Department of History
University of California, Irvine
 Instructor:    Dr. Barbara J. Becker

Lecture 11.  Seeing Old Things in New Ways.


Changes in Saturn's appearance based on observations by Galileo (~1616)

Comparison of observations of Saturn by Galileo, Scheiner, Hevelius and others from 1616-1655,
from Christian Huygens' Saturnian System (1659).

Huygens was fascinated by Saturn's unusual and continually changing appearance.  While making regular observations of the planet in 1655, his attention was drawn to a bright star that appeared to tag along with Saturn.  Just like the so-called Medicean stars orbited Jupiter, this star moved around Saturn in a regular periodic way leading Huygens to conclude that it was a moon (now known as Titan). 

Huygens' comparison of Saturn's size with that of Earth (Tellus) and the Moon (Luna).

To see more detail in this unusual planet, Huygens constructed a telescope with greater magnifying power. In 1656, he noted that Saturn is surrounded by a detached ring.  In 1659, he published Systema Saturnium in which he announced his explanation for Saturn's changing appearance:  unlike all other planets that had been observed telescopically, Huygens claimed that Saturn is encircled by a detached ring tilted about 27° to the plane of the ecliptic.

In this famous diagram, Huygens showed how the axial tilts of both Earth and Saturn combine with their orbital motions around the Sun to produce the cyclical pattern of change in Saturn's appearance.

New and Improved Telescopes
After reading of Galileo's remarkable telescopic discoveries, many individuals wanted to see these celestial wonders for themselves.  And they wanted to see them better.  To do this required improving the quality of the lenses in a telescope's optical system. 

Lenses have curved surfaces.  When light travels through a lens it is dispersed, that is, it separates into a rainbow of colors.  Dispersion interferes with the creation of a clear image.  This distortion is called chromatic aberration.

One way to reduce the effect of chromatic aberration in a telescope is to use an objective lens (the large lens closest to the object being viewed) that has very little curvature.  A lens with nearly-flat surfaces will have a very long focal length, that is, it will form images at a great distance.  It's a bit of a challenge to view an image formed by such a lens.

Holding an eyepiece near an image is easier when it has been produced by a shorter focal length objective.  You can construct a tube to keep the objective lens and the eyepiece at just the right distance from one another to produce the best images.

But with a long focal length lens, the observer must stand far away from the lens in order to view the image it has formed.  Encasing an optical system like this in a tube would create a heavy and unwieldy instrument.

So why not make a telescope without a tube!  These so-called "aerial" telescopes became all the rage in the second half of the seventeenth century.

Christiaan Huygens' 123-foot "aerial" telescope.

The 150-ft aerial telescope of astronomer Johannes Hevelius (1611-1687) of Danzig (Gdansk)

William Herschel (1738-1822)
"The King's Astronomer"

Notable Events in William Herschel's Life

1738 born in Hanover, Germany
1757 emigrated to England

pursued career as music teacher and performer

compositions include:

  • 24 symphonies
  • 7 violin concertos
  • 2 organ concertos
1766 took position as organist in Bath

made his first entry in an astronomical diary

1772 sister, Caroline (1750-1848), joins him in Bath
1773 purchased book on astronomy

began constructing his own telescopes

William Herschel's 7-ft reflecting telescope

Hoping to detect and measure a slight shift in stellar positions (parallax) as the Earth orbits the Sun, Herschel began "sweeping" the heavens in search of close pairs of stars:

Albireo in Cygnus

Mizar and Alcor in Ursa Major

On 13 March 1781, during one of his routine "sweeps", Herschel observed "a curious rather Nebulous star or perhaps a Comet" in the constellation Taurus.

Careful observation of this unusual object over the next few weeks convinced Herschel that he had found a new planet.  He initially suggested naming the planet Georgium Sidus (George's Star) in honor of King George III of England.  French astronomers, meanwhile, called it Herschel.  German astronomer, Johann Elert Bode (1747-1826) suggested naming it Uranus, after the mythological god of the heavens and father of Saturn.

Seeing is in some respects an art that must be learnt.  To make a person see with such a power is nearly the same as if I were asked to make him play one of Handel's fugues upon the organ. 

Many a night have I been practicing to see, and it would be strange if one did not acquire a certain dexterity by such constant practice.

--William Herschel (1782)

In 1783, Herschel completed work on his new 20-ft reflecting telescope and undertook a celestial census of the entire sky in order to determine the "construction of the heavens."

William Herschel's 20-ft telescope.

Various methods may be pursued to come to a full knowledge of the sun's place in the sidereal stratum, of which I shall only mention one as the most general and most proper for determining this important point, and which I have already begun to put in practice.  I call it Gaging the Heavens, or the Star-Gage

It consists in repeatedly taking the number of stars in ten fields of view of my reflector very near each other, and by adding their sums, and cutting off one decimal on the right, a mean of the contents of the heavens, in all the parts which are thus gaged, is obtained....

--William Herschel (1784)

Herschel's "star-gaging" assumptions:
  • all fuzzy little objects are resolvable into clusters of stars
  • our Sun is part of a similar cluster of stars
  • stars in our cluster are roughly the same brightness

  • (variations in brightness are due to variations in distance)
  • stars in our cluster are distributed uniformly

  • (thickness of the cluster in any given part of the sky can be deduced from the numbers of stars)
  • we can see to the edge of our cluster
In 1788, Herschel completed work on a 40-ft reflecting telescope.

Herschel's 40-ft telescope

With this instrument, Herschel found he could now see MORE STARS!!

  • earlier observations had not revealed the edge of our cluster after all
He also discovered that some fuzzy little objects remained unresolvable.
  • Are these objects really clusters?
  • Do some objects appear fuzzy because they are made of fluid or gas?
When Herschel began his observations in the early 1780s, all the fuzzy little objects (about 100) that were known to northern hemisphere observers were listed in a catalogue created by the French comet hunter, Charles Messier.  In 1786, Herschel published a catalogue of 1,000 nebulae.  Three years later, he added another thousand to the list.  And in 1802, he published his third and final catalogue of 500 nebulae.

This method of viewing the heavens seems to throw them into a new kind of light. 

They now are seen to resemble a luxuriant garden, which contains the greatest variety of productions, in different flourishing beds; and one advantage we may at least reap from it is, that we can, as it were, extend the range of our experience to an immense duration. 

For, to continue the simile I have borrowed from the vegetable kingdom, is it not almost the same thing, whether we live successively to witness the germination, blooming, foliage, fecundity, fading, withering, and corruption of a plant, or whether a vast number of specimens, selected from every stage through which the plant passes in the course of its existence, be brought at once to our view?

--William Herschel (1789)

In 1811, Herschel published the drawings shown below in the Philosophical Transactions of the Royal Society to exhibit the rich variety of nebula types.
Cycle of Growth and Decay in the Celestial Garden
  • stars and nebulae emit light (shining fluid)
  • light gathers together and forms nebulosity
  • nebulous matter is gravitationally attracted to star
  • some material falls into star; replenishes star
  • some material forms into comets
  • comets can form seeds of future planets
  • a star is just a large planet with outer atmosphere of luminous clouds
  • our Sun is inhabited; sunspots are holes in the Sun's luminous clouds
Go to:
  • "The Construction of the Heavens" (1784-1791), by William Herschel (1738-1822)
  • "Great Astronomical Discoveries Lately Made by Sir John Herschel" (1835), Richard Adams Locke, New York Sun
  • The Unparalleled Adventure of One Hans Phaal (1850), by Edgar Allan Poe (1809-1849)
Weekly Readings
Lecture Notes