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Caroline Herschel as an Observer

by Michael Hoskin

Michael Hoskin is a Fellow of Churchill College, Cambridge. He has written many books on
the history and philosophy of science. He edits Journal for the History of Astronomy, which he
founded in 1970.

June 2006

In the last quarter of the eighteenth century, William Herschel designed and built reflectors large
and small, and he used them in observational campaigns that lasted for years. His socalled
sweeps for nebulae and clusters extended over two decades; and by his daring interpretations
of his specimens of nebulae, he did more than anyone to transform astronomy from
the mathematical study of the unchanging, clockwork planetary system of Newton and
Leibniz, to the exploration of a universe in which everything from the individual stars to
the cosmos itself has a life story.

It would have been impossible for William to do this without the selfless help of his sister
Caroline, born in Hanover in 1750 and twelve years his junior. While William was at the
eyepiece of the south-facing telescope waiting for another of the mysterious nebulae to be
brought into his field of view by the rotation of the Earth, she was at a nearby window ready
to write down his shouted observations. It was she who wrote up a fair copy next day, and
later compiled the catalogs for publication. It was she who prepared a star catalog by zones
of north polar distance so that she could call out to William and tell him the stars that would
next come into view and which he could use as reference points for the positions of any nebulae.
And later in life it was she who arranged their two-and-a-half thousand nebulae into similar
zones of north polar distance so that William's son John could systematically re-examine his
father's nebulae, work that led to his General Catalogue and so to the New General Catalogue
that we use today. In all this, and much more, Caroline's contribution was crucial; William
could not have done it without her, and she fully deserved the Gold Medal and the honorary
membership of the RAS that she was awarded in old age. But she was also an observer on her own
account and we must ask, What did she achieve by her observations?

William, who joined his father in the band of the Hanoverian Guards when he was fourteen,
had fled from Hanover to England during the Seven Years War and by 1772 was well established
in the musical life of fashionable Bath. Caroline was then a household drudge in the family home
in Hanover, in the clutches of a mother who hoped to keep her forever as an unpaid servant,
and she was badly in need of rescuing (see box). William proposed that she come to Bath to see
if she had the voice that might make of her a solo singer in Handel's oratorios (as indeed she
had); this, we may think, was a pretext, and that the real motive was to acquire a housekeeper to
manage his bachelor household.

Caroline was thrilled to escape to England, but less so when she found that her arrival had
by chance coincided with William's development of an amateur passion for astronomy. In the time
he could spare from his own musical duties he was simply too busy making telescopes to give
her music lessons, and she was even roped in to help with the construction work, sometimes by
putting food into his mouth while his hands were engaged in holding a mirror he was polishing.

In 1781, William's systematic examination of the brighter stars led him to the discovery
of the planet we know as Uranus. His friends were anxious that he should be free to dedicate
himself to astronomy, and they persuaded him to name the planet the Georgian Star in honor of
King George III, that most enlightened of British monarchs. The custom of patronage would then
require the King to make a financial gesture in return, and George hit upon the idea of
conferring a pension on William, on condition that he live near Windsor Castle and be willing
to show the heavens to royal guests after they had dined at the castle. And so William and
Caroline arrived at Datchet near Windsor in the fall of 1782.

While they were still in Bath William had made her a small reflector, but there is no
evidence that she ever used it. Now that they had abandoned Bath with its vibrant musical life
and lived in a tiny village where there was little employment for sopranos, Caroline's musical
career was curtailed and she had (William thought) time to spare. He therefore gave her a
little refractor mounted about a vertical axis so that she could use it for horizontal sweeps of the
sky when she had nothing better to do, and he told her to look out for interesting objects, such
as double stars, clusters, nebulae, or comets.

At the end of the following year, 1783, Caroline found herself having to give priority
to acting as amanuensis to William as he swept for nebulae and clusters. It was therefore during
1783 that she had ample leisure to observe on her own account, at first with the little refractor,
then from the summer with the ingenious Newtonian sweeper that William made for her.
During that year she came across some double stars, she watched Algol drop from second
to fourth magnitude, and she compiled some simple sequences of stars in order of brightness.
But her main interest was in nebulae and clusters, searching out those listed by Messier
while at the same time on the lookout for new ones. A turning point occurred on 26 February
1783, when she first saw M 47 and M 41, and then found two clusters for each of which she
proudly noted: "Messier has it not." William was nearby and happy to interrupt his own searches
for double stars in order to examine the objects his little sister had discovered, and he confirmed
they were new discoveries.

Study of her observing books, and of her newly-discovered draft Catalogue of Nebulae
and Star Clusters, shows that in the course of 1783, Caroline in fact found 11 nebulae and
clusters previously unknown to astronomers, and she found one more in 1784 and a final one
in 1787. Two of these 13 were in fact galaxies: the second companion to the Andromeda nebula,
sometimes referred to as M 110, which Messier had in fact observed but not yet published; and
the bright edge-on spiral in Sculptor, NGC 253. The remaining 11 were clusters, and these
included the so-called ‘missing’ M 48 whose position Messier had miscalculated. When one
remembers that Messier's final list consists of 103 objects, for Caroline single-handedly to
discover 13 was a fine achievement. In fact she found a fourteenth, the cluster IC 4665, but
for some reason she credits it to William (and William to her, but correctly so).

