In Orbit
Overview
The celestial bodies of our solar system move according to rules
governed by gravity, Newton’s Laws of Motion, and mathematics.
In this computer-based simulation developed by the University of
Colorado, students have the opportunity to experiment with a limited
number of variables that impact the movement of planets around a
star. By manipulating the mass of objects and their velocities, students
can observe the changes in elliptical motion of these objects relative
to their gravitational foci. These variables add up to the challenges
and outcomes in launching an object into orbit.
Newton’s Laws of Motion:
First law: An object at rest remains at rest unless acted on by a
force. An object in motion will remain in motion with the same
velocity unless acted upon by a force.
Second law: Force acting on a mass produces acceleration. The
greater the mass of an object, the greater the amount of force is
required to accelerate the object.
Venn Diagram
Positioning
Space Science
Mathematics
Physical Science
Time Required
90 minutes
Materials
Required
•A computer connected to
the Internet per student/
group
Third law: For every action, there is an equal and opposite
reaction.
In Orbit
[ 1 ]
Background & Connection to the ISS
Newton’s Laws of Motion are at the heart of so many physical forces
that we experience on Earth. Though we are not as close to the illustrations of those Laws as expressed in orbiting bodies, orbits are
tangible for students. Moreover, the International Space Station is
an excellent opportunity to introduce and investigate Newton’s Laws
of Motion via orbits. The ISS weighs nearly one million pounds and
is essentially free falling around Earth. The primary force that keeps
the ISS from falling to Earth or leaving orbit is the gravitational force
of Earth. Its orbital path maximizes energy efficiency to keep the ISS
from crashing to the Earth’s surface or leaving the Earth’s atmosphere
and drifting off into space.
Standards
Addressed
•Most objects in the solar
system are in regular and
predictable motion. Those
motions explain such
phenomena as the day, the
year, phases of the moon,
and eclipses.
•Gravity is the force that
keeps planets in orbit
around the sun and
governs the rest of the
motion in the solar system.
Gravity alone holds us to
the Earth’s surface and
explains the phenomena of
the tides.
•The Earth is the third
planet from the sun in a
system that includes the
moon, the sun, eight other
planets and their moons,
and smaller objects, such
as asteroids and comets.
The sun, an average star,
is the central and largest
body in the solar system.
In Orbit
[ 2 ]
Activity Steps
Inquiry
1. Prompt students to think about what they
already know about orbits/orbiting. Ask
questions such as:
• Why do we have 365 days in a year?
(Why is there a leap year every four
years?) It takes approximately 365 days
(365 and ¼ days, thus a leap year) for
the Earth to orbit the sun.
• Why does the moon’s cycle between full
moons/new moons take approximately
28 days? It takes approximately 28 days
(27.3 days) for the moon to orbit the
Earth.
• How do satellites travel (or stay) in
space? They orbit the Earth from varying
altitudes.
2. Whether it’s through discussion or through
writing, students should arrive at the
common thread among the questions is
orbiting. Ensure that students have at least a
rudimentary understanding of the concept of
orbiting.
3. Share the multimedia definition of “orbit”
on the CASIS Academy microsite [www.
casisacademy.org]. Additionally, the
multimedia definitions for “microgravity” and
“satellites” help illustrate aspects of orbiting
and Newton’s Laws of Motion.
4. As a transition into the main part of the
lesson, ask students how we figured out
that satellites could orbit the Earth, or even
perhaps how Newton was able to develop
his ideas around motion long before we
launched vehicles into space. Students
should be able to articulate that it was
through observing orbits in our solar system
— that it’s orbiting that keeps the universe
organized and functioning. This lesson is a
chance to delve deeper into that idea.
Investigate & Manipulate
5. Divide students into small groups, so that
each group has access to a computer with
Internet access. (Note: you may want to
divide students earlier so their groups can
view the multimedia definition(s) in Step 3
above.)
6. Guide students through the following
experience on this website: http://phet.
colorado.edu/en/simulation/my-solar-system
• Click “Run Now” — unless you’re able to
download it.
