It sounds like this“Things on Earth fall by gravity, but the Moon and planets are held up by some other, heavenly rule.”
Thinking the heavens follow different rules from Earth
Why it feels right
The sky looks different from the ground. Things on Earth fall and stop; things in the sky wheel round for ever, so smoothly and regularly that for most of history they seemed to belong to another order. For about 2,000 years the dominant view, from Aristotle, was that the heavens were made of a different material and moved naturally in perfect circles, while things on Earth moved up and down. It's an old, natural and long-lived idea, and it still shapes how many people picture space.
Why it’s wrong
That was the standard view until Newton. He showed that one law, a pull that weakens with the square of the distance, explains both the falling apple and the Moon. At 60 Earth radii the apple's pull should be 9.8 divided by 3,600, about 0.0027 m/s², and the Moon's measured acceleration comes out the same.
A worked example
Newton's test, which he began around 1666, went like this. An apple falls with an acceleration of 9.8 m/s². If the same gravity reaches the Moon, 60 times further from Earth's centre, the inverse-square rule says the pull there is 9.8 divided by 3,600, which is 0.0027 m/s².
Now measure the Moon directly. It goes round a circle of radius 384,400 km once every 27.3 days, which is a speed of about 1,022 m/s. A body going round a circle accelerates toward the centre by v squared divided by r: 1,022 x 1,022 / 384,400,000 = 0.0027 m/s².
The two numbers agree. The apple's gravity, thinned by distance, is exactly what the Moon's motion needs. There is no separate heavenly rule. The same law, applied to the Sun and planets, explains their orbits, the comets that return on schedule and the tides the Moon raises.
(Newton reportedly got a mismatch the first time, because the figure he had for Earth's size was off. With better measurements it matched.)
How to catch yourself
- You describe planets as moving by 'natural motion' or in a different way from falling things.
- You find it surprising that a falling apple and the Moon obey the same rule.
- You assume space physics is a separate subject from everyday physics.
What to hold instead
One law covers both: every mass pulls on every other, more weakly the further apart they are, in proportion to the square of the distance. It makes an apple fall and keeps the Moon in orbit, and the Moon's measured motion matches the prediction. Newton called it universal, and nothing since has found a place where it fails on these scales.
Where you’ll meet it
- Calculating a spacecraft's path years ahead.
- Understanding why tides follow the Moon.
- Seeing how Newton connected Kepler's laws of planetary motion to gravity.
Questions people ask
- What was Newton's moon test?
- A check that the force making an apple fall is the same one that keeps the Moon in orbit. Newton compared the Moon's measured acceleration with what the inverse-square law predicts from the apple's 9.8 m/s² divided by 3,600, and found they matched at about 0.0027 m/s².
- Is gravity the same everywhere in the universe?
- As far as we can tell, yes. The same law explains the Moon, planets, tides and the motion of stars and galaxies, with refinements from Einstein's general relativity for very strong gravity or very high precision.