We often hear people explaining that things float around in space because there is no gravity. This is not true.

Gravity extends across the whole universe and more locally, it keeps the Moon in orbit around the Earth and the Earth and other planets orbiting the Sun. What is really going on?

A good way to start is to imagine a hugely scaled-up version of the CN Tower. The rotating restaurant and observation decks are 410 kilometres above the ground. We chose this height because as we eat we can occasionally see the International Space Station whiz past. Since this is an imaginary tower we can ignore the very obvious safety issues.

As you enjoy your meal you can also enjoy the amazing panorama of the Earth and ever-changing cloudscapes far below.

The only obvious difference will be that the restaurant and viewing galleries will be airtight and pressurized. As we get higher above the ground, the air pressure drops. Above about 5 km, the air pressure is low enough for each breath not to bring in enough oxygen, so we have trouble breathing.

At the top of our tower, there is virtually no air. One thing you might not notice, because it is so familiar, is that the restaurant tables are laid out exactly as they are at restaurants closer to the ground. Knives, forks, plates and wineglasses show no tendency to float off the tables, and you can walk around more or less normally. In fact, you weigh almost 90% of what you weigh at sea level.

Imagine you are wearing a space suit and are on the special outside viewing platform. Would you step off it and go for a space-walk? If you did, you would find yourself falling towards the Earth with plenty of time to wonder what you had got wrong. One thing you will notice is that you are now weightless. This is what we experience if we are falling freely under gravity.

We get to experience this for a short time when bungee jumping or on one of the more thrilling amusement park rides. Astronauts on space walks or doing acrobatic things inside the International Space Station experience weightlessness because they too are falling freely under gravity. In fact this condition is often described as “free fall.”

To make the situation clearer, let’s imagine that on top of the tower there is a very big cannon, along with a good supply of gunpowder and cannonballs.

Load the cannon and fire it. As you expect, the cannonball follows a downward curving path under gravity until it hits the ground. Load it again, using more powder. The cannonball is moving faster and follows a more gradual downward curve until it hits the ground further away. If you keep firing the cannon over and over again, using more powder each time, the ball will hit the ground further and further away.

However, you will reach a point where something weird happens. As expected, the Earth's gravity pulls the ball into a downward curving path, but underneath, because the Earth is a sphere, the ground is curving away underneath it. The result is that ball does not hit the ground, it just keeps curving around the Earth; it is in orbit.

To make this happen, the cannonball needs to leave the muzzle of the gun at 27,600 km/h. That is the speed the International Space Station is whizzing past the restaurant windows, because like that cannonball, the station, along with its passengers, are freely falling around the Earth.

In practice we don't only choose to avoid having our spacecraft hit the ground, we need to keep them above the atmosphere too, which will slow them and bring them down.

With no convenient tower or cannon, putting something into orbit involves lifting it above the atmosphere and then accelerating it horizontally so that it is moving fast enough so that as it falls, it keeps missing the Earth. So remember, those astronauts performing their acrobatics are just as subject to gravity as we are on the ground.

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Jupiter dominates the eastern sky after dark. Saturn rises around 10pm. The Moon will be New on the 2nd, and will reach First Quarter on the 9th.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton

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