4
4
0.00
B
4.2
4
0.30
C
5
4
0.60
D
10
4
0.92
3.
a)
Using the eccentricities you just calculated for the four ellipses, which ellipse has the
lowest eccentricity?
Ellipse A
b)
Which ellipse has the highest eccentricity?
Ellipse D
c)
Do your answers match your answers in Question 1?
Yes
4.
a)
The Earth has an eccentricity of 0.016.
Which of the four ellipses on the first page best
matches the orbit of the Earth around the Sun?
Ellipse A
b)
Halley’s Comet has an eccentricity of 0.967.
Draw an ellipse that looks like the orbit of
Halley’s Comet.
The ellipse will look a lot like Ellipse D which has an
eccentricity of 0.92, but just a little more squished.
5.
Since planets do not orbit the Sun in perfect circles, their distance from the Sun is constantly
changing.
We can calculate the closest and farthest distances that an object gets from the Sun
using the following equations:
Closest Distance
=
(
1
−
eccentricity
)
×
(
Semimajor axis
)
Farthest Distance
=
(
1
+
eccentricity
)
×
(
Semimajor axis
)
The semi-major axis is just the average distance of the planet from the Sun.
For instance, the
semi-major axis of the Earth is 1 AU.
Calculate the closest and farthest distance that each of these objects gets from the Sun:
Object
Average Distance
(in AU)
Eccentricity
Closest Distance
(in AU)
Farthest
Distance (in AU)
Earth
1.00 AU
0.016
0.984
1.016
Mars
1.52 AU
0.093
1.379
1.661
Neptune
30.1 AU
0.008
29.859
30.341
Pluto
39.5 AU
0.248
29.704
49.296
Halley’s Comet
17.8 AU
0.967
0.587
35.013
Comet Hyakutake
1700 AU
0.9999
0.170
3399.830
a)
Your friend claims that there are times in its orbit that Mars gets just as close to the Earth as
the Moon does.
The Moon never gets farther away from the Earth than 0.003 AU.
Using the
table above, explain whether you agree or disagree with your friend.
I disagree.
The distance from Earth to Mars will be the smallest when Mars is the closest it
gets to the Sun and the Earth is the farthest it gets from the Sun.
The closest Mars ever
gets to the Sun is 1.379 AU.
The farthest the Earth gets from the Sun is 1.016 AU.
They are
still 0.363 AU apart, which is more than 100x bigger than the distance from the Earth to the
Moon.
b) A different friend claims that there are times when Pluto (the old 9
th
planet) is actually closer
to the Sun than Neptune (the 8
th
planet).
Based on the table above, could this be a possibility?
Explain.
Yes it could.
The closest Pluto gets to the Sun is 29.704 AU which is closer than Neptune
ever gets to the Sun.
So Pluto will be closer to the Sun than Neptune for part of its orbit (it
turns out that Pluto is closer than Neptune for about 20 years in its 240 year orbit).
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Please solve and show solution. Please include ILLUSTRATION TOO. Thanks
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Table of Data for Kepler’s Third Law:
Table of Data for Kepler’s Third Law:
Planet aau = Semi-Major Axis (AU) Actual Planet Calculated Planet
Period (Yr) Period (Yr)
__________ ______________________ ___________ ________________
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Distance from the sun
Period of orbit around the sun
Earth
150 million km
___ Earth years
Mercury
58 million km
___ Earth years
Venus
108 million km
___ Earth years
Mars
228 million km
___ Earth years
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M
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A.U.,
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