Flamsteed Astronomy Society

Comets — Messengers from deep space

October 10, 2005

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Dr Robert Massey — Royal Observatory Greenwich

Following the Flamsteed AGM, Robert talked to us about comets and gave a ‘make your own comet’ demonstration.

We may be smug that we can predict the movement of the planets and their eclipses, but not everything in the solar system is so predictable.   The exact number of sunspots, appearance of aurorae, and many comets, all come as surprises — pleasant we hope now.  By their very nature ‘great’ bright comets are unpredictable.  Jogged out of their home in the Oort cloud, they travel in round the Sun on very long orbits maybe not to return for hundreds or thousands of years.  Many we see only once.  Comets remain one of the few opportunities for amateur astronomers to achieve immortality — comets are named for their discoverers and there is still scope for amateur discoveries of new comets.  People are now doing this by analysing pictures from the SOHO satellite.

Comets of course are prominent in historical accounts.  They appear from the beginning of written records.  They were usually taken as bad news — certainly true for King Harold as in the apparition of Halley’s Comet shown on the Bayeux tapestry, but how did Duke William feel about it after the Battle of Hastings?  Giotto also put comet Halley in his ‘Adoration…’ although it is unlikely that the comet was the Star of Bethlehem.  Giotto may have seen Halley’s comet in 1301 just before painting the work in the year 1304.

Halley’s comet was the first to have its return predicted using Newton’s law of universal gravitation.  Edmund Halley (later second Astronomer Royal) had encouraged Newton and indeed paid for the publication of Newton’s Principia in 1687.  By analysing historical records, Halley deduced that the comets of 1531, 1607, and 1682 were one and the same.  He predicted its return after 76 years in 1758 when he knew he would most likely be dead.  In due course it was again detected on Christmas Eve 1758, a magnificent vindication for Halley and Newton’s laws.   Halley’s comet was disappointing on its return in 1986 and will probably be so again in 2061.

The orbits of long-period comets are relatively thin ellipses which can have any inclination to the plane of ecliptic where the planets broadly have their orbits.  Comets can come at the Sun from any angle, indeed Halley has a retrograde orbit.  Its direction of travel is opposite to that of the planets.   Most long-period comets seem to originate in the Oort Cloud around 50,000 AU from the Sun (The mean distance of the Earth from the Sun is 1 AU ‘astronomical unit’; the Kuiper Belt beyond Pluto is 30-100 AU away).   As comets near the Sun they can develop two tails: a gas tail which may shine (fluoresce) and a curved dust tail.  The tails are ‘blown’ away from the Sun by light pressure and the solar wind, so a comet will arrive ‘head first’ but depart tail-first.   Spectroscopic analysis of comets’ tails shows poisonous compounds (cyanogens) so there was some considerable anxiety when the Earth was due to travel through the tail of Halley’s comet in 1910.  The material in the tail is so tenuous though and no ill effects have been proven (yet!).



Robert Massey

Comets depicted in ancient Chinese script

Halley’s comet in the Bayeux Tapestry

Giotto’s Adoration...

Oort Cloud

Comet Halley 1910