Sir John Herschel
William Herschel Society 35th Annual Lecture
Saturday 14th March 2015 at Bath Royal Literary and Scientific
Institute
Notes taken by William
Gaskell
Lecture given by Allan Chapman MA DPhil DSc
DUniv FRAS
Dr Allan Chapman became
William Herschel Society 2nd President after Sir Patrick Moore
Introduction by Dr Peter Ford
William Herschel, astronomer
and musician, discovered the planet Uranus in Bath on 13th March 1781. The
discovery was made at William Herschel’s house in Bath at 19 New King Street.
This house was saved from demolition and was instead converted to the William
Herschel Museum in Bath in 1981 and this lecture was inaugurated at that time
to honour this discovery.
Who was John Herschel?
John Herschel was The Leading Natural Philosopher of his day
in Europe and the world. He was fluent in French and German as well as English
and well-travelled in Europe. He is a philosopher as the term “scientist” was
only coined in 1840. He was exceedingly cultured for example he translated Homer
into German in his 60s for fun.
Although he is not notable for any particular achievement he
furthered his father, William Herschel’s work, as part of his inheritance. He
was the only son of William Herschel who was already wealthy when his son was
born from telescope sales, selling instruments for upto £4,000 to Spain in the
1790s.
William Herschel was an esteemed member of British society
but was born of humble origins to an Army drummer in Hannover. He was a friend
of the leading financier Sir Joseph Banks by his 40s. He married Mary Baldwin
and Sir John Herschel was born in 1791 in Slough.
Education
Sir John Herschel’s real passions in life were optics and
mathematics. He only took up astronomy to honour his father’s work. He was a
genius born of a genius, mostly educated at home due to poor health caused by a
weak chest but still lived to 79. He grew up in an environment with many
notable houseguests including the Archbishop of Canterbury. He started off at
Eton College – his local comprehensive of the day – but dropped out due to
harsh regime of floggings by then headmaster, Dr Keates. He was up at Cambridge
University at 18 where he notably formed the Analytic Society to bring modern
French mathematics from leading mathematicians such as Laplace to the
curriculum at Cambridge (this furthered Isaac Newton’s work, Principia).
He started off training as a barrister to have a serious
career to support himself but gave up to become an astronomer to further his
father’s work.
He learnt from William Herschel:
·
How to manage telescopes
·
How to best use the 18 ¼ inch reflector mirror
·
No need to use the 40 foot telescope as it was
too unwieldy
·
Stuck to the 20 foot telescopes
·
Use of the “Sweeping the Heavens” technique
(which William Herschel called Gauging), allowing star drift over the course of
a night to study 2 degrees of the sky per night.
Work
He duplicated his father William Herschel’s work before his
father passed away and his father’s assistant, his Auntie Carolyn, moved back
to Hannover in Germany.
He contemplated of what the objects known as nebulas could
be composed and some other questions of the day:
·
Are nebulas stars or gaseous?
·
Why are there inconsistencies in the Milky Way?
·
There appear to be many nebulas in Virgo cluster
but none elsewhere
Answering these questions by furthering his father’s work by
studying the sky from more angles helped him to further understanding of
natural phenomena:
·
Postulated Steady State Theory of Universe
Family and South Africa
John Herschel married Margaret Brodie Stewart, he had three
children with her before deciding to embark for South Africa to study the
heavens from the southern hemisphere - work his father never managed to
complete.
·
Sets of for South Africa in November 1834
·
Passage costs £500, a large amount, for whole
household and equipment, he chartered a 600 ton vessel used to carry Imperial
Viceroys to India.
·
Arrives in South Africa January 1835
On arrival in South Africa he rents a large estate from the
Boer’s called Veldhausen which he then buys in a shrewd move that ultimately
nets him a tidy profit on top of his return passage to England 4 years later
when he sells the property.
In South Africa he encounters new phenomena such as the
Magellenic Clouds. At this time he invented Photometry to measure the brightness
of starlight to help him determine the distance of the objects he is studying.
·
He uses a device called the Astrometer
o
Using a shielded gaslight with a tiny prick of
light allowed to shine through along a variable distance to a prism
o
He could vary the brightness he saw in eyepiece by
varying the distance of the gaslight of known brightness to the star or object
he observed through a telescope with the neighbouring eyepiece.
o
When the two eye-pieces showed the same
brightness he could then accurately record the brightness of the object he was
observing
His family enjoyed a great social life in South Africa at
the tie there was already established a Royal Observatory at the Cape in 1835.
There are accounts that he befriends a surgeon who on one occasion was able to
remove his wife’s painful tooth in their living room.
In South Africa he used guns for protection to keep wild
animals at bay whilst picnicking, also at night he had to escape from wild
feral dogs after observing the sky and would use a double barrelled pistol
loaded with buckshot to scare away the barking dogs and make it home from his
work safely.
Return to England
He was recognised by the newly coronated Queen Victoria as a
celebrity with a Baronet’s title. He decided to quite Slough and move to the
country and rents out Herschel House in Slough.
One of his great friends of the time was Lord Ross from the
middle of Ireland who has a 6 foot telescope and they had great fun together
through their shared interest of astronomy.
He used a spectroscope to discover the relative distances of
objects and realises that we are not located at the centre of the Milky Way
Galaxy as his time in South Africa raised the following questions:
·
Is the Universe asymmetric?
·
Why are more Planetary Nebulae in the Southern
Hemisphere rather than the Northern Hemisphere?
·
Stars seem to be denser at the centre of the
galaxy
The product of his career in astronomy is a rigorous study
of the Sky at Night in the Southern Hemisphere.
Later in life he used a spectroscope to discover space
contains gas later discovered to be ionized Nitrogen gas.
Work on Gravity
William Herschel had published a list of binary stars in
1800. In 1820 Sir John Herschel discovers that these stars had changed
orientation.
·
Postulates that gravity is responsible for the
observed changes in 1830s
Notable friendships
He had a well-documented friendship with the great
mathematician and astronomy Mary Somerville, who was 12 years his senior and
married to a surgeon.
·
They discuss using the Spectroscope – Huggins
was doing work on this
He was friends with Fox Talbot of Lacock Abbey who was a
photographic pioneer of the calotype method of photographic recording.
·
One of Sir John Herschel’s discoveries was
photographic fixation
Sir John Herschel died in 1871
after a short illness. His wife had written to Mary Somerville a few months
earlier saying “their philosopher was in decline”.
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