# Physics KS4 / GCSE: How Halley’s Comet inspired Newton’s Law of Gravity

Tim Peake introduces Simon Shaper.

In 1680 Halley’s Comet appeared brightly in the sky.

Isaac Newton noticed it on November 19th and over the next week it disappeared behind the Sun.

A comet was also observed coming out the other side of the Sun in December 1680.

If it was the same comet then its path must have bent a great deal.

Simon shows a diagram of an elliptical orbit drawn by Newton.

Newton realised that a force, he called gravity, must have been responsible.

Newton’s discovery of gravity was nothing to do with an apple falling! It was made by making careful observations of a comet.

Newton and Halley set out to predict when the comet would come back.

They predicted it would return 75 years later in December 1758.

Although neither lived to see the prediction fulfilled. Halley’s comet last visited Earth in 1986 and we will see it again in 2061

### Teacher Notes

Key Stage 4

Could be used to introduce comets and the elliptical orbit.

It could be used to set the gravity section of the forces topic in historical context.

Students could be challenged to describe and explain the changing speed of a comet in its path around the Sun.

GCSE/Higher Levels

Could be used to introduce and set Newton’s Law of Gravitation in historical context.

Students could go on to use Newton’s Law of Gravitation to make calculations of the mass of the Sun, the radius of a planet’s orbit around the Sun or the velocity/time period of a planet’s orbit.

Students could also be challenged to describe and explain the changing speed of a comet in its path around the Sun.

As an extension, students could research the general equation of an ellipse and show how it reduces to the familiar form for a circle.

### Curriculum Notes

This clip will be relevant for teaching Physics/Science at KS4/GCSE in England Wales and Northern Ireland. Also at National 4/National 5 and Higher in Scotland.