Archive for the ‘astronomy’ category

Orbs of flame

15 April 2010

Off on a comet – Part II (Find Part I here)

Giovanni Battista Donati was an Italian astronomer who, on Aug. 5, 1864, was first to observe the spectrum of a comet (Comet 1864 II) now named after him. This observation indicated correctly that comet tails contain luminous gas and do not shine merely by reflected sunlight. Note the Big Dipper to the right. The bright star near the comet's head is Arcturus in the constellation Bootes.

", dry exhalations gathered and occasionally burst into flame..." - Aristotle

"..were hot, dry exhalations which gathered and occasionally burst into flame..." - Aristotle

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.

However, till the advent of  the telescope and the “Thinking Man” of Renaissance times, mankind had no way to understand what exactly a comet was composed of.

Aristotle, the great Greek philosopher, who studied under Plato and taught Alexander the Great, in his work “Meteorology“,  speaks across two millenia to us :

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.

Types of cometary forms, illustrations from Johannes Hevelius' Cometographia (Danzig, 1668)

An ancient bust of Seneca the Younger in the Antikensammlung, Berlin.

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.

This drawing of the comet of 1577 by a Turkish astronomer appeared in the book "Tarcuma-I Cifr al-Cami" by Mohammed b. Kamaladdin written in the 16th century. The yellow Moon, stars and comet are shown against a light blue sky.

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.

A curious case. Halley's Comet and portrait on a Grenada stamp, but the drawing is by the great Tycho Brahe himself. The caption of the stamp and that of another stamp in the series have been exchanged. It should read: “Tycho Brahe’s notes and sketch—Comet of 1577."

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.

Sir Isaac Newton's depiction of the orbit of the Comet of 1680, fit to a parabola. (From ''The Mathematical Principles of Natural Philosophy''. London: Benjamin Motte, 1729. )

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.

Immanuel Kant thought not just about reason but cometary science too.

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.

The external parts of a comet

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.

The annual Perseid meteor showers are created by the dust plumes of Comet Swift-Tuttle which visited Earth in 1992 and next comes in 2126.

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”).

Sources :

  • “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 –
  • Brahe/Halley stamp : copyright: -Dan from

Looking into the distant past – the Ultra Deep Field.

11 December 2009

This is truly the only way to travel into the past!

The time machine - Hubble Space Telescope.

The image

The Ultra Deep Field is an image of a small region of space in the constellation Fornax. This image was created, or rather composited, from Hubble Space Telescope data accumulated over a period from September 24, 2003 through to January 16, 2004.

Locating the Ultra Deep Field!

The unique feature of this image is that it is the deepest image of the Universe ever taken, looking back approximately 13 billion years. It is being to search for galaxies that existed between 400 and 800 million years after the Big Bang.

The image was taken in a section of the sky which had a low density of nearby bright stars. This low density allowed the light from more distant parts  of space to penetrate to Earth. It provided much better viewing of dimmer, more distant objects.

The image below contains an estimated 10,000 galaxies!

The Hubble Ultra Deep Field Image! Look on my works, ye mighty, and despair!

The galaxies are so distant that from the red shift of their light it is calculated that the light began its journey at a period of time between 400 and 800 million years after the Big Bang.

We are looking through a time machine at events of 13 billion years ago. What the condition there is now, we can come to know only after another 13 billion  years have passed.

Truly, the awesome time scale, the massive number of galaxies just in this one shot and the fact that we saw something which happened so very, very long ago is truly mind-blowing.

See more images here!

The video

As if this image was not amazing enough, Mike Gillis of Pennsylvania State University created an animation of the Hubble Ultra Deep Field. Using the measured redshift of all 10,000 galaxies in the Hubble Ultra Deep Field image, he extrapolated the location of each galaxy in the field and created a 3D flythrough.

Tony Darnell who runs an amazing astronomy video collection on Youtube wrote a short script of the history of the deep field images, wrapped it around the fly-through and put it up on Youtube here. You can also view it below though in a smaller window.

Every galaxy in the image is in its proper distance as viewed from the telescope line of sight.

Credits – Jeremy A. in this post on his blog ‘The life of a priest is HELL” brought the video to my notice.

Off on a comet!

11 December 2009

This post (hopefully first of a series) was inspired when a chance twitter about the Scattered Disc, Oort’s Cloud et al triggered requests from my Facebook friends to blog about the new Solar System.

The new Solar System

The mind can journey far beyond where the eye can see, or the ear can hear, and much much farther than the spear can be thrown.

The mind effortlessly soars beyond humanity’s farthest frontiers – the material frontier where Voyager now transits through the Heliopause some hundreds of millions of miles away – or the energy frontier – the outward traveling wavefront of the very first radio transmission recognisable as emanating from intelligent human life.

