Off on a comet – Part II (Find Part I here)
Of all heavenly phenomena, none have fascinated man more than the appearance of comets in the sky. These objects behaved (to the ancients) so strangely and will fully that they were considered signs from Gods to mortals on Earth.
What were the gods trying to say? Some cultures read the message by the images that they saw upon looking at the comet. For example, the tail of the comet gave it the appearance of the head of a woman, with long flowing hair behind her. This sorrowful symbol of mourning was understood to mean the gods that had sent the comet to earth were displeased. There are many such myths which have emerged over the millenia in many cultures, both oriental and occidental.
We know that the dry and warm exhalation is the outermost part of the terrestrial world which falls below the circular motion. It, and a great part of the air that is continuous with it below, is carried round the earth by the motion of the circular revolution. In the course of this motion it often ignites wherever it may happen to be of the right consistency, and this we maintain to be the cause of the ‘shooting’ of scattered ‘stars’.
We may say, then, that a comet is formed when the upper motion introduces into a gathering of this kind a fiery principle not of such excessive strength as to burn up much of the material quickly, nor so weak as soon to be extinguished, but stronger and capable of burning up much material, and when exhalation of the right consistency rises from below and meets it.
This was completely in harmony with Aristotle’s view of the “Geocentric Universe“, i.e. the stars and the Sun revolved around the Earth. He argued that they could not be heavenly bodies as they did not move across the sky with the stars. Hence, to suit his world-view, he postulated them as creatures of the atmosphere.
Aristotle’s views were questioned even in antiquity. The Roman Stoic philosopher, Lucius Annaeus Seneca, more popularly known as Seneca the Younger, writes in “Natural Questions VII“, his only opus on Natural History, that :
Are they a concentration of flame as our
vision avers, and as the very light that streams from
them, 1 and the heat that descends from them suggest ?
6 Or are their orbs not of flame, but, as it were, solid
bodies of earth that glide through tracts of fire,
and having no light of their own draw thence
their brightness and heat ? That is an opinion that
has been held by great men who have believed
the stars to be compact of hard material, and to be
nourished by fire that is not their own. Flame
by itself, they argue, would be dissipated and would
have nothing to hold or to be held by. If it were
merely massed and not attached to a solid body,
the universe would assuredly long since have
scattered it in its impetuous whirl…
Seneca held that comets moved regularly through the sky and were undisturbed by the wind, behavior more typical of celestial than atmospheric phenomena. While he conceded that the other planets do not appear outside the Zodiac, he saw no reason that a planet-like object could not move through any part of the sky.
Aristotle’s views drowned out voices like Seneca’s and were pre-eminent through the mighty march of centuries till the dawn of the Renaissance.
In 1577, a bright comet was visible in the sky for several months. The Danish astronomer Tycho Brahe used measurements of the comet’s position taken by himself and other, geographically separated, observers to determine that the comet had no measurable parallax. Within the precision of the measurements, this implied the comet must be at least four times more distant from the earth than the moon. This was the first proof that comets were extra-terrestrial creatures and not of the Earth itself. Hence they had to be “real’ bodies, not atmospheric apparitions as propounded by Aristotle.
The next person to comment on comets was none other than the great natural philosopher and mathematician, Isaac Newton himself. He described comets as compact and durable solid bodies moving in oblique orbits, and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the sun. Newton suspected that comets were the origin of the life-supporting component of air. Newton also believed that the vapors given off by comets might replenish the planets’ supplies of water (which was gradually being converted into soil by the growth and decay of plants), and the sun’s supply of fuel.
Newton was right in many ways. Comets were solid objects trailing vapour emitted by nuclei emitted because of solar heating. Comets are also considered to be a source of extra-terrestrial water in the Solar Sytem. However they did not add to the sun’s store of nuclear fuel.
Newton made another great contribution to cometary science with his treatise “Philosophiæ Naturalis Principia Mathematica“. In book 3, “De mundi systemate” (On the system of the world) , he describes the celestial mechanics of cometary orbits.
Newton was approached by Edmond Halley for guidance in the understanding of celestial mechanics. Newton sent him a document which, though untitled in reality, is today known under the name of “De motu corporum in gyrum“ (Latin: “On the motion of bodies in an orbit”). Using Newton’s mathematical principles, Edmond Halley deduced that the great comets of 1456, 1531, 1607, and 1682 were one and the same. In 1705, Halley published “Synopsis Astronomia Cometicae”, in which he stated his belief that the comet sightings were of the same comet. He further predicted that it would return in 1758. Halley did not live to witness the comet’s return, but when it did, the comet became generally known as Halley’s Comet.
In 1755, the great German philosopher Immanuel Kant, who is not very widely known for his astronomical studies, hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion. In his work “Allgemeine Naturgeschichte und Theorie des Himmels” (English: Universal Natural History and Theory of Heaven), Kant writes :
Their atmosphere and tail, which expand through the heat of their close approach to the sun, are only consequences of the eccentricity, although they have always served in times of ignorance as uncommon images of horror, announcing to the common folk imaginary destinies….
He was right as regards the nature of the atmosphere and tail – it is solar radiation that causes the volatile materials within a comet to vaporize and stream out of the nucleus, carrying dust away with them. This stream of dust and gas forms a huge, extremely tenuous atmosphere around the comet called the “coma”, and the force exerted on the coma by the Sun’s radiation pressure and solar wind cause an enormous “tail” to form, which points away from the sun.
However Kant’s theory which would have required comets to comprise mainly of volatile material were overshadowed, not by another philosopher’s views but by events which drew another, partially correct explanation of the the nature of a comet’s substance.
In 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its visit in 1846, and was never seen again after 1852. Earlier, the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors over the period 1864–1866. Based on orbital similarities, he correctly hypothesized that the Perseid meteors were nothing but fragments of Comet Swift-Tuttle. This not only linked comets and meteor showers but also gave rise to the “gravel bank” model of comet structure, according to which comets consist of loose piles of small rocky objects, coated with an icy layer. That is, it is mostly hard matter but with some ice and other volatile material.
By the middle of the twentieth century, this view of a comet’s composition suffered from a number of shortcomings. For example, how could a body, that contained only a little ice, continue to put on a brilliant display of evaporating vapor after several perihelion passages around the Sun.
In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. So matters stood till the turn of the Twentieth Century when, to answer these and other such questions, NASA began launching space missions to intercept comets and interact with them.
To learn about the trysts of spacecraft with comets, wait for “Of Deep Space and Stardust” (part III of “Off on a Comet”).
- “Meteorology” by Aristotle. (Read the English translation of Book 1 here.)
- “Natural Questions” by Seneca the Younger. (Read the English translation of Book VII here.)
- “Universal Natural History and Theory of the Heavens” by Immanuel Kant (Read Part II, Section 3 here.)
Images : Click the image to reach the source.
- Wikimedia Commons – Comet Donati (p.d.), Aristotle (cc sa 3.0), Seneca (cc sa 3.0), Newton’s diagram (p.d.), Kant postage stamp (p.d.), Perseids meteor shower (cc sa 3.0),
- Hevelius’ comets : NASA/JPL.
- Blue Turkish Comet : copyright – Erol Pakin.
- Cometary structure : copyright – http://www.splung.com.
- Brahe/Halley stamp : copyright: -Dan from http://www.danstopicals.com/.