The light gathering power of an optical telescope is proportional to the square of the diameter of its primary mirror. So a 200 inch would have been able to see galaxies and stars four times fainter (1.5 mag) or twice as far away, as the 100 inch model. The larger the telescope the more could be seen, the limitations were the technology available to maintain precision.
The mirror of the 200 inch Hale telescope took many years to make from Pyrex glass which is resistant to temperature changes, with an aluminium reflecting surface. The 10-metre mirror of the Keck Telescope, is made up from 36 hexagon segments. Modern telescope mirrors need not be made in one piece as they are adjusted by computer.
But mirror problems have not been overcome. The distortions of the l,500 million dollar 2.4 metre Hubble telescope were discovered only after it had been launched into orbit in 1990. It cost many more millions to design a tiny system of mirrors to correct the image distortions of the primary mirror and fit this on a service mission by shuttle. But it was worth it for the images it sent back gave us new information about the formation of stars and solar systems, the formation of galaxies and the true nature of black holes which has gen us new insight and drastically changed and increased our knowledge of the universe.
- More on the 18th century instrument makers and the discoveries made in "John Harrison and the Problem of Longitude" - See the Cosmic Elk main page for information on this book.
- A booklet is to acompany these astronomy websites, further illustrations and extra links to more information added.
Transit of Venus
Because Mercury and Venus have orbits between us and the sun, we can only see them either just before dawn or just after sunset. The exception is on the rare occasions when their orbits lie in direct line of sight between us and the sun - so we can see them as a little black dot crossing the sun's disk. But we could only see that after the invention of the telescope, when it could be used to project the image of the sun onto a white card on which the transit could be viewed.In 1639, Jeremiah Horrocks (1618-41), and William Crabtree (1610-44) who were penfriends, were the first to use telescopes this way, to observe the transit of Venus, which they found predicted in the astronomical tables of Philip von Lansberg (1561-1632). Horrocks, had recently graduated from university and was working as a private tutor.
He set up his equipment in an upstairs bedroom. The eyepiece of the telescope was shrouded by the curtains at the window to keep out unwanted light, and the image was to project on marked out white card, which he had prepared and set up ready.
But the day was cloudy and Horrocks was busy looking after the children. Luckily the sky cleared just in time. And Horrocks was able to see and record the transit. Crabtree was also able to observe the transit. In 1641, Horrocks died, aged 23, the day before he was to have visited Crabtree for the first time. Crabtree died in the Civil War.
By timing the exact moments that the planet entered and left the Sun's disk, from different places on Earth, it was thought possible, by using parallax, to determine the distance of the Earth from the Sun, and use that as a baseline, the Astronomical Unit, to find the distance of other objects in the Solar System, and the nearest stars. Once the distance of the Earth to the Sun was known, the comparative distances of other bodies in the solar System and nearby stars could be calculated. It would also be possible to determine the longitude of the position of the observer.
The transit of Mercury occurs more often than the transit of Venus and had been used to fix the longitude of some places, as had been suggested by Halley. But as the transit of Venus is rarer, major international preparations were made for astronomers despite political problems and even wars, to travel to remote parts of the world to observe the transits of Venus in 1761 and 1769. They knew a similar opportunity would not re-occur again until 1874 and 1882, then 2004 and 2012. The astronomers had some amazing and awful adventures on their travels to their destinations in Canada, Tahiti, South Africa, Central Siberia, India, Mexico, and other far-flung parts of the Earth.
In 1761, Nevil Maskelyne was sent to St.Helena in the Atlantic Ocean, setting out in January to arrive in time for the June transit. Maskelyne was to have been accompanied to St. Helena, by Charles Mason, an astronomer at the Royal Observatory, Greenwich, but Mason was sent instead, with Jeremiah Dixon to Bencoolen in Sumatra. Mason and Dixon had barely started out when a French frigate in the Channel attacked their ship. They set out again, the Royal Society who sponsored the expedition refused to admit they now had no chance of getting to Sumatra in time. When they reached Cape Town, they heard that Bencoolen had been taken by the French, so they remained in South Africa. It was as well, for no other observations were made in the South Atlantic. Maskelyne, with astronomer Robert Waddington on St.Helena, had a cloudy day.
Jean-Baptiste Chappe d'Auteroche, an astronomer at the Paris observatory, was invited to observe the transit from Tobolsk in Western Siberia. He had to travel overland, right through the winter, to reach Tobolsk in time for the transit on June 6th. On the way he had to stay with poverty stricken peasants in grim conditions. Fortunately the big day dawned bright and clear. He asked to go somewhere sunny for the 1769 transit. He was sent to Southern California but died in an epidemic soon afer observing the June 3rd transit.
