[glenc (Glen)]

THE TWENTY FOOT REFLECTOR
The 20-foot (6.1 m), 18.5-inch aperture telescope with its standard 39 mm eyepiece had a power of
157 times and a field of view of 15.1 arc-minutes.
(The 20’ https://www.sciencephoto.com/media/1309187/view)

“Like most of the Herschel telescopes, the 20-foot reflector was constructed on the
Newtonian principle, but without a secondary… It was suspended by a system of ropes from
a framework mounted on movable rollers. A movable platform gave the observer access to
the eyepiece in any position. There was no secondary mirror; the Herschel’s looked straight
down the tube. Of the three interchangeable mirrors constructed for this telescope, William
Herschel had made one, another had been ground and polished by father and son together,
and John Herschel had produced the third on his own. All three mirrors had the same aperture
of 18.5-inches and the same focal length of 20 feet, so that they were identical in their optical
performance.”

Salt spray sometimes spoiled the mirror within a week and it had to be repolished. John estimated that the 20-foot telescope could see 5.3 million stars in both hemispheres using a power of 180 times.
William Herschel calculated that a new speculum mirror absorbed 33% of the light and hence reflected 67% while modern aluminium mirrors reflect 89% of the visible light. This means John’s 18.5-inch had the same magnitude limit as a 16.8-inch aluminium mirror since there was no secondary in Herschel’s telescope.

On March 5, Herschel began regular sweeps. He searched for star clusters, nebulae and
double stars in a series of zones 3 degrees long (north-south) in Declination. In four years he catalogued 1708 clusters and nebulae and 2102 double stars in the southern sky. He also studied the structure
of the Milky Way by counting a total of 68,948 stars in 3000 areas of the sky. His father
William believed that there were a vast number of cosmic gaseous masses “a shining fluid, of
a nature totally unknown to us,” but John thought all nebulae were made up of stars. As a
result John sometimes catalogued several parts of one nebula with different numbers instead
of seeing the nebula as a single object. The invention of the spectroscope proved William
correct.

John made an astrometer to measure the magnitudes of 191 bright stars by comparing them
with the light of the moon. He also arranged the naked eye stars by magnitude using his step
method. To do this, he divided the star charts made by the German astronomer Johann Bode
into triangular fields and “all stars visible to the naked eye were arranged in sequences. This
was done by writing down a list of stars having very marked differences in brightness
between them, leaving enough space between successive entries so that more stars could be
interpolated.”

William Herschel correctly thought the Milky Way was a lens-shaped system, but John
thought it was ring-like. He noted some dark patches in the Milky Way, such as the Coal
Sack, and he discovered a large number of nebulae outside the Milky Way, but did not know
that these were distant galaxies. He also made detailed catalogues of the Magellanic Clouds,
which he called the Cape Clouds. John listed 244 objects in the small cloud and 919 objects
in the large cloud. All the catalogues he made were arranged by Right Ascension and he
used north polar distance (NPD) not Declination.

A map made of the area around the Orion nebulae contained 150 stars and a map of the area
around Eta Argus included 1216 stars. The star Eta Argus (eta Carinae) was fourth magnitude according to Halley and second magnitude according to others but it suddenly brightened at the end of
1837 to be equal to Alpha Centauri (magnitude –0.1). By January 20, 1838 it had faded to the
magnitude of Rigel (magnitude 0.1) and by April 14, it was similar to Aldebaran (magnitude
0.8). Thomas Maclear, who was the director of the Cape Observatory at Cape Town, thought
it was only slightly fainter than Sirius (magnitude –1.5) at its maximum in early 1843.
Herschel observed this very unusual occurrence between 1834 and 1838.

Maclear and Herschel worked jointly on a number of projects, including measuring the
61” difference in longitude between Feldhausen and the Cape Observatory (5.55 km apart).
Maclear provided Herschel with the exact positions of a number of fundamental stars and
Herschel helped Maclear with financial support for tidal and meteorological observations,
especially at the solstices and equinoxes.

John observed Halley’s Comet between October 28, 1835 and May 5, 1836. It was magnitude
2.4 when he first saw it in the evening near M14, and magnitude 12.7 at the end. He moved the 7-foot telescope to the sand hills on the Cape Town flats, south-east of the city, and cut down some oak trees near the 20-foot telescope in order to study the comet. It was visible in the evening sky until November 10 (elongation 33 deg from the sun) and in the morning sky after January 26 (elongation 66 deg from the sun). Herschel speculated on the cause of the comet’s tail, and made the radical (and correct) proposal that it was due to positive and negative charges. He also tracked the positions of Saturn’s satellites including two faint moons that were discovered by William in 1789, and subsequently lost.

Herschel made drawings to record the positions of sunspots by projecting the sun with a power of 105 or 179, and he speculated on the cause of them, believing sunspots were holes in the solar atmosphere, which allowed a view of the dark solid core. He inherited his belief in the dark core from his father.

As already mentioned three children were born in Cape Town, Margaret Louisa on
September 10, 1834, Alexander Stewart on February 5, 1836 and John on October 29, 1837.
On Sundays the family attended church at the nearby village of Rondebosch. Herschel
developed an interest in botany and collected tuberous and bulbous plants to take back to
England. Using his camera lucida he drew the plants and his wife coloured the drawings.
Herschel’s other interests included poetry and music and he played the flute and violin. On
June 15, 1836 he was visited by Charles Darwin who described him as “a very modest man,
rather shy and even gauche (socially awkward), despite his lively intellect.”

While Herschel was at Cape Town, the New York Sun published a series of false articles
about his “discoveries” on the moon. These included paradise-like woods and meadows, hills
and valleys and moon men and women. It was some time before Herschel found out about
these articles and denied them.

On March 11, 1838 the Herschel family left Cape Town for England on the Windsor. The voyage, including a stop over at St Helena, took nine weeks. A stone obelisk commemorating their stay was later erected at Feldhausen, in the grounds of the current Grove Primary School, 10 km SE of Cape Town (latitude 33.983 deg S, longitude 18.460 deg E). John believed his time at the Cape was the happiest time of his life.

BACK IN ENGLAND, 1838 - 1840
Back at England Herschel visited Lord Glenelg, Secretary of State for the Colonies regarding
education in South Africa. While in Cape Town, Herschel had campaigned for public education, demanding “fundamental improvement in the social and economic position of teachers and the creation of a secure professional status.”

On June 28, 1838, Herschel was made a Baronet, at the time of the coronation of Queen Victoria. Soon after this John paid another visit to his aunt Caroline Herschel in Hanover and also met with other scientists on the Continent. In London he discussed constellation reform with the Astronomer Royal, George Biddell Airy. John proposed a “complete rearrangement of the constellations in the southern sky.” “Herschel’s plan was not accepted by astronomers on the Continent, and he withdrew it,” but in 1928, the IAU (International Astronomical Union) used a similar method to draw constellation boundaries for the whole sky.

In October 1838 Herschel dined with the Queen and Lord Melbourne at Windsor Castle and
they discussed a planned trip to the south magnetic pole. Herschel was a member of a
committee convened to study geomagnetism and meteorology, using geomagnetic stations
established at St Helena, Van Diemen’s Land (Tasmania), Cape Town and Madras (India) in 1840 and 1841. Another forty-seven stations were set up in other countries. These found a correlation “between disturbances of the compass needle and events in the solar atmosphere, after the discovery of the eleven-year periodicity of sunspot numbers.” As part of this project voyages were made to the north and south magnetic poles.