5.5: Measuring Orbital Speed of Planets
Introduction
A Boeing 747 can fly 624 miles per hour. That’s pretty fast! Right now, Earth and other objects
in the solar system are orbiting the Sun, but how fast are these objects moving? In this
assignment, you will calculate the orbital speeds of objects in our solar system.
Part A: Planetary Periods
In the 1600s, Johannes Kepler discovered three laws that govern the motion of objects around
the sun. Kepler’s first law explains that objects travel in an elliptical, or oval-shaped, path
around the sun. Kepler’s second law explains that an object travels faster in its orbit when it is
closer to the Sun and slower in its orbit when it is farther away. In this activity, you will
investigate Kepler’s third law: the relationship between an object’s distance from the sun and
the time it takes the object to complete an orbit around the sun.
To access the Gizmo needed for this activity, open your Connexus curriculum to page 3 of
Unit 5 Lesson 5 and click on the link titled “Solar System Explorer Gizmo.”
Part B: Calculating Orbital Speed
1. Every object moves at a different average speed in its orbit around the Sun. In this activity,
you will use calculate the average orbital speeds of each planet and of Pluto. Using the
Gizmo, click on the Additional data display to find the mean orbital radius for each planet.
(Toggle between the different planets by clicking on the planet name.) Record this data in
the Mean Orbital Radius (AU) column of the table.
2. Recall that the path an object takes around the Sun is an ellipse, and not a circle. However,
to approximate the circumference, or length, of each orbit, you will treat the orbits as
circles.
a. Using the mean orbital radius (R) that you recorded, calculate the circumference of
each planet’s orbit and record it in the table. Use the following formula to calculate
the circumference:
3. The Period is the amount of time that it takes for a planet to make one complete orbit. To
make it easier for us to understand, we will record this measurement in Earth Years. It
takes the Earth 1 Earth Year (365 days) to make one complete orbit of the Sun. Planets
with a Period less than 1 Earth Year take less than 365 days to make an orbit of the Sun,
while planets with a Period greater than 1 Earth Year take more than 365 days to make an
orbit of the Sun.
a. Refer back to the Additional data display to find the Period (in Earth Years) for each
planet. (Toggle between the different planets by clicking on the planet name.)
Record this data in the Period (Earth Years) column of the table.
4. Using the periods and the circumference of orbit that you just calculated, calculate the
average orbital speed of each planet and record it in the table. Use the following formula to
calculate speed:
a. The units of your answer will be in AU/year.
5. To give you a better perspective of how fast these orbital speeds really are, convert the
orbital speeds you just calculated from AU/year to miles/hour.
1 AU/year = 10,604 miles/hour
Record your answers in the table.
Table
Object
Mean Orbital Circumference Period (Earth Orbital
Radius (AU) of Orbit (AU) Years)
Speed
(AU/Earth
year)
Orbital
Speed
(miles/hour)
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Pluto
Drawing Conclusions
1. Which planet takes the longest amount of time to travel once around its orbit?
2. Which planet takes the shortest amount of time?
3. How does an increase in orbital radius affect the period?
4. How does an increase in orbital radius affect average orbital speed?