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An
information leaflet
by
Dr. Gregory Beekman
of
Keele
University
Comets Should you wish to contact the
society or require general information please contact ASTRA
using the following Email address: Should you encounter any problems
with this Web
Page please
email: ------------------------------------ The first observations of
comets were made not for their astronomical (i.e.
scientific) worth but for their astrological significance:
they were believed to be omens in the sky. For example, the
Bayeux Tapestry depicts Halley's Comet above the battle of
1066, which the Norman's won. They believed that the comet
had foretold of their victory over the English King Harold.
Atahuallpa, the last native ruler of Peru, linked the death
of a man to a comet that had appeared in the constellation
of Perseus. But the astrological connection of comets with
disasters was a tenuous one at best. Comets were associated
with Mars, and it was Mars that caused wars and the
destruction of peoples .. It wasn't until the 15th
century that comets were systematically observed, by
Toscanelli, Regiomantanus and Walther. The main emphasis now
was on their precise co-ordinates and orbit in space.
However, people of those days subscribed to the Aristotelian
view of the Universe and assumed (wrongly) that the comets
were closer than the Moon, and that they were of atmospheric
origin. Fracastro, in 1538, discovered that cometary tails
always point away from the Sun, and led to the idea that a
comet was actually a lens, the tail being explained as the
focussing of the Sun's rays through it. There were many
criticisms of this model, but the idea of a comet as a
spherical lens took a long time to die out. The 16th and 17th centuries
saw the destruction of the Aristotelian view of the
Universe, but it took a long time for this to happen. Tycho
Brahe (born in Denmark in 1546) was the greatest naked-eye,
pre-telescope observer that has ever lived, achieving
unprecedented accuracy in his astronomical measurements. In
1572 he was the first to observe a "new" star that had
appeared in the W-shaped constellation of Cassiopeia.
(Nowadays we would call this "new" star a supernova, a
violent explosion marking the end of a stars life.)
According to Aristotle, the "spheres" beyond the Moon were
unchanging but here was a new star clearly in contradiction
with that view. In 1577, another comet
appeared. This became as bright as Venus, and had a huge 22
degree-long tail. Michael Mastlin, Kepler's teacher,
observed this comet and placed it beyond the distance of the
Moon; instead, he placed it on the same sphere as Venus.
Tycho Brahe also observed this comet, and was the first to
calculate the comets trajectory. He placed it beyond even
the sphere of Venus, describing it "as if it were a
fortuitous and extraordinary planet". Although he praised
Mastlin, he emphasised that there were no real spheres in
the heavens. Another comet appeared in
1580, for which Mastlin could find no measurable parallax
(which depends on distance). He thus concluded that the
comet was further away than the Moon, indicating that the
Aristotelian view of physics was wrong, and began openly
attacking it. All of these arguments helped to create an
atmosphere that would eventually lead to the destruction of
the old physics over the next two centuries. In the early 1600s there
were discussions of the (then unknown) laws that governed
the paths of the planets and their relation with the newly
emerging science of dynamics. It was believed that there was
some sort of central (sun-directed) force of attraction that
controlled the motion of the planets but what this was and
how it operated, no-one was entirely sure. This discussion
was stimulated by the comet of 1644. There was an attempt to
derive the new comets path by assuming (following Kepler)
that it followed a straight line at constant speed, but they
could not explain it's orbit. A few months later, another
comet appeared which helped to stir a flagging interest in
the problem of orbital motion. It soon became evident,
however, that this problem was beyond the powers of all
concerned. For example, Robert Hooke tried a circular orbit
to the comet, but favoured the straight line idea with some
sort of solar attractive power. He also suggested the
existence of an all-pervading ether that was vibrating, with
the vibrations diminishing as the distance from the Sun
increased. It wasn't until Edmond
Halley visited Isaac Newton in Cambridge in 1684 that the
problem was solved. Halley asked Newton what path a planet
would follow if the solar attractive force decreased in
strength with the square of the distance out from the Sun,
explaining that Wren, Hooke and he had been unable to solve
the problem. Newton, equipped with his newly invented
mathematical calculus, was able to show that the path taken
would be elliptical (i.e. egg-shaped). He also showed that
comets would follow similar paths. After Halley had seen the
brilliant work that Newton was doing, he pushed hard for
Newton to publish it. It is said that if it were not for
Halley, Newton's Principia would never have been published:
it became the founding stone of modern physics. However,
Newton's work aroused some philosophical hostility. Leibniz
objected to the idea of "action at a distance" (i.e.
gravity) and likened it to magic. Many others were extremely
uneasy by the idea that gravity could act through empty
space. Halley himself was also a
brilliant thinker and contributed much in his day, but he is
best known for his work on comets. He analysed cometary
orbits and came to the conclusion that the comets of 1531,
1607 and 1682 were actually sightings of the same comet. He
was then able to predict, even taking into account the
perturbing affect of Jupiter's gravity, that it would return
in December 1758. It duly did and served to emphasise, in
blazing fashion, the correctness of Newtonian over
Aristotelian physics. -------------------------------- It is believed that comets
(from the Greek, meaning "long-haired star") come from the
Oort Cloud. This is envisaged as a roughly spherical shell
thought to lie about 1000 times further from the Sun than
Pluto, with a radius of perhaps up to one third of the
distance to the nearest stars. It would therefore contain
billions of comets. Disturbances caused by the passage of
nearby stars sends some of these comets towards the planets,
where they are captured by the solar gravity and forced into
an orbit around the Sun. Around 800 comets have been
observed sufficiently well enough for these orbits to be
determined, showing that most are elliptical. About four
fifths of these are long-period comets that take hundreds,
thousand or even millions of years to complete one orbit of
the Sun. "Old" comets are those that have been perturbed by
the giant planets, principally Jupiter and Saturn, into
smaller orbits. Sometimes, these perturbations cause the
comet to be ejected from the solar system altogether (e.g.