• Move the slider all the way to accurate,
click on the tape measure and the grid.
(mid/lower right of interface)
• Click the radio button for 4 objects (lower
left of interface) and run the simulation
until the purple planet (body 2) has made
one complete orbit (one year). (Students
need to click Stop.)
• After the first orbit (year), turn off the
traces (deselect “Show Traces” box) and
watch another orbit (year) of the purple
planet (body 2).
7. Pose the following question to students
(have them answer aloud or write it down):
Is blue moon (body 3) circling the yellow
sun (body 1) or the purple planet (body 2)?
Explain your answer.
In Orbit
[ 3 ]
8. Now on the website, prompt students to:
• Increase the mass of the sun (body 1) to 400 and allow the simulation to run for one complete
orbit of the purple planet (body 2).
• Decrease the mass of the sun (body 1) to 175 and allow the simulation to run for one complete
orbit of the purple planet (body 2). (~90 seconds)
9. Pose the following questions to students (have them answer aloud or write it down):
How do the orbits of the planets change when the mass of the sun is increased or decreased?
Why? Explain your answer.
Why does the sun (body 1) follow a circular path? How does the path change as its mass changes?
Why? Explain your answer.
10. Have students continue manipulating data on the website and answering questions:
• Choose the preset for Sun and Planet from the pull-down menu.
• Complete the data table below by changing the mass as shown and recording the length of the
year in seconds, and also measuring the distance from the planet to the sun at the closest point
(perihelion) and farthest point (aphelion). (Make sure slider is set to most accurate)
Mass of Sun
(body 1)
Mass of Planet
(body 2)
200
10
400
10
600
10
800
10
1000
10
150
10
200
1
200
20
200
50
200
100
200
200
Time of One Orbit Closest Distance Farthest Distance
(planetary year) to Sun (perihelion) to Sun (aphelion)
In Orbit
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Q: When is the planet moving fastest? Why?
Q: What makes the length of the year increase and decrease? Why?
Q: A planet in a circular orbit would always be the same distance from the sun. What do you notice
about orbits with the shortest years? Why?
•Choose the ellipses preset from the pull-down menu.
•You may move the slider bar about 2/3 of the way towards fast for this simulation.
•Run the simulation until the green planet (body 4) returns to its starting point (one planetary year)
Planet
Time of One Orbit
(planetary year)
Closest Distance to
Sun (perihelion)
Farthest Distance to
Sun (aphelion)
Purple Planet (body 2)
Blue Planet (body 3)
Green Planet (body 4)
•Change the y velocity of the blue planet (body 3) to 90 and the green planet (body 4) to 70.
•Run the simulation again until the green planet (body 4) returns to its starting point (one planetary
year)
Planet
Time of One Orbit
(planetary year)
Closest Distance to
Sun (perihelion)
Farthest Distance to
Sun (aphelion)
Purple Planet (body 2)
Blue Planet (body 3)
Green Planet (body 4)
estimate
Q: How does the year of a planet closer to the sun compare with one that is farther away? Why?
Q: How can an orbit be made more circular? Explain your answer.
Q: In your own words describe what an orbit is and what factors affect the size, speed and time
(period) of an orbit.
In Orbit
[ 5 ]
Extensions & Modifications
• Give students the opportunity to have a more open-ended experience in which they use this web
resource to manipulate data and try to detect patterns or seeming anomalies.
• Extend the lesson into language arts by having students write explanatory essays that teach
readers about orbits/orbiting.
• To abridge the lesson, you can complete the online prompts and manipulation as a class on your
smart board. Your familiarity with the website and with the steps should greatly speed up the
process.
Attribution
Adapted from Chris Cochran’s Orbits-Effects of Mass and Distance,
https://phet.colorado.edu/en/contributions/view/3015
In Orbit
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Orbit! Or … Is a Bit too Confusing?