The position of Voyager 1 in 2007 as it approaches the heliopause. At the heliopause, the Sun's solar wind is stopped by the interstellar medium and the satellite is subject to the the stellar winds of the surrounding stars. (Click to enlarge).

The stars above me on a clear moonless winter night, the necklace of luminescent stars that make up the Milky Way, the pinwheel spiral galaxies where man may never reach – all these have fascinated me since I was a child. I adored fiction about space, especially space opera. Like most of us, for a long time all I knew was what I had learned in school – that we had a star and nine planets…yadda,yadda, yadda… Jupiter is the largest, Pluto is the furthest,,,yadda, yadda, yadda…

The old view of the Solar System - sun with 9 planets!

The old view of the Solar System - Sun with 9 planets, outermost of which was Pluto!

As a young child, I found nearby planets boring! I yearned for the deeps of outer space. I wanted to be on the bridge of the Enterprise, boldly going where no man had gone before. Galaxies were great stuff, so were pulsars and supernovas. Mars, Moon, Jupiter? Huh!

The Andromeda Galaxy portrayed on a German stamp of 1999.

I much preferred the mysteries of black holes and the looming terror of an event horizon to the rings of Saturn. It was during my early days of editing Wikipedia. I had not yet received a barnstar, a kind of award on Wikipedia. I learnt that if I reviewed five nominee articles for the status of ‘Good Articles‘ and shepherded them through a process, I was eligible to receive a barn star.

One of the articles I reviewed was Deep Impact (space mission)!  You probably would not have heard of 9P/Tempel – a minor periodic comet which circles around our Sun every five and a half years! A fairly frequent and predictable comet, it was selected by NASA as the first comet to be explored by their pioneering venture into space after comets – the unmanned space mission called ‘Deep Impact’.

While reviewing this article I once again discovered the magic and mystery in that mundane grouping of planets around a yellow dwarf that we call our Solar System.

Deep Impact, a spacecraft which sent an impactor to collide with Comet 9P/Tempel in July 2005.

Wikipedia tells us that –

A comet is a small solar system body that has a coma and/or a tail and is bigger than a meteoroid. When close enough to the Sun, a comet exhibits a visible coma (fuzzy “atmosphere”), and sometimes a tail, both because of the effects of solar radiation upon the comet’s nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, ranging from a few hundred metres to tens of kilometres across.

Comets have long fascinated mankind – in the Western civilisation they have been regarded as harbingers of doom or omens of upheaval and change! The people of the past had no idea of geography, no concept of the Universe as we know it today. They found it fearsome to see a strange heavenly object appear amongst the familiar patterns of the constellations.

The tail of a comet always points away from the Sun!

This strange visitor moved from constellation to constellation and had a plume that ALWAYS pointed away from the Sun! For a period it would vanish as it would circle the Sun and reappear as a blaze on its return path. Gradually as it went into the deeps of space it would lose its plume, its shine and then disappear ostensibly forever.

No one could predict when such an apparition would return until the English astronomer Edmund Halley corelated historical data from a variety of sources and realised that in a cosmic game of Vikram and Vetaal, the comets of 1456, 1531, 1607, and 1682 were one and the same. He further predicted that this comet 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.

One of the earliest photographs of Halley's Comet taken during its visit in 1910.

Since then hundreds of comets have been mapped. Each year, large numbers of minuscule comets traverse the Solar system unknown to all but astronomers.

It is only those comets which are visible to the naked eye that draw the imagination of mankind. Such comets are called ‘Great Comets‘. Some of the recent great comets have been Halley’s Comet of 1986 which was visited by the Giotto space mission, Comet Hale-Bopp in 1997 and Comet McNaught in 2007.

On a gruesome note, members of a religious cult in the United States, called Heaven’s Gate committed mass-suicide after their leader proclaimed that a space-craft followed in the wake of comet Hale-Bopp to transport them to ‘the next level of existence’.

Besides this, Comet Shoemaker-Levy 9 made history in 2004 when it broke apart and crashed into Jupiter.

Comet Hale-Bopp seen over Croatia. It's arrival triggered a mass-suicide of the Heaven's Gate cult in the US.

But what is a comet composed of? What are the properties of its ‘earth’ or ‘soil’? To answer these and other such questions, many space missions have been launched to date. Vega 1 and 2, Giotto, Deep Space 1, Stardust and Deep Impact all helped us piece together what we know today.

To learn more about comets, read “Orbs of flame” (Part II), and for even more wait for ‘Deep Space and Stardust’ (Part III)…