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 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.
The family fortunes were dented in 1666 when Edmond’s father lost several properties to the Great Fire.
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.
In 1676 Halley visited the South Atlantic island of Saint Helena….
….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.
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…..
….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.
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’.
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.
This diagram clearly illustrates the comet’s topography.
Swirling dust clouds in this image of the ‘coma’ clearly shows the nucleus rotating as the comet is propelled towards the sun.
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.
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.
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.
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.
Twice year, in May and October, earth’s orbit takes us across the route Halley’s Comet took in both 1910 and 1986.
Twice a year, in May and October, earth’s force of gravitational attraction pulls comet debris downwards into our atmosphere…
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.
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.
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.
In 1696 he made a liquid filled compass designed to reduce the swing and wobble of magnetized needles of ship compasses.
He produced a chart showing where magnetic north varied from geographic north in different parts of the Atlantic Ocean.
In 1720 he tried using scientific methods to date Stonehenge.
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.
The memorial plaque dedicated to Edmond Halley in Westminster Abbey, London commemorate his considerable achievements.
Comet facts,Myths and Legends at Amazing Space.stsci.edu