On orbits

We've all heard the expression “What goes up, must come down.” Yet how does this apply to the Moon that is both definitely up, and is most definitely not coming down?

A history of going around in the sky

For centuries the planets, the Moon and Sun were all viewed as going around a stationary Earth which was in the center of it all.

Not only that, the planets were thought to be crystal spheres being pushed around in their orbits by angels. Then came Sir Isaac Newton. The simplistic story is that he sat under an apple tree, saw an apple fall and said: "I've discovered gravity!" The problem is, people had known about falling things for a very long time.

His big idea was in fact that the same gravity that causes objects on Earth to fall also causes the orbits of the Moon and planets.

Objects in orbit are also falling.

Huh?

How falling keeps an object up

In his book Principia, Newton explained how. He imagined a planet that was featureless except for one large mountain. On this mountain sits a cannon that shoots cannonballs at different speeds.

I will follow his logic, replacing the mountaintop cannon with a tall tower and a ball on top.

Suppose the ball is given a small tap, just barely strong enough to get it off the tower. What path would it take? Simple, it would more or less fall straight down. (Reference the apple from the tree.)

OK, kind of boring. So instead, give it more of a kick. Now it lands a little further out.

Since this is so much fun, (some people have a strange view of fun) repeat with a stronger whack.

Yet, a funny thing has started happening. As the ball falls, the planet's natural curve moves away from it, causing the ball to have to "chase after" the surface. Also, note that the path of the ball is beginning to match the curve of the planet.

Give a little larger initial whack.

The ball's path has now been stretched out such that it almost matches that of the planet. The ball lands halfway around the planet.

Just a tiny more initial push.

Now the ball has missed the other side and can fly back to the start, repeating the process.

The ball is in orbit.

To summarize, orbits in orbit are still being pulled down by gravity as we would expect. However, when an object moves fast enough the path can be stretched out enough that the object cannot actually reach the ground.

Consider the quote by the physicist Richard Feynman:

"The next question was - what makes planets go around the sun? At the time of Kepler, some people answered this problem by saying that there were angels behind them beating their wings and pushing the planets around an orbit. As you will see, the answer is not very far from the truth. The only difference is that the angels sit in a different direction and their wings push inward."

Sweet spot

There is a special speed in which the orbit path exactly matches the shape of the planet. The path becomes a circle.

How fast is fast enough? Consider the following examples:

• Low-Earth orbit (100 miles high) – 18,000 mph (miles per hour)

• Moon around the Earth – 2,300 mph

• The Earth around the Sun – 66,000 mph

So orbits pose a strange and, it might seem, contradictory story. One the one hand, gravity is not being ignored. The object in orbit must still be pulled down by gravity. Yet the high speed of the orbiting object keeps it from actually falling down to the ground.

Strange but true.

On the web

How do Orbits Work?

This video demonstrates, in simple terms, how orbits work using air tables and buckets swung on ropes.

Stuff in space

Very nice!

The background of this page shows a 3-D animation tracking all the objects in orbit. Put your mouse on one and it will display the object name and show it's the orbital path of the object. Click and drag and the view will be rotated. Click on an object and the view will focus on the object and it will show information on it.

Some useful orbital terms:

• Apogee – The greatest height the object goes up to.

• Perigee – The lowest.

• Inclination – How “tipped” the orbit is. A 0º means the object orbits over the equator, a 90º means it goes over one pole than another.

• Period – Time for one complete orbit around the Earth.

When I tried it I had fun watching as a satellite went over the north pole, or passed over where I live.

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