For the 1769 transit of Venus, astronomer William Wales also asked to be sent somewhere warm and not too remote - he was sent to Hudson Bay. Meanwhile Captain James Cook was sent in his specially modified research ship to observe the transit of Venus in Tahiti. (See our book John Harrison and the Problem of Longitude ISBN 1871443253 price £10 for more information).
The next two transits were in 1874 and 1882. Astronomers travelled for the 1874 transit to Hawaii, New Zealand, New Caledonia, Australia, Tasmania, Japan, Eastern Siberia, China, Kerguelen, Persia, the Seychelles, Southern Europe, Egypt, etc.
The 1882 transit is notable for having a number of women astronomers making the observations, and excellent photographs.
As you can see the transit of Venus is likely to be a once in a lifetime event so we were lucky this time - early morning June 8th 2004 for not only did we get the opportunity to see this - just over a year after we could observe the transit of Mercury on the 7th May 2003, but we could observe both early in the morning from our own back garden. No need for expensive expeditions. Or expensive equipment. Or even time off work. In fact the transits now have little scientific use - but were fascinating to observe.
- More on the history and observations of the transit of Mercury
- More on the history and observations of the transit of Venus
Achromatic lenses
Achromatic lenses which reduced chromatic aberration - the coloured fringes caused by the different wavelengths of light scattering as they passed through the glass, were developed in the 18th century.
Achromatic lenses were made by passing the light through different materials. The original inventor was Chester Moor Hall, a barrister. His achromatic lens was made from two lenses sandwiched together, a convex lens of crown glass and a concave lens of flint glass. In 1733, he commissioned two different opticians each to make one of the lens, but by chance they both sub-contracted the work to the same man, George Bass. Chester Moor Hall then continued to keep his invention secret.
Some twenty years later, Bass told optician John Dollond about the achromatic lens he had made. Dollond's son Peter, saw the commercial advantages and once his father had made the test lenses, patented the invention. Chester Moor Hall twice attempted to challenge the patent on the grounds that he was the inventor, but lost his case on the grounds that the person who should profit by the invention was the one who benefited the public by it, not the one who kept it locked in his desk draw.
Achromatic lenses changed the design of refractor telescopes which could now be portable, or more easily mounted for greater precision in direction.
The Sicilian astronomer Giuseppi Piazzi came to England to order an achromatic refractor from London instrument maker Jesse Ramsden. The telescope which had a 5 ft. focal length was fitted in an alt-azimuth mounting and the position of the telescope was measured on circular scales. The whole instrument which was called the "Palermo circle" was designed as rigidly as possible for accuracy.
The Herschels
While Piazzi was waiting for Jesse Ramsden to get on with his telescope, (Ramsden was notoriously slow at finishing anything), he visited Caroline and William Herschel and their latest, largest telescope.
William Herschel had been a musician in the army in Hanover (then part of Britain) and moved to Britain and settled in Bath. Bath was the major resort of those wealthy enough to afford it, so he could be sure of a demand for playing and teaching music. He was joined by his brother Alexander and his sister Caroline was a small mousy woman who had remained with her mother, and once in Bath, then became housekeeper for her brothers. But she was herself a talented singer with a promising career, now she had left home and her mother.
Increasingly William Herschel's hobby of astronomy took over their home, part of a rented terraced house shared with another family, 19 New King Street, Bath, which is now a museum. Alexander helped build William's telescopes and Caroline helped with the observations and data. The Herschels started making bigger and bigger reflecting telescopes, bigger than anyone had made before.
They made a successful 18-inch reflector, then attempted to make a telescope with a mirror three feet in diameter that would have three times the light-gathering power of their 18 inch. They tried to cast the mirror themselves in the scullery of the house which led off from the kitchen at the back. They made the mould with the help of a friend William Watson, out of horse poo (which was often used for this sort of thing). Caroline had the horrid job of sieving the poo and getting it prepared. Then they poured in the hot molten metal. The mould cracked and the hot metal spilled out all over the floor exploding flagstones and chasing the Herschels into the garden. While we were visiting in 2001, the decorators were in – and when I was exclaiming – “There are the famous cracked tiles in the scullery!” – the lady showing me round was reminded to tell the workman “not to replace the cracked tiles!”
