Earth, planets and orbits

1705–1758

A comet returns on schedule

Edmond Halley; refined by Alexis Clairaut and colleagues

Could a comet be the same visitor, coming back?

Halley used Newton’s laws to show that comets seen in 1531, 1607 and 1682 were one object, and predicted it would return around 1758. He died before it came. It was spotted on Christmas night 1758, and gravity became a tool for predicting the future.

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How they figured it out

Using Newton’s new methods, Halley calculated the orbits of 24 comets from old records. Three, seen in 1531, 1607 and 1682 (he had watched that one himself), had nearly identical orbits, about 76 years apart. He predicted the comet would return around 1758, knowing he would not live to see it.

As the date approached, Alexis Clairaut, Joseph Lalande and Nicole-Reine Lepaute spent months calculating by hand how much Jupiter and Saturn would delay it, and predicted its closest approach to the Sun for mid-April 1759, give or take a month. A farmer and amateur astronomer, Johann Palitzsch, spotted it on Christmas night 1758. It reached the Sun on 13 March 1759.

The equation

T≈76 yearsT \approx 76 \text{ years}

A comet on a long ellipse comes back every 76 years or so, nudged a little by Jupiter and Saturn.

Everyday echoHalley’s comet is due back in 2061.

See it in real life

One train on a very long loop

If a train with exactly the same route passed your station every 76 years, you would guess it was one train on a loop, not three trains. Halley’s comet is next near the Sun in 2061. If you are reading this, there is a good chance you can see it.

Where engineers use it

  • Predicting returns

    Space agencies predict comet and asteroid approaches decades ahead, including possible impacts.

  • Perturbation methods

    Small corrections from other planets (like Jupiter delaying Halley) are how engineers handle any small extra effect.

Where it came from