But individually her discoveries counted for nothing, for they were published only if William
chanced upon one of her nebulae in the course of his regular sweeping, and Caroline afterwards
realized it was already in her own list. Then William's catalog entry would carry the initials
C.H. to acknowledge her priority. Every one of Caroline's nebulae that was published had been
independently rediscovered by William, and the inclusion of the initials C.H. made no difference
to the scientific value of his catalogs.

Collectively, however, their significance was immense, nothing less than epoch-making. We
saw that on 26 February 1783 William had to interrupt his own searches for double stars to
examine two clusters that his sister had found. One was a genuine discovery, the cluster NGC 2360,
while the other was in fact M 93. But Caroline's observations that evening evidently alerted
William to the fact that the mysterious nebulae, whose nature was one of the unsolved mysteries
of deep-sky astronomy, were so numerous that a novice observer equipped with what was little
more than a toy could find new ones.

Until then, William's own observing books are mainly filled with observations of double
stars, a subject that he had made his own. But on 4 March 1783, a week after Caroline had
brought him these two nebulae to examine, he made the momentous decision "to sweep the
heaven for Nebulas and Clusters of stars". And the entries in his observing books suddenly
change: the double stars largely disappear, and nebulae and clusters take their place.

That fall, William completed his magnificent new 20ft and with it he began his great campaign
of sweeps for nebulae. At first he worked alone, but he was continually frustrated by the need to
go into artificial light to record his observations. At the year's end he decided that he must work
in partnership with Caroline as his amanuensis, and over the next two decades they were to
sweep most (but not quite all) of the sky visible from Windsor, and to discover 2507 nebulae
and clusters.

In the early years they worked intensively and Caroline had almost no time for her own
observations. But in the late summer of 1786 William was away in Germany, and his absence
allowed Caroline time to observe once more on her own account. On 1 August she discovered
what she suspected was a comet, and observations the following night confirmed this.

In William's absence it was up to her to make her discovery known, and she sent details
to Dr Charles Blagden, Secretary of the Royal Society. A few days later the President of the
Royal Society, along with Blagden and Lord Palmerston, arrived at the Herschel home and
begged the favour of sight of Caroline's comet. And when William returned he was summoned to
Windsor Castle to demonstrate Caroline's comet to the Royal Family. The novelist Fanny Burney
was there: "The comet was very small, and had nothing grand or striking in its appearance; but
it is the first lady's comet…"

Caroline was doing everything in her power to repay William for rescuing her from the
scullery in Hanover. She had sacrificed her musical career to his ambitions in astronomy,
she ran his household for him, and she partnered him in his researches day and night. To her
anger and dismay, in May 1788, at the age of 49, he married; Caroline was no longer needed
as housekeeper, and was banished to the cottage in the garden. William offered her money in
compensation, but instead she persuaded him to ask the King for a salary as his assistant, and
so she became the first salaried female in the history of astronomy.

William now had better things to do at night than observe the stars, and so Caroline found
herself with plenty of time and a flat roof from which she could sweep for comets to her heart's
content. This state of affairs continued for nine years, until October 1797. In those nine years
she found no fewer than seven comets: three with her existing sweeper, three with a larger
version that William made for her, and one with her naked eye. Male astronomers throughout
Europe were charmed, and even the public got to hear of Caroline.

The larger sweeper that William made for her was a Newtonian of 5ft focal length, and
so to use it she had to stand on a stool, whereas its predecessor she could use sitting down. This
was less than welcome, but the 5ft was equipped with wires that allowed her to measure relative
positions as opposed to estimating them. We have an account by the Astronomer Royal as to
how she worked:

I paid Dr & Miss Herschel a visit 7 weeks ago. She shewed me her 5 feet Newtonian telescope
made for her by her brother for sweeping the heavens. It has an aperture of 9 inches, but
magnifies only from 25 to 30 times,…being designed to shew objects very bright, for the better
discovering any new visitor to our system, that is Comets, or any undiscovered nebulae. It is a very
powerful instrument, & shews objects very well. It is mounted upon an upright axis, or spindle,
and turns round by only pushing or pulling the telescope; it is moved easily in altitude by strings
in the manner Newtonian telescopes have been used formerly. The height of the eye-glass is
altered but little in sweeping from the horizon to the zenith. This she does and down again in 6
or 8 minutes, & then moves the telescope a little forward in azimuth, & sweeps another portion of
the heavens in like manner. She will thus sweep a quarter of the heavens in one night… Thus you
see, wherever she sweeps in fine weather nothing can escape her.

