Edmond Halley and his Comet

This science blog investigates discoveries made by the 17th century English astronomer Edmond Halley. In particular we learn about the famous comet that bears his name.

 Edmond Halley by Richard Phillips

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Edmond Halley was born in Shoreditch London in 1656, the son of a wealthy soap maker. Edmond studied at St Paul’s School in London where he demonstrated a considerable talent for learning mathematics and the classics.

Panorama of London by Claes Van Visscher,1616

London before the Great Fire of 1666

The family fortunes were dented in 1666 when Edmond’s father lost several properties to the Great Fire.

Great Fire of London as seen from a boat in vicinity of Tower Wharf.

In 1673 at the age of 17 and already an accomplished astronomer, Edmond was admitted to Queen’s College at Oxford University. In 1675 he began working with the Astronomer Royal John Flamsteed at the Greenwich Royal Observatory in London.

Royal Observatory Greenwich

The Queen’s House and the Royal Observatory, Greenwich by Jan Griffier

In 1676 Halley visited the South Atlantic island of Saint Helena….

St Helena shown on map of the world in 1777

….where he set up an observatory to catalog the stars of the southern hemisphere. He stayed on St Helena for two years during which time he discovered 341 southern stars unknown to astronomy.

St Helena in 1790 showing Jamestown and the island with sailing ships in the foreground

On returning to England he was elected a fellow of the Royal Society. With an established reputation and with considerable personal wealth inherited from his father behind him, Halley was able to devote himself to a life of scientific research. In 1703 Halley was appointed Professor of Geometry at the University of Oxford and in 1720 became Astronomer Royal, a position he held until his death in 1742 at the age of 85.

In 1705 Halley published Synopsis Astronomia Cometicae…..

Synopsis of astronomy of comets- front cover of Halley's work

English translation of book in Latin

….in which he analyzed historical data of previous comet sightings. He concluded that sightings of a comet in 1456, 1531, 1607 and 1682 were all of one and the same comet.

wood carving of a comet over medieval Europe presages doom

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Halley concluded that this comet returned every 76 years. He predicted that the same comet would return in late 1758 or early 1759.

Halley’s prediction turned out to be correct; the comet did return after 76 years; it was first spotted over Germany on 26th December 1758. Thereafter it became universally known as ‘Halley’s Comet’.

Comet Halley over the river Thames near London, England in 1759. Painting by Samuel Scott.

Halley’s Comet over London in early 1759

Halley’s Comet-a description

1) The nucleus

The nucleus of Halley’s Comet has a diameter of 9.3 miles (or 15 kms.) As solar radiation heats the comet, gas and dust flow out of the nucleus. The comet’s topography including hills, small valleys and an impact crater.

  • What direction is the sun?

Comet Halley's Nucleus close up

This diagram clearly illustrates the comet’s topography.

Comet Halley image and diagram showing topographical features

Swirling dust clouds in this image of the ‘coma’ clearly shows the nucleus rotating as the comet is propelled towards the sun.

dust ejected from a rotating Halley comet nucleus

  • Can you spot the comet’s gas and dust tails?

comet halley showing ion and dust tail

2) The 76 year elliptical orbit

The comet has a 76 year elliptical orbit which takes it beyond the orbit of Neptune. The comet last reached perihelion in 1986 and will reach aphelion in 2024.

Orbital period of Halley's Comet showing full 76 year cycle© Steven Dutch University of Wisconsin

At perihelion Comet Halley moves incredibly fast at a velocity of 55km/sec. As it travels further away from the sun its velocity steadily decreases. At aphelion its velocity slows down to only 1 km/sec.

Of the 76 years it takes to complete one full orbit the sun, the comet spends a little over 2 years, or roughly 3% of its time, close enough for solar radiation to warm the comet and wake it up from its ‘deep sleep’.

