First look
The idea, in plain words
Throw a ball and it curves down to the ground. Throw it harder and it lands further away. Isaac Newton asked what would happen if you kept going.
Picture a cannon on a mountain so high that it stands above the air. Fire a ball level with the ground and it falls in a long curve. Fire it faster and it lands further round the world.
Now fire it at about 28,000 km/h. The ball still falls, but the Earth is round, and at that speed the ground curves away beneath the ball exactly as fast as the ball drops towards it. It never gets any closer. It goes all the way round and would hit the cannon from behind.
That is an orbit: falling, and missing, for ever.
Nothing holds the Moon up. It is falling round the Earth, and the Earth is falling round the Sun.
Go deeper
That was the idea. The same subject is written again below for readers who want more, each on a page of its own.
Words used here
- Orbit
- The path one body follows around another, held there by gravity.
- Gravity
- The pull that every mass has on every other mass.
- Free fall
- Moving under gravity alone, with nothing pushing back.
- Orbital velocity
- The speed a body needs to stay in a given orbit.
- Escape velocity
- The speed needed to leave a body for good without any further push.
Worth knowing
The Earth goes round the Sun at 29.8 km/s, about 107,000 km/h. You have travelled about 2.5 million km since this time yesterday.
Worked out from measurements. NASA NSSDC Earth fact sheet
The Earth and Moon both circle a shared balance point. It lies inside the Earth, about 4,670 km from the centre, so the Earth wobbles round it once a month.
Worked out from measurements. NASA NSSDC Moon fact sheet
Jupiter is heavy enough that it and the Sun circle a point just outside the Sun's surface. Seen from another star, the Sun would appear to wobble once every 11.9 years.
Worked out from measurements. NASA NSSDC Jupiter fact sheet
Where this comes from
The words are written for this site. The pictures are drawn by the page from the figures in the lesson, and each one says underneath what it is based on.