But cometary discoveries at that period, though of widespread popular and professional
interest, had little impact on the history of astronomy. With rare exceptions, all that could
be computed for most comets was a simple parabolic orbit; and in the case of Caroline's
discoveries, for this to be done accurately much depended on how quickly Maskelyne and other
astronomers were notified of the visitant. So for example the letter to Nevil Maskelyne
announcing her second comet had taken the best part of 48 hours to travel from Slough to
Greenwich "owing to the slowness of our penny post", and by then the weather had turned bad,
with the result that Maskelyne had not been able to observe the object for several nights.

By ill chance William was absent when she found her eighth comet. To make sure the
news reached Maskelyne promptly this valiant woman decided on direct action. She allowed
herself an hour's sleep and then she saddled a horse, and road the twenty miles to London
and then the six or seven more to Greenwich, to present herself at the front door of an astonished
Astronomer Royal.

Her comet of 1793 had already been seen by Messier, and that of 1797 was a naked-eye
object seen the same night by Eugène Bouvard and Stephen Lee. But her 1795 comet proved
to be of considerable interest because in 1819 it was recognized as being identical with comets of
1786, 1805 and 1818, and so having a period of only 3.3 years (we of course know it as Encke's
comet); and her 1788 comet surprised observers by returning in 1939.

In October 1797, for reasons that remain obscure to this day, she left her brother's home
and moved into lodgings. Her observing platform was now some considerable distance away, and
even if she expected a clear night and planned to visit her former residence and sweep for comets,
she would need to arrange in advance to hire a man to escort her safely back to her lodgings when
her observations were ended for the night. It was all so different from the years when her sweepers
were yards away and available for use whenever the skies were clear. And so the move effectively
ended her career as an observer. From that time on she was forever on the move. When William
died in 1822 she returned to her native Hanover where she lived, an honored figure in the town,
until her death in 1848 at the age of 97.

Individually, as we have seen, Caroline's nebulae and clusters counted for nothing. They
were known only to her brother, who had little interest in occasional discoveries of nebulae
(whether by Caroline or himself) but only in those he came across in his systematic sweeps.
If a nebula of Caroline's was published, this was because it had been independently rediscovered
by William. But by demonstrating to William that these mysterious objects were so easy to
find, she triggered his great campaign and changed the course of history.

Her eight comets brought her fame. One of them contributed to the recognition of Encke's
comet, another returned in 1939 and so is known to be periodic. For the others we have
little more than a parabolic orbit.

But there is a coda to the story. As I examined Caroline's and Willliam's observing books and
sought to identify her nebulae, I was baffled by two entries. On 24 August 1783, Caroline
saw "Between γ & δ of Equule[us] a rich spot". These two stars are little more than one degree
apart, and so the location of the "rich spot" is well defined. Yet inspection of the Palomar Sky
Survey shows that today no such object that would have been visible to Caroline is to be
found there.

Remarkably, a few weeks earlier, on 30 July, she writes that there is a "rich spot"—a term
she uses only on these two occasions—not far away, near the little triangle of stars, 3, 4 and 7
of Pegasus. This triangle is again a well-defined location, and again no such "rich spot" is to be
seen there. It is likely, therefore, that Caroline's rich spots were a comet that she saw on 30 July
and again on 24 August. Brian Marsden has kindly studied the possible elements of such a
comet; and it seems that we now know a ninth comet that should be credited to Caroline.

The Caroline Herschel Visitor Program

The Space Telescope Science Institute has created a visitor program, named after this extraordinary woman astronomer, to enhance the representation of women and minority astronomers at the Institute. This program has been designed with two goals in mind: provide a stimulating and productive environment for distinguished women and minority scientists to spend time at the Institute working and lecturing
on their scientific projects and providing active mentoring to the Institute’s junior scientists, especially women and other underrepresented groups.

The Caroline Herschel Visitor Program is intended for distinguished women and minority scientists from the international community who are committed to mentoring junior colleagues. STScI will offer them a scientific base for a sabbatical period or long research leave, typically 1-3 months and invites them to participate fully in the life of the Institute, including events organized by our mentoring
program and memberships in short-term committees.

Antonella Nota, Science Division Head at STScI says “We believe the Caroline Herschel Visitor Program will bring fresh perspectives from other international institutions to the Institute and provide benefits both to the visitors and the Institute staff as a whole. We are delighted to report that the program has already generated considerable interest.”

Readers interested in this program should contact Antonella Nota (nota@stsci.edu).


Further reading:

For a detailed study of Caroline as an observer, see Michael Hoskin, "Caroline Herschel as
Observer", Journal for the History of Astronomy, vol. 36, pages 373-405, 2005.

For Caroline's life-story, and in particular her role in the great partnership with her brother,
see the author’s two books: Caroline Herschel's Autobiographies and The Herschel Partnership:
As Viewed by Caroline, both published by Science History Publications Ltd, Cambridge,
2003 and available via www.shpltd.co.uk.

Also http://www-history.mcs.st-andrews.ac.uk/ Mathematicians/Herschel_Caroline.html

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