Bowell 192I). The best description of a
comet is that of Fred Whipple's dirty snowball: ices of ammonia,
hydrocarbons, carbon dioxide and water that cement
meteoritic particles of stone together. Far from the Sun,
the comet is cold and shows no head or tail. In this cold
state, comets are only a few tens of kilometres across. As
their elliptical orbit brings them closer to the Sun, the
ices begin to vapourise, releasing the dust particles they
contained. As the comet gets closer still to the Sun, more
material is stripped from it's surface, flying off in all
directions. A huge ball of gas and dust then surrounds the
cometary nucleus, known as the head or coma of the comet:
this coma is usually hundreds of thousands of kilometres
across. As already mentioned,
Fracastro in 1538 noticed that the tail of a comet always
points away from the Sun. This shows that it is something
emanating from the Sun and colliding with the coma of the
comet that causes the tail to be produced. It was originally
thought that pressure from the Sun's light was the mechanism
doing this, but this is only half the answer. Ludwig F. Biermann in the
1950s showed that the pressure from Sunlight was not enough,
and that the force must be provided by a stream of particles
travelling away from the Sun at speeds of hundreds of
kilometres per second. Observing cometary tails allowed this
supersonic streaming of material to be studied. When the
first early space probes were flown (e.g. Lunik 1 and
Mariner 2) they were able to record this stream of material
high above the Earth's atmosphere, proving its existence.
This invisible stream of subatomic particles is now know as
the solar wind, and is continuously "blown out" from the Sun
in all directions. We are now in a position to
explain cometary tails. Comets usually display two types of
tail: one is curved and made of dust; the other is usually
straight and made of gas. The curvature of the dust tail
shows that the force pushing the dust out of the coma is
only just larger than the attractive force of the Sun's
gravity, i.e. it is collisions with the Sun's light which
makes the dust tail. Consequently, the dust particles must
be incredibly small, of order one micron (it would take
about 1000 of these to make a ball the size of a
bread-crumb). As the gas tail is very straight, the force
pushing against these ionised molecules must be a few
hundred times greater than the attractive force of the Sun's
gravity, i.e. it is collisions with the solar wind which
causes the gas tail. Over the centuries, there
have been some spectacular comets. Charles Messier,
nicknamed the "Ferret of Comets" by Louis xiv of France
because he had discovered so many of them, observed a
six-tailed comet in 1744. He was not the only great comet
hunter, though. Schiaparelli (made infamous by his
observations of canali on Mars) observed the Great Comet of
1861, which was bright enough to cast shadows! Schiaparelli
observed another comet (Swift-Tutle) in 1862 and was the
first person to link comets with meteors. The particles of
dust released from the comet spread out along the cometary
orbit. When Earth passes through this orbit, the tiny specs
of dust burn up in the atmosphere and are seen as shooting
stars. It is this comet which produces the Perseid meteor
shower in August. The body classified as minor planet No.
3200 (Phaethon) is the source of the Geminid meteor shower
in December, and is probably a defunct comet. Some comets do not die as
gracefully as this, but meet a more catastrophic end. One
example is Beila's comet. This had an orbital period of only
seven years, and so was seen on numerous occasions during
the 19th century. During the 1846 return, the coma became
unusually bulbous near closest approach to the Sun, and
eventually split into two. It was seen again in 1872, with
the two parts well separated, but has never been seen since.
In 1886, comet Brooke was perturbed by the gravity of
Jupiter. Before it's encounter with the planet, Brooke's
comet had an orbital period of 29.2 years that took it out
beyond the orbit of Jupiter. After the encounter, the orbit
was shrunk to lie totally within the orbit of Jupiter with a
new orbital period of 7.1 years. Jupiter was also the
culprit in more recent times. During the summer of 1995,
comet Shoemaker-Levy 9 was broken into numerous pieces
("strung out like beads on a piece of string") by the
gravitational influence of Jupiter, eventually colliding
with this huge planet. During the 1985/86 return
of Comet Halley, numerous space probes were sent to
investigate it. The European craft Giotto passed through the
tail of the comet on the 13th March 1986. It recorded
several thousand dust impacts, and large particles were
found to be more abundant than previously thought. Pictures
of Halley's blackened nucleus confirmed the dirty snowball
interpretation of these spectacular long-haired
stars. The author of this leaflet,
Dr Gregory
Beekman, is an
honours graduate of The University of Glasgow. When he wrote
this article he was studying for a post doctorate degree at
the Department of Astronomy at Keele University.
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