3) After warming the comet nucleus becomes unstable and breaks up 

Comet Halley has orbited the sun a total of 2,000 times in the last 152,000 years. During those last 152, 000 years it has spent only c.4,560 years in regions of space in which solar radiation is strong enough to warm its nucleus. Solar radiation is strong enough to heat the nucleus only when the comet is inside the orbit of Jupiter.

A warm nucleus becomes an unstable nucleus; chunks of ice and rock break off and leave a trail of comet debris behind. It has been calculated that every time Halley approaches the sun a 6-meter deep layer of ice and rock is jettisoned into space. During the last 152,000 years Halley may have reduced in size by 80%! Within the next hundred thousand years Halley could even break up into small pieces and cease to exist.

This is exactly what happened to Comet West in March 1976; this time lapsed image shows Comet West breaking up over a period of only 16 days.

The splitting of the nucleus of Comet West photographed on 8, 12, 14, 18 and 24 March 1976

4) Comet leaves cosmic debris in its wake

You know where a jet aircraft has been flying because you can see the vapour trail it leaves.

vapour trail left by jet aircraft flying high in the sky

Similarly you know what route Halley’s comet has taken because it leaves a trail of cosmic debris- cosmic debris which includes fine dust, pebbles and rocks.

In this infrared image you can see comet debris marking the route taken by Comet 73P/ Schwassman-Wachmann 3. Along with the debris trail you can see how this comet has already broken up into several pieces.

comet crumbs from comet 73P Schwassman-Wachmann 3

Twice year, in May and October, earth’s orbit takes us across the route Halley’s Comet took in both 1910 and 1986.

Debris left by comet Halley showing Orionids and Eta Aquarids

Twice a year, in May and October, earth’s force of gravitational attraction pulls comet debris downwards into our atmosphere…

Path of comet Halley and elliptical orbit round earth
…resulting in spectacular meteor showers.

Snowy Range Wyoming Perseids meteor shower© David King Photography

Edmond Halley- dispelling the myths

Edmond Halley, a confirmed atheist, was the first astronomer to explain how comets with elliptical orbits returned periodically after defined intervals of time. In delivering this explanation he was able to debunk some of the myths and superstitions that have been associated with the appearance of comets through the ages.

For instance the appearance of Comet Halley in 87 BC was considered a good omen for the reign of King Tigranes of Armenia. The king commemorated the event by including an image of Comet Halley on his crown.

Comet Halley on crown of Armenian King Tigranes

But the appearance of Halley’s Comet in February 1066 was interpreted as a bad omen for the reign of Harold as king of England.

Bayeux Tapestry showing Halley's comet

Edmond Halley- his other achievements

Edmond Halley was a polymath who excelled in different areas of Science. Here are some of his other notable achievements:

In 1691 he built a diving bell which he used to dive 60 feet (18 m) under the River Thames . Together with five companions he remained submerged for an hour and a half. His oxygen was replenished by weighted barrels of air sent down from the surface. The bell proved to be too heavy to be of any practical use to the Royal Navy.

Halley inside diving bell under the River Thames suspended by ships

In 1696 he made a liquid filled compass designed to reduce the swing and wobble of magnetized needles of ship compasses.

liquid filled compass in box

He produced a chart showing where magnetic north varied from geographic north in different parts of the Atlantic Ocean.

Halley magnetic compass variations shown on map of Atlantic Ocean

In 1720 he tried using scientific methods to date Stonehenge.

John Constable painting of Stonehenge

‘Stonehenge’ painted by John Constable

He financed the publication of his friend Isaac Newton’s ground breaking work Mathematical Principles of Natural Philosophy (originally written in Latin) explaining the force of gravitational attraction.

Page of Newton's Philosophiæ Naturalis Principia Mathematica written in Latin

The memorial plaque dedicated to Edmond Halley in Westminster Abbey, London commemorate his considerable achievements.

Edmond Halley memorial in Westminster Abbey listing his achievements

Further Reading

Comet facts,Myths and Legends at Amazing Space.stsci.edu

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