The Herschels concentrated on the mysterious fuzzy blobs which could be seen now telescopes had improved. As telescopes got better, more and more faint fuzzy blobs, like luminous clouds or "nebula" could be seen. Some of these were comets. To help distinguish comets from other nebulae, French astronomer Charles Messier, compiled a catalogue. The nebulae in his catalogue still have his numbers eg. M.1., M.42 etc. The Herschels got a copy of his catalogue in 1781.
Caroline found many comets herself - although at first only William got the credit for their new discoveries. In 1781 the Herschels thought they had found a new comet. William carefully noted its increasing size as he thought it approached Earth. But when astronomers in other observatories checked his observations they found that the Herschels had discovered a new planet.
William hastily revised his data. The Herschels called the new planet Georgium Sidus - George's Star, after their king, but this was not a name popular abroad. Many astronomers thought the planet should be called "Herschel". The name that was finally accepted was suggested by Bode, - Uranus, after the Greek God of Heaven who was castrated by his son Chronos (Saturn to the Romans). The rings were not discovered until 1977, if Bode had known about them perhaps he might have changed his mind. Also the Herschels might not have started the tradition of naming Uranus's moons after fairies, when they discovered Titania and Oberon in 1787. More on Uranus.
Naming the new planet after the king had been a good idea. George III was interested in science and had his own observatory at Kew. He gave the Herschels a grant to build the world's largest reflector telescope, by their home in Slough, with a 48 inch mirror that weighed a ton in a tube forty feet long. To reach the eyepiece they had to climb a scaffolding that rose 50 feet into the air. By now William had married and had a son. Caroline had been given her own income from the King who recognised her acheievements as an astronomer in her own right, so she could have her own home, but still worked for her brother.
Pulleys and gears enabled the telescope to be turned and lifted into position, and the platform raised so the astronomer could look through the eyepiece at the end of the tube as described on page 8. In the pitch dark the structure was hazardous. Piazzi fell off a ladder and broke his arm.
In 1789, the Herschels discovered two more of Saturn's satellites, Mimas and Enceladus. Piazzi's ordeal in England paid off on the 1st January 1800 when while surveying the sky with Ramsden's telescope he made the first discovery of a minor planet between Mars and Jupiter, which he called Ceres.
The Herschel telescope was the biggest in the world until William Parson, Third Earl of Rosse, completed his 72-inch reflector in Ireland in 1845.
With these large telescopes it was noticed that some of the nebulae which were not comets were irregular clouds like the Orion nebula M.42 which can be seen below Orion's "belt", but more and more of a different type were being found. Small, elliptical in shape. In many of these such as the largest in the northern hemisphere's skies, M.31 (in Andromeda), spiral arms radiating from the centre could be discerned. The nature of these diffuse elliptical spirals was controversial. Some speculated, like the philosopher Kant, that these were island universes of stars and the Milky Way was just one of these in which we happened to live, scattered through eternity. The establishment view was that the Milky Way was the centre of the whole universe and the spiral nebulae were solar systems in the making.
Two typical types of nebulae are illustrated in 19th century astronomy book. One is the Orion Nebula (M.42) which we now know is a cloud of gas and dust in our own galaxy in which new stars are forming, the other is Andromeda (M.31) which we now know is another galaxy similar to our own, the Milky Way.
Until the distances of the spiral nebulae could be measured, the possibility that they were other star systems remained speculative and controversial. They were thought to be new solar systems forming around new stars. But by the beginning of the 19th century, optical astronomy had gone as far as it could go on its own.
The next developments would be made with the use of spectroscopy, and photography - William Herschel's son John invented the word photography, and one of his photos was of the by then derelict telescope at Slough.
The Astronomers Drinking Song
This was apparently sung at a dinner in 1798 given by the Mathematical Society in London, to honour their solicitor, Mr. Fletcher.
The Mathematical Society was founded in 1717 in Spitalfields in the East End of London, where a number of Hugenots and Jewish refugees had settled. Unlike the many "Gentlemen's Societies" founded in the early 18th century, the Mathematical Society was intended for working people "the studious artisan". One of its rules was - "it is the duty of every member, if he be asked any mathematical or philosophical question by another member, to instruct him in the plainest and easiest manner he is able." Those that refused to do this were fined one penny.
Members also included many eminent mathmaticians including for example, Nicholas saunderson, Lucasian Professor of Mathematics at Cambridge (who had been blind since a child).
In 1798, the Mathematical Society started giving lectures, open to the public for an admission charge of one shilling payable at the door.
An action was brought against the society for £5000 for holding unlicensed public exhibitions for payment at the door. Mr. Fletcher, the solicitor, successfully defended the Society. A collection was made by the members, but he refused payment so a dinner was held in his honour.
