o Write on the board: “Relative Motion” and “Motion of the Earth, Moon, and Sun”. o Begin by telling the class that you are going to “blow a hole in what they know” and fill it with new knowledge. Explain “there are some things in science you probably think you know, but you don’t know everything. ” Ask, with a show of hands, “Who knows how the Sun, Earth and Moon move? ” o Ask a volunteer to draw on the board the motions of the Sun, Earth, and Moon. o They will probably draw a diagram that looks similar to this:
Remind the students that these “paths” are called orbits, they are a result of gravitational pull and orbital velocity, and all of the orbits are counterclockwise. o Explain that this diagram is one way to think about the motion of the Sun, Earth, and Moon, and it is not wrong, but this diagram does not tell the entire story. Relative Motion o Discuss relative motion. How do we know that something is moving? Show video disc of two trains and a man walking on a boat close to shore. Write on the board “All motion is relative to whatever frame of reference is chosen, because there is no motionless frame. Discuss, asking the following questions: 1) What is a frame of reference? (A frame of reference is a set of reference points with respect to which motion is measured.
These points move together and keep their relative distances and angles of view. Examples include interior of a house, a ship, airplane, car, railcar, spaceship, surface of the Earth, a moving elevator, a river carrying a swimmer) 2) Why is there no motionless frame? (There is nothing in the Universe that is not moving) 3) What frame of reference do we usually use when we are describing motion? (We use the surface of the Earth. Give the following example where we have 2 different frames of reference: Frame of reference A is the inside of an elevator rising with constant velocity u, while frame of reference B is the outside of the building in which the elevator is located. Is the velocity of the penny the same as seen from A and B? No, in A the penny looks like it is falling down, while in B the penny is actually moving up with respect to the building. Motions of the Sun and Earth o Tell the students that first of all we will talk about the motions of the Sun and the Earth. o Ask the students, “Does the Sun move?
How do we know? ” Students might answer that they see the Sun moving across the sky every day, but they should be asked, how do we know that the Sun is moving around the Earth and not vice versa. This can be explained by looking at how the Sun’s diameter changes during different parts of the year, by the changing locations of the stars, by applying some of Kepler’s laws and understanding gravity. We will have touched on this when we learned about the different historical models of the universe. o Explain that the Sun rotates every 27 days at its equator ( How do we know?
We have watched the location of sunspots) and moves through the universe at 19. 7 km/sec (44,000 mph) as it revolves about the Milky Way galaxy. It is tilted at a 25 degree angle and headed toward the Hercules constellation. This is the Sun’s motion relative to the other stars in our galaxy. o Model the Earth’s and Sun’s motions using the grapefruit and lime. Show the Sun (grapefruit) moving linearly through space with one hand, while the Earth (lime) moves around the Sun. Ask the students, “Do you still think the Earth moves in a circle around the Sun? Confirm that the Earth moves in a spiral around the Sun relative to the other stars. o Discuss the limitations of this model, namely the incorrect distance and sizes represented between the Earth, Sun, and Moon. Why then do we use this model? Although this model is incorrect in some ways, it is still appropriate and more convenient to show relative motions. If we represented the distance correctly, as in the previous inquiry lab about relative size and distance, the lime and grape would be too far away or too small to understand their relative motions.
When would it be inappropriate to use this model? When we are demonstrating eclipses, because it would appear that there is an eclipse of the moon every month. Motions of the Earth and Moon o Ask students, “Does the Moon revolve around the Earth? ” How do we know? Does the Earth revolve around the moon? ” Explain that the Earth and Moon revolve around each other because of the relatively large size of the Moon’s mass. They revolve around a point called the barycenter, which is their center of mass. The barycenter of the Earth/Moon system is slightly below the surface of the Earth.
Compare this motion to two dancers who are holding hands and spinning. Ask two students of different size to come to the front of the room to demonstrate. The larger person has more influence on where the smaller partner goes. Model using the lime and grape. Ask a student to hold the Sun (grapefruit) and move it slowly through space. With one hand move the Earth (lime) around the Sun and with the other hand move the Moon (grape) around the Earth. Explain that the barycenter revolves about the Sun and the Earth and Moon revolve about the barycenter.
This means that the Earth and Moon each have orbits around the Sun, but they are intertwined with each other. The Earth’s and Moon’s orbits are said to have a wobble. o Scientists discovered the planet Neptune by noticing that Uranus’ orbit was not as it should have been as predicted by Newton’s laws. They predicted that another planet was changing Uranus’ orbit because of its gravitational effect. o Show animation on the computer of the motion of the Moon around the Sun and Earth and explain how this motion appears to us on Earth as the Moon rotating about Earth. http://www. pbs. rg/wgbh/nova/tothemoon/puz2n3. html If the animation does not work, draw the relationship on the board and explain. Conclusion o Conclude the lesson, by asking students to explain the relative motion of the Earth, Moon, and Sun. Refer to the student’s original drawing as you clarify and summarize. Discuss how a student correcting their preconceptions is similar to ancient astronomers figuring out their misunderstandings about the solar system. o Complete worksheet (See Assessing Student Understanding below). o Ask the student to do research on the internet to find support for these ideas.