[glenc (Glen)]

EUROPEAN TRAVELS, 1821 - 1826
During the summer of 1821 John, aged 29, made the first of several trips to the continent, accompanied
by Babbage. There they met with other scientists doing similar experiments on physical
optics in France, Switzerland and Italy. During his travels John measured the temperature, the
altitude (with a barometer) and made drawings with his camera lucida. This pocket sized
instrument, invented by Herschel’s friend Wollaston in 1807, consisted of two inclined sheets
of plain glass arranged so that “the image of an object produced by it is seen by the eye as if
lying on a sheet of paper placed beneath and the image can be traced out on the paper.”

In the summer of 1822 he made a second trip to Europe, touring Holland and Belgium. His
father died on August 25, 1822 while John was in Europe and his Aunt Caroline later
returned to Hanover, Germany. That same year John described a new method for calculating occultations of the stars by the moon and provided tables to determine the places of fundamental stars.

John’s third continental trip to France, Italy and Sicily began in April 1824. Within eight
hours of his arrival in Paris, he met with 7 famous people, Arago, Laplace, Humboldt, Thénard, Gay-Lussac, Poisson and Fourier. He also visited Guiseppi Piazzi who was famous for making a catalogue
of 7500 stars from Sicily and discovering the first asteroid, Ceres (now a dwarf planet). In Munich,
Joseph von Fraunhofer presented him with a large prism of flint glass, which he later used to
study photochemistry. The six and a half month journey concluded with a visit to Aunt
Caroline at Hanover, before his return to England in October 1824. He also made a short
journey to France in the autumn of 1826 where he used the actinometer he had invented to
measure radiation from the sun.

Between 1825 and 1833 John Herschel produced two great catalogues. The first contained
2306 nebulae and clusters including 525 newly discovered objects, and also included 100
drawings of remarkable objects. The second was a six-part catalogue of double stars,
containing 5075 pairs. This was an amazing feat considering that clear, moonless nights are
rare in England. Some nights he was very discouraged as the following quote from his notes
shows. “Two stars last night, and sat up till two waiting for them… Ditto the night before.
Sick of star-gazing – mean to break the telescope and melt the mirrors.”

John now concentrated on measuring position angles between double stars. He wanted to
measure the parallax shift and hence the distance to the stars. In 1826 Herschel published a
paper entitled “On the parallax of the fixed stars” which contained a table giving the approximate annual parallaxes for some 70 double stars. Two astronomers measured stellar parallaxes by the late 1830s. Thomas Henderson, in Cape Town, measured the first stellar distance by the parallax method (to Alpha Centauri), but he did not publish it timeously. Friedrich Bessel accurately measured the distance to 61 Cygni, publishing first in 1838.

In July 1825, John helped supervise a whole detachment of artillery, deployed to measure the
difference in longitude between Paris and Greenwich, using rockets. The measurement for
Paris was calculated to be 2 deg 20’ 24”, out by only 10.5” according to modern measurements.

However John tried to avoid paid employment like this and he was fortunate to be able to do
so, because “his father had left him a very considerable fortune, which enabled him not only
to fill all material needs but to finance his sometimes quite expensive scientific enterprises
without requiring a professional salary.”

Herschel was elected president of the Astronomical Society in February 1827, but found this
seriously interfered with his research. “He wanted above all to keep his personal freedom and
scientific independence, and not to have the scope of his private researches restricted by any
official and public engagements.” He was offered academic positions at Cambridge and
the University of London, but refused both. “It was partly out of vanity that he would prefer
his contribution to knowledge to be regarded as that of an amateur rather than a professional
scientist,” preferring he said to “loiter on the shores of the ocean of sciences and pick up
such shells and pebbles as take my fancy for the pleasure of arranging them and seeing them
look pretty.”

John did much for the adoption, in England, of the wave theory of light. This was at odds
with Newton’s particle theory. (Light turned out to be a wavelike particle). His 1827 treatise on light, written for an encyclopedia, was translated into French in 1830 and into German in 1831. He also wrote a book called Preliminary Discourse on the Study of Natural Philosophy, which gave a general introduction to the nature and method of science and short surveys of astronomy, physics, chemistry, geology and mineralogy. The book also included much of Herschel’s philosophy of life.

MARRIAGE AND CHILDREN
Herschel’s friends, Whewell, Grahame, Peacock and Babbage were all involved in scientific
pursuits and apart from them he had few social contacts outside of science. He was becoming
a “crotchety and eccentric old scholar” and Grahame, urging him to marry, introduced him
to a widow by the name of Mrs Alexander Stewart. On March 3, 1829, when John was about to turn 37, he married 18 year old Margaret, one of Mrs Stewart’s two daughters. She was almost twenty years younger than him.

In the late 1820s, after James Dunlop had catalogued the southern sky, John decided to travel to the southern hemisphere to make a catalogue of the southern skies. However the expedition was delayed because his mother, Mary, was dying. She died on January 4, 1832 and was buried at St Laurence’s Church, Upton with her husband.

John’s first child, Emilia Mary was born on March 31, 1830 and his second daughter, Isabella was born in 1831, both before his mother’s death. His first son, William James was born in 1833, six months before they left for Cape Town. Altogether John and Margaret had twelve children. John and Margaret’s other children were Margaret Louise (1834), Alexander Stewart (1836), Colonel John (1837) (3 born at the Cape), Maria Sophie (1839), Amelia (1841), Julia Mary (1842), Matilda Rose (1844), Francisca (1846) and Constance Ann (1855). (6 were born after their stay at the Cape)

After the death of his mother, John and his family moved from London back to the old family home at Slough. In June 1832 he paid a farewell visit to Aunt Caroline in Germany.

CAPE TOWN, 1834 - 1838
Herschel planned to collect all his father’s published papers, written over 40 years, into a
single volume, but this was not done until 1912 when J L E Dreyer published them in two
volumes called The Scientific Papers of Sir William Herschel. Instead Herschel decided to
explore the southern sky with his father’s 18.5” aperture, 20-foot reflector. He considered traveling to Parramatta but decided to go to Cape Town instead for a number of reasons, including the fact that
Margaret’s brother was there. The Royal Society and the British Admiralty offered to help
with this expedition but Herschel refused because he did not want to compromise his independence.

On November 13, 1833 John Herschel, his wife and 3 children sailed from Portsmouth on the ship
Mountstuart Elphinstone, bound for Cape Town. The journey took nine weeks. They arrived
on January 16, 1834 and spent several days unloading their apparatus and baggage and
moving into a large property called Feldhausen, ten kilometers southeast of Cape Town on
the eastern side of Table Mountain. Orchards and a grove of trees surrounded it. The weather there was much better than in England.

Herschel’s assistant, John Stone, helped him set up two telescopes; the 18.5-inch aperture 20-
foot long reflector and the 5-inch aperture 7-foot refractor. On February 22, Herschel looked
at the triple star Alpha Crucis and the nebula Eta Argus (Carinae) for the first time using the
20-foot, but the 7-foot was not used until May 2. Thus John became the first person to explore the southern sky using a large telescope. James Dunlop had explored it in 1826 with a 9” aperture reflector.
(John’s 20’ telescope was set up here. https://maps.app.goo.gl/nBkTGkWHS4Q7u8rr9)