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Guided Notes

Astronomy

Orbits

1. What is an orbit?

• Orbit: path an object takes around another object

2. What are the shapes of orbits?

•Circle vs. Ellipse:

o Circle: a geometric figure constructed around a single point called the focus in which all possible diameters are equal.

o Ellipse: a geometric shape constructed around 2 center points called foci (plural of focus) in which all diameters have different measurements.

•Each planet in our solar system revolves around the Sun in a slightly elliptical (eccentric) orbit.

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3. Is the Sun located in the center of a planet’s orbit?

• NO! Our Sun is not located at the center of a planets orbit.

• The Sun is located at one of the foci of the ellipse. The other focus is an imaginary point in space.

Focus 1 Focus 2 (Imaginary)

4. How do we know that the Sun is not in the center of Earth’s orbit?

• Evidence: the apparent diameter of Sun changes throughout the year.

Graphing Apparent Diameter of the Sun: The data table below gives the apparent diameter of the Sun as it appears from Earth. The apparent diameter is measured in minutes (‘) and second (‘’).

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32’40’’

32’20’’

32’00’’

31’40’’

31’20’’

Ang

ular

Dia

met

er o

f Sun

 

Apparent Diameter of the Sun

Practice #1

a. According to your graph, during which season does the Sun appear largest?

b. According to your graph, during which season does the Sun appear smallest?

c. Aphelion is when the Earth is farthest from the Sun in its orbit. According to the graph, what are the approximate dates of aphelion? How do you know?

d. Perihelion is when the Earth is closest to the Sun in its orbit. According to the graph, what are the approximate dates of perihelion? How do you know?

• Conclusion: We know the Sun is not in the center of the Earth’s orbit because…

Answer in packet!

5. How elliptical (eccentric) can an orbit be?

• Eccentricity: the measured amount of “ovalness” an ellipse has

o Calculating eccentricity: Equation on ESRT Front Cover

Eccentricity= distance between foci (d) length of major axis (L)

o Measure distance between foci (d) and length of major axis (L).

▪ Major axis: largest possible diameter that can be drawn across an ellipse (diameter that passes through both foci)

6. What do the values of eccentricity mean?

• Eccentricity is a number between 0.0 and 1.0 without units that is always be rounded to the nearest thousandth place.

o The closer the eccentricity is to 0.0, the closer the

ellipse is to being a perfect circle. (Eccentricity of a perfect circle = 0.0)

o The closer the eccentricity is to 1.0, the more

elongated and flattened the ellipse is.

6. What do the values of eccentricity mean?

• Rounding Practice: Round the following numbers to the nearest thousandth:

a. 0.045889 = _______________

a. 0.73819 = _______________

Practice #2 Eccentricity: Determine the eccentricity of each orbit below by following the directions in each box.

Orbit A&B Orbit A&B: Calculate the eccentricity of the object’s orbit as shown in the diagram by completing the following steps: a) Write the equation. b) Substitute the data into the equation

with correct units. c) Solve the equation and round to the

nearest thousandth.

Eccentricity = distance between foci Length of major axis

Answer in packet!

Go back to packet for next steps

Stop!

7. How do the eccentricities of celestial objects in our solar system compare?

• Each celestial object in our solar system has a different eccentricity.

• See ESRT page 15 Solar System Data – “Eccentricity” column to compare the eccentricity of the planets in our solar system.

Practice #3 Comparing Eccentricities: Use the “Solar System Data” column labeled Eccentricity of Orbit page 15 of the ESRT to answer the following questions.

a) Which planet has the most circular orbit? b) Which planet has the most eccentric orbit? c) What is the eccentricity of the Earth’s orbit? d) How would you describe the shape of Earth’s

orbit? e) Which planet’s eccentricity is closest to the

eccentricity of Earth? f) What is the eccentricity of Earth’s moon? g) Which planet’s orbital shape is closest to

orbital shape of Earth’s moon?

8. Why do celestial objects, including Earth, have orbits?

• Gravitational attraction (gravity): force of attraction between two objects

o Two factors affect the gravitational attraction between two objects:

▪ Mass: the greater the mass of the objects, the more gravity between them.

▪ Distance: the closer the objects, the more gravity between them.

Practice: The symbols below represent star masses and distances.

a) Which diagram shows two stars that have the greatest gravitational force between them?

a) Which diagram shows two stars that have the weakest gravitational force between them?

9. Why and how does the speed of a planet change throughout its orbit?

• Gravitational attraction changes as a planet orbits the sun. This causes the orbital velocity to change.

• As gravitational attraction increases, orbital velocity increases.

o The closer the planet is to the sun, the greater the gravity, the greater the orbital velocity.

o The further a planet is from the sun, the weaker the gravity, the lower the orbital velocity.

11. How long does it take for a planet to revolve around our Sun?

• Period of revolution: the amount of time it takes for a planet to complete one orbit around the Sun.

• The further a planet is from the sun, the greater its period of revolution.

Example: Which planet on the diagram below would have a shorter period of revolution?

Planet A would have a shorter period of revolution than Planet B because Planet A is closer to the star (has a shorter path and greater orbital velocity).

Practice #4

See ESRT page 15 Solar System Data - “Period of Revolution” and “Mean Distance from the Sun” column.

1. Which planet has the shortest period of revolution?

2. Which planet has the longest period of revolution? 3. What is the period of revolution for Earth? 4. Approximately how many times longer is the period of revolution for Mars compared to the period of revolution for the Earth?

5. What is the period of revolution for Earth’s Moon?