Apparent Distance
A fourth grade student recently asked me why the Moon and the Sun seem to follow her no matter where she goes.
This is a question asked by many kids, and it is one that is not the easiest to explain. After all, Einstein’s Theory of Relativity is based on where the observer is and what is being observed as well as the motion of both.
OOPS! Just the mention of the name Einstein generally puts fear into a reader. After all he was a genius. But you don’t have to be a genius to understand that when you walk around in your yard, very distant things remain visible while things near you are no longer visible once you pass by them.
Since the Sun and Moon are so far away from Earth, as long as the sky is clear, when I see a full moon, my children who also live in Texas, see the same full Moon at the same time that I do.
But the position of the Moon in the sky is different for friends living in other places, such as New York, London, and South Africa. This is because the Earth is round and rotates on its axis.
The diagram is a model for moonrise for observers in different locations on Earth. As drawn, the Moon is not visible to either observer. This is because the Earth is blocking the observer’s view of the Moon. Observer B in the diagram will see the Moon before observer A. This is because as the Earth rotates toward the East, observer’s position on Earth rotates so that the Moon, which is presently hidden by the Earth, becomes visible.
Find Out For Yourself
You can model a moonrise by securing a white paper circle on a wall (represents a full moon).
Stand a few feet from the wall with a basketball held in front of your face. Note that you cannot see the white paper circle.
Stilling holding the ball, slowly turn counterclockwise. Notice that as you turn, part of the white paper circle comes into view until all of the paper is visible.
How does this information relate to the Moon following you?
The Moon rises above the eastern horizon and sets below the western horizon. This is because the Earth rotates towards the east, thus giving the illusion that the Moon is moving across the sky.
Since the Moon is at such a distance from the Earth, moonlight shines on a large area of Earth, including the Northern and Southern Hemisphere. Thus, if you face the Moon, it appears as if you are heading toward it and at the same time the Moon seems to move away since it never appears to get closer no matter how fast you move.
Turn and face away from the Moon and it seems to follow you –the apparent distance between you and the Moon never seems to change.
apparent distance: This is not a measured distance. Instead, it is how far away an objects appears to you.
For more Einstein related activities see, Albert Ei
nstein.