Can you put the moon in orbit?


Some scientists think the moon was formed when the Earth's gravity pulled space dust together into a ball. Others think a lump of matter was passing the earth and it was captured into orbit. Still others would say a comet smashed into the earth causing the earth to squirt lava into space with exactly the right force to put it into orbit.

Now thanks a bit of JavaScript you can have a go at putting the moon into orbit with this simulation of Newtonian gravity. The big circle represents the Earth, the small one the moon. The grey circle represents the start position of the moon. The dashed red circle represents the orbit we are trying to get the moon into.

Some people think the moon came from outer space and as it approached the Earth at the correct speed and angle it was pulled into its present almost circular orbit. To test this theory click the outer space button. The launch angle is measured in degrees with zero being parallel to the Earth's surface. You should never need a force more than 10 units (you can use a decimal point if you want). Try a launch angle of -35° then adjust the force until the moon just grazes the dashed red line. What happens? Can you get the moon into the lower circular orbit? You can try any angle and force you like. Top

You cannot get the moon into an orbit that is lower than its starting position because the nature of every orbit is that the object must return to the place where it started. So if the moon starts in outer space it wants to return there. The only circular orbit you can get the moon into from outer space is an orbit that is in outer space. Try a launch angle of zero and a launch force of 4.83. Top

The moon orbit simulation was originally coded as an example of Scalable Vector Graphics (SVG is a W3C web standard which has yet to be adapted by all mainstream browsers). You can see the original graphics demo here. It only works well in Firefox browsers:

Can You Put The Moon in Orbit? - SVG

Other people think the moon was formed when an asteroid smashed into the earth sending lava into space with the correct force so that when it solidified it became the orbiting moon. Let's give that a try. Click the ground button to start the projectile on the ground representing matter being ejected out of the Earth . Adjust the force until the moon just grazes the red orbital path (somewhere between 8 and 10 units). Can you get the moon into orbit from the ground? Why not? Try any angle and force? Top

The problem is the same as outer space. Namely, because every orbit is either circular or elliptical it always returns to its starting position. If it starts on the ground and follows an elliptical path it will end up on the ground.

Some people think the moon was formed by space dust clumping together. But, how did the dust get into orbit? Where did each particle come from - outer space or Earth? Introducing more particles does not solve the problem, it just gives you more problems.

If you want to get the moon to follow the red orbital path then you have to start it somewhere on the red orbital path. Now click the orbit level button and see if you can get the moon into orbit. Top

Did you manage? Not too hard when you can start it in the right place.

I discovered this for myself when I was writing a network java game called "Interplanetary Marbles". It was a bit like Tanks but throwing marbles between planets (perhaps I should do an AJAX version). No matter how hard I tried I could not throw my marble into orbit.

When we look at the universe we see orbits at multiple levels. The moon orbits the Earth and many of the other planets have moons. Some have stable ring structures made up of orbiting particles. The planets orbit the sun and the sun along with multitudes of other stars orbit the centre of the galaxy.

There are some people who think that the moon was actually placed into orbit. It neither came from the Earth or outer space. It began its journey in a circle on the circle. But they are just unscientific, aren't they?


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