# How does the earth move?

My son who is 5 years old is asking me a question about how the earth moves around the sun. What answer should I give him?

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Are you talking about the shape of motion, or the physics behind why it revolves? – MGZero Sep 20 '11 at 0:13
Good question- I guess the physics is harder to describe to a 5 year old so I will accept an answer that addresses the physics at the level of a 5 year old. – Algo Sep 20 '11 at 0:31

The earth goes around the sun kind of like a ball on a string goes in a circle when you swing it around. Instead of the string holding the earth, the sun's gravity holds it. As the earth goes around the sun, it also spins. This makes day and night. You can see this with a flashlight on the ball when you spin it with your hand.

Part 2: Since gravity is a little flexible, in a sense, the earth's orbit around the sun is not a perfect circle. Sometimes we're closer to the sun, and sometimes we're farther away.

The Earth's spin is not directly lined up with the sun. The Earth is tilted. So sometimes, the sun hits our part of the Earth more directly. When this happens, we get more heat and that makes summer. That's why in summer the sun is higher in the sky at noon.

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PLEASE don't say "Earth is tilted" to the child as this will serve no purpose unless you say with respect to what. It will mean nothing to the child and will only serve as a placeholder for actual understanding should the child ever take an astronomy or physics course. – user11266 Feb 25 '13 at 21:22

Okay, take this thought experiment..

Prerequisites - knowledge about gravity i.e. not more than that things fall towards earth.

Thought experiment - take a cannon and fire form the surface of earth horizontally from a high tower. Initially, the cannon ball would fall on the ground. Now gradually increase the velocity of the cannon, this will make the cannon ball fall further and further. Now a point will come when due to the spherical (ovoid to be more precise) the cannon ball would actually try revolving around the earth.(btw this velocity is called orbital velocity)

Now take this analogy and explain this happens with the Sun and Earth.

You will need the basics of how earth got separated form Sun( i.e. how it is acting like a cannon ball, anyway there is no consensus about how the satellites are present!).

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I will use Vineet Menon explanation. The main issue to comprehend, to my understanding is why does are the sun and earth interacting to begin with. I would explain that by letting an orange (or an apple, or a soccer ball, or his favorite regile toy or what ever) fall to the ground and ask him what connects the two (the ground and they toy). This should give you about 5 minutes of silnece before he comes up with any conclusion.

Once your son understand there is a gravity, it should be simpler to explain to him that the earth constantly falls into the sun but it also has an additional velocity which holds the earth in orbit (Vineet's explanation).

Note: I'm a father to a 2.5 years old and a 8 month old.

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I would start with the How by observing how the earth moves using, e.g., NASA's new Eyes on the Solar System site. Even at this point you can fascinate the kid by explaining the lunar cycle.

Then, in order to explore the Why you can attach an object (preferably a sphere) to a string and rotate it. Then explain by analogy focusing on the following points

1. The earth is attracted to the sun by gravity like the sphere is attracted to your hand by the string
2. Since it has been already in motion it will continue in a motion that is similar to the circle formed by the sphere.

Remember that at this age it is not crucial to be rigorous on every detail (like ellipse vs. circle). It is more important to provide a qualitative understanding and to nurture the kid's curiosity

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My suggestion would be to simply show him a picture or better yet animation of how the earth moves around the sun. There are also a number of interactive gravity simulations like "Universe Sandbox", that let you play around with gravity.

If the question is "why?" rather than "how?", then I would go with: No one really knows, but physicists (or some more age apropriate word) can predict the motion by using either Newtons theory of gravity or Einsteins theory of general relativity (What is a theory? You could probably just say there is one explanation/method by Einstein and one by Newton).

According to Newton massive objects attract each other by a force proportional to the inverse square of their distance from each other. (At this point you could use marbles to represent the earth and sun, then you can measure the distance with a string. To explain what the inverse of the square is, you could resort to some constructive geometry, circle inversion comes to mind. In Feynmans Messenger Lectures he gives a constructive proof of Keplers law, that you could take as inspiration).

According to Newton, this attraction is what causes the sun and earth to revolve around each other (If your son were a few years older you could use Eulers method to solve the equations of motion by hand and make a table to compute the motion, as it is done in Feynmans lectures).

To explain Einsteins theory of general relativity I would use several baloons, cut one into a rubber sheet (or let your son do it). Then take some markers and mark some points lines and circles. Now distort the rubber sheet and observe that the distance between points changes and how previously straight lines are bend and the area of the circles changes. Do the same with a baloon while pumping air into it. This demonstrates several features of curvature. According to Einstein space-time is a bit like a rubber sheet and massive objects distort it in such a way that things that would go straight, go in bend lines (again marbles would be helpful). The sun is like a massive marble at the center of the rubber sheet and the light planets speed along the indentation it causes.

Both those explanations assume a significant amount of knowledge on your part, to flesh them out and answer all the questions a 5 year old might have.

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It moves in an ellipse, which is a shape that's like a circle that's been stretched out a little bit. The amount of stretching for the earth's orbit is tiny, so if you looked at an accurate drawing of the orbit you would not be able to tell it was not a perfect circle without measuring.

If you want to draw an ellipse, you can use this method. Just remember that for the Earth's orbit, the two thumbtacks would so close that they would be on top of each other.

As for why it goes around the sun, think about a cannon shooting cannonballs horizontally on earth. As the speed of the cannonball coming out of the cannon increases, the ball goes further and further. Eventually, it goes so far that the roundness of the earth causes the ground to "get out of the way", and the ball is able to go even further because of that. Eventually it can make it to the opposite side of the planet. As you add more and more power to the cannon, it can make it even further than the opposite side and starts coming back the other way. Eventually, it comes all the way back to the cannon that it was fired out of.

In space, there's no air to slow the ball down, so if it were shot off of a high platform, it would reach the cannon it came out of at the same speed it came out of the cannon. It would then go around and around, repeating the same path over and over.

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That was useful. However, I would appreciate it if you could explain the physics of revolution at the level of a 5 year old (a challenge, I admit). – Algo Sep 20 '11 at 0:33
I hope this helps. I don't know a lot about how much 5 year olds are able to process. Let me know what needs elaboration or simplification if this is too complicated. – Dan Sep 20 '11 at 1:11

For a child of only 5 years old, I.m a science Teacher, it's enough to say that the Earth has 2 basic movements, the way it moves in it's place or axis, which is called the rotation that takes 23 hours and 56 seconds (remains for leap year), and the way it moves around the the Sun, which is called revolution that takes 365 1/4 days. There are other movements that are not understandable for his age level. That the tilting of the axis is what makes the seasons, that there is equinox 2 times a year when we have same hours of day and night and while he is in daylight others are sleeping.....blessings

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The sun is bigger than the earth and the earth is trying to run away"ie velocity" but because the sun is a big bully it just will not let go. Rather bland explanation but may be effective without to concept of gravity the bigger bully has more gravity so can hold on alot longer and a smaller sun would not be able to hold on to the earth so we would "fly away" instead of being stuck trying to run away

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I'm not sure how psychologically productive it would be to assign a negative quality to the sun/earth interaction. – Jason C Jul 18 '14 at 2:49

## protected by Qmechanic♦Jun 25 '14 at 5:50

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