The astronomer's drinking song had been tidied up if not invented, by mathmatician and historian Augustus De Morgan who claimed to have found it amonst the papers of a deceased friend. By 1845, the Mathedmatical Society was failing, so an arrangement was made with the Royal Astronomical Society to make all remaining 19 member Fellows of the Royal Astronomical Society. The last President of the Mathematical Society was Benjamin Gompertz, who had been elected to membership of the Mathematical Society when he was only 16, bending the rules that members had to be over 21.
The RAS was conerned that the new Fellows from the Mathematical Society should be "of that class of educated men who could associate with the Fellows of the Astgronomical Society on terms agreeable to all parties." De Morgan, one of the organisers of the merger (and founder of a new Mathematical Society in 1866) claimed "We found that the artisan element had been extinct for many years; there was not a man but might, as to education, manners, and position, have become a Fellow in the usual way."
Science and mathematics were considered suitable pursuits for ladies in the 18th century but they only had a chance of attending meetings of societies on "ladies nights" by invitation from a member, or, in London they could go to lectures at the Royal Institution - the British Astronomical Association was the first scientific organisation to admit women members on equal terms - although that has retained its "amateur" status.
The astronomers drinking song gives us a summary of astronomy at the end of the 18th century:
Whoe'ver would search the starry sky,
Its secrets to divine, sir,
Should take his glass - I mean, should try
A glass or two of wine, sir!
True virtue lies in golden mean,
And man must wet his clay, sire;
Join these two maxims, and 'tis seen
He should drink his bottle a day, sir!Old Archimedes, reverend sage!
By trump of fame renowned, sir,
Deep problems solved in every page,
And the sphere's curved surface found, sir;
Himself he would have far outshone,
And borne a wider sway, sir
Had he our modern secret known,
And drank a bottle a day, sir!When Ptolemy, now long ago,
Believed the earth stood still, sir!,
He never would have blundered so,
Had he but drunk his fill, sir:
He'd then have felt it circulate,
And would have learnt to say, sir,
The true way to investigate
Is to drink your bottle a day, sir!Copernicus, that learned wight,
The glory of his nation,
With draughts of wine refreshed his sight,
And saw the earth's rotation;
Each planet then its orb described,
The moon got under way, sir;
These truts from nature he imbibed
For he drank his bottle a day, sir!(vodka perhaps)
The noble Tycho placed the stars,
Each in its due location;
He lost his nose by spite of Mars,
But that was no privation;
Had he but lost his mouth, I grant
He would have flet dismay, sir,
Bless you! he knew what he should want
To drink his bottle a day, sir!(He lost his nose as a student in a duel with another student in an argument on who was the better mathematician. Later his pet elk drunk a barrel of beer and fell down the stairs.)
Cold water makes no lucky hits;
On mysteries the head runs:
Small drink let Kepler time his wits
On the regular polyhedrons:
He took to wine, and it changed the chime,
His genius swept away, sir,
Through area varying as the time
At the rate of a bottle a day, sir!Poor Galileo, forced to rat
Before the Inquisition,
E pur si muove was the pat
He gave them in addition:
He meant, whate'er you think you prove,
The earth must go its way, sirs;
Spite of your teeth I'll make it move,
For I'll drink my bottle a day, sirs!Great Newton, who was never beat
Whatever fools may, think, sir;
Though sometimes he forgot to eat,
He never forgot to drink, sir:
Descartes took nought but lemonade,
To conquer him was play, sir;
Thefirst advance that Newton made
Was to drink his bottle a day, sir!(This was just pointless Brit patriotism supporting Newton's theories against the French Descartes. However Newton is the only Brit who gets a mention in the song.)
D'Alembert, Euler, and Clairaut,
Tough they increased our store, sir,
Much further had been seen to go
Had they tippled a little more, sir!
Lagrange gets mellow with Laplace,
And both war wont to say, sir,
The philosophe who's not an ass
Will drink his bottle a day, sir!Astronomers! What can avail
Those who calumniate us;
Experient can never fail
Withsuch an apparatus:
let him who'd hve his merits know
remember who I say, sir;
Fair science shines on him alone
Who drinks his bottle a day, sir!How light we reck of those who mock
By this we'll make to appear, sir,
We'll dine by the sidereal clock
For one more bottle a year, sir:
But choose which pendulum you will,
You'll never made your way, sir,
Unless you drink - and drink your fill, -
At least a bottle a day, sir
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