Question 1

Why don’t planets fall into the Sun?

Accepted Answer

They are falling. They are also moving sideways. By the time a planet has fallen, the Sun is no longer where it was falling toward. Falling and missing, without end, is an orbit. There is no outward force holding them up.

Question 2

How does an orbit work?

Accepted Answer

At every instant a planet has a velocity — where it is already going — and gravity adds a small pull straight toward the Sun. Bend a path by the same small amount at every step and it closes. That closed path is the orbit.

Question 3

What happens when you change one part of an orbit?

Accepted Answer

The shape changes, not just the speed. Slow a planet at one point and it drops onto a longer ellipse that swings closer in and climbs back out. Speed it up and the far side lifts away. It does not spiral into the Sun unless almost all of the sideways speed is gone.

Question 4

Why do we have seasons?

Accepted Answer

Not because Earth gets closer to the Sun. When it is summer in the United States it is winter in Australia, on the same planet, at the same distance. Seasons happen because the axis is tilted 23.4° and keeps pointing the same way all year.

Question 5

Why does the Moon change shape?

Accepted Answer

The Moon makes no light of its own. One half is always lit by the Sun. Phases are your changing view of that lit half as the Moon goes around Earth — about 29.5 days from full moon to full moon.

Question 6

What causes tides?

Accepted Answer

The Moon’s gravity pulls harder on the water nearer to it than on the Earth’s centre, and harder on the centre than on the far-side water. The ocean stretches into two bulges. Earth then turns under those bulges, so most coasts see two high tides a day.

Question 7

Why does Jupiter have so many moons?

Accepted Answer

Gravity’s reach grows with mass, and Jupiter has more of it than anything in the solar system except the Sun. A wide region around it is a place where a small body stays with Jupiter instead of orbiting the Sun on its own.

Question 8

Why is the night sky dark?

Accepted Answer

If stars went on forever, every line of sight would hit a star and the night would be as bright as a surface of suns. It is not. The dark is a clue that the universe is not an infinite, unchanging hall of lights.

Big Questions

1 · Why don’t planets fall into the Sun?

2 · How does an orbit work?

3 · What happens when you change one part of an orbit?

4 · Why do we have seasons?

5 · Why does the Moon change shape?

6 · What causes tides?

7 · Why does Jupiter have so many moons?

8 · Why is the night sky dark?

The ninth question is yours