View Full Version : Moon's Orbit

2004-Nov-14, 02:50 PM
This may be a bit elementary for some of you, but I'm an English teacher, so it's all very new to me.

My son, Avery, is 7 years old and is researching a question of his choice, which is:

Why does the moon orbit the earth instead of crashing into it?

Seems logical enough to him and to me. We've searched the 'net for some user-friendly articles (that is, articles that the English teacher or the 2nd grader could understand) but we haven't found a suitable explanation.

Can anyone offer any help as to where we could go to find the answer?

Avery's Dad

2004-Nov-14, 03:23 PM
Can anyone offer any help as to where we could go to find the answer?
I don't know if you've rejected this one as too diificult, but it was how I was introduced to orbits as a youngster: the ol' cannon on a mountaintop (http://www2.jpl.nasa.gov/basics/bsf3-4.html).

So... the moon orbits earth because someone shot it out of a cannon on top of a high enough mountain with enough gunpowder to make it go as fast as it does -- or something else, some set of circumstances, got it to the right speed in the right place.

Joel Clifton
2004-Nov-14, 06:06 PM
If the moon were stationary, it would fall straight down. But because of its speed, by the time it falls enough to reach the Earth, it has moved past it. It is literally falling around the Earth.

Another way of thinking about it is the "centrifugal" force is in equilibruim with the Earth's gravity.

Imagine if you had weight at the end of a spring. If you let it go, the spring will pull the weight toward your hand. The spring can be equated to Earth's gravity. When you swing it around, the centrifugal force pulls the weight away from your hand, and stretches the spring. The spring is trying to pull the weight toward your hand, but weight's vector is keeping the spring extended.

A Thousand Pardons
2004-Nov-14, 06:28 PM
In other words, it's the same reason that the television and communication satellites stay up--they are moving fast enough that they go halfway around the world before the force of gravity can pull them to the earth. Then the earth pulls them back the other way. :)

AGN Fuel
2004-Nov-14, 11:33 PM
Welcome to the BABB, AverysDad! :D

Some of the ideas behind understanding this can be a little difficult for a 7 year old, but the site given by 01101001 is a useful place to start. It is the concept that the moon (and satellites, astronauts in the shuttle etc, etc) are literally falling around the Earth that I find most kids have difficulties with. If an object is moving fast enough, it will move forward quickly enough that the curvature of the Earth matches the distance that the object is falling - in other words: although the object is falling, the Earth curves away underneath it at the same rate and so the object doesn't get any closer to the surface! For a satellite in Low Earth Orbit, the velocity necessary for this to happen is around 28,000 kph!

As a starting point, I sometimes find that asking the question "Why do astronauts in the space shuttle float?" is useful. 99.9% of the kids (& adults too for that matter!) say 'Because there's no gravity in space'. The follow up question is then, "But what keeps the moon in orbit around the Earth if there is no gravity in space?' The kids suddenly realise that the glib answer of 'No Gravity in Space' is not really correct.

You can then introduce the idea that although gravity attracts the moon to the Earth, the moon is also travelling in a (near) circle around the Earth. So if you suddenly 'turned off' gravity, the moon would fly off at a tangent (explain this with drawings!). In other words, the moon has a velocity at right angles to the line between Earth/Moon.

So now we can combine these ideas: the moon is being attracted to the Earth, but it also wants to fly off into space. The combination of these two opposing factors is that the moon literally 'falls' around the Earth - over and over and over again.

The next question is what gave the moon the initial velocity to reach this orbital speed? (e.g. for satellites, they are given the necessary velocity by the rockets that launch them into space.) However, I don't know how detailed your son's report needs to be!