The Earth orbits the sun every 365 days. Technically, it’s every 365 and ¼ days — that’s why we have
a Leap Year every for years to make up for that quarter day. The moon orbits the Earth roughly every 28
days (27.3 to be exact). Gravity is the key to those orbits. Thanks to gravity, smaller bodies are attracted
to larger bodies. The Earth orbits the sun because the sun is much bigger, so its gravitational pull keeps
us in its orbit. Our moon orbits the Earth because our home planet is much bigger than the moon, so the
Earth’s gravitational pull keeps it in orbit.
Of course, the International Space Station also orbits the Earth (along with hundreds and hundreds of
other satellites.) The same forces help keep the ISS in orbit.
In this activity, you’re going to go to a website where you can manipulate the forces that affect orbits by
changing the size of the objects or the velocity of the orbiting object. Don’t worry. It’ll make more sense
when you go to the site. Speaking of …
1. Go to this website: http://phet.colorado.edu/en/simulation/my-solar-system
• Click “Run Now” — unless it’s already downloaded.
• Move the slider all the way to accurate, click on the tape measure and the grid. (mid/lower right
on screen)
• Click the radio button for 4 objects (lower left on screen) and run the simulation until the purple
planet (body 2) has made one complete orbit (one year). (Click Stop.)
• After the first orbit (year), turn off the traces (deselect “Show Traces” box) and watch another orbit
(year) of the purple planet (body 2).
2. Answer this question:
Is blue moon (body 3) circling the yellow sun (body 1) or the purple planet (body 2)?
Explain your answer.
3. Now on the website:
• Increase the mass of the sun (body 1) to 400 and allow the simulation to run for one complete
orbit of the purple planet (body 2).
• Decrease the mass of the sun (body 1) to 175 and allow the simulation to run for one complete
orbit of the purple planet (body 2). (approximately 90 seconds)
4. Now, do your best to answer these questions:
How do the orbits of the planets change when the mass of the sun is increased or decreased?
Why? Explain your answer.
Why does the sun (body 1) follow a circular path? How does the path change as its mass changes?
Why? Explain your answer.
In Orbit
[ 7 ]
5. Now that you have a good feel for what to do, continue to manipulate data on the website and
answering questions:
• Choose the preset for Sun and Planet from the pull-down menu.
• Complete the data table below by changing the mass as shown and recording the length of the
year in seconds, and also measuring the distance from the planet to the sun at the closest point
(perihelion) and farthest point (aphelion). (Make sure slider is set to most accurate)
Mass of Sun
(body 1)
Mass of Planet
(body 2)
200
10
400
10
600
10
800
10
1000
10
150
10
200
1
200
20
200
50
200
100
200
200
Time of One Orbit Closest Distance Farthest Distance
(planetary year) to Sun (perihelion) to Sun (aphelion)
Q: When is the planet moving fastest? Why?
Q: What makes the length of the year increase and decrease? Why?
Q: A planet in a circular orbit would always be the same distance from the sun. What do you notice about orbits with the shortest years? Why?
• Choose the ellipses preset from the pull-down menu.
• You may move the slider bar about 2/3 of the way towards fast for this simulation.
• Run the simulation until the green planet (body 4) returns to its starting point (one planetary year)
In Orbit
[ 8 ]
Planet
Time of One Orbit
(planetary year)
Closest Distance to
Sun (perihelion)
Farthest Distance to
Sun (aphelion)
Purple Planet (body 2)
Blue Planet (body 3)
Green Planet (body 4)
• Change the y velocity of the blue planet (body 3) to 90 and the green planet (body 4) to 70.
• Run the simulation again until the green planet (body 4) returns to its starting point (one planetary
year)
Planet
Time of One Orbit
(planetary year)
Closest Distance to
Sun (perihelion)
Farthest Distance to
Sun (aphelion)
Purple Planet (body 2)
Blue Planet (body 3)
Green Planet (body 4)
estimate
Q: How does the year of a planet closer to the sun compare with one that is farther away? Why?
Q: How can an orbit be made more circular? Explain your answer.
Q: In your own words describe what an orbit is and what factors affect the size, speed and time
(period) of an orbit.
In Orbit
[ 9 ]