Transit Instruments
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Index:
Galpin;
Jones;
Dolland;
Greenwich;
Airy;
Fallows
Circle;
FallowsTransit;
Bradley
Zenith
Description: Galpin Transit Instrument
Used
by Galpin
in 1848.
Location: Observatory
Museum
in Grahamstown.
Source:
-(R ) Observatory Museum acq. no. OL81 (17)(21) 1870
Description: Dollond Transit Circle
(1828 - 1855)
Location: Cape
Observatory.
Installed in 1828 (one of the first instruments installed at the Cape Observatory.)
At Greenwich Observatory stands the original transit instrument
that was designed and built by the foremost instrument builder
of England at the time, Edward Troughton. The British Admiralty
decided to equip the Observatory at the Cape with instruments
of the same calibre. This transit instrument is a copy of the
original, but the manufacturing of the instrument was contracted
out to George Dollond. The assistant to Fallows,
Captain Ronald, brought the instrument out to the Cape and arrived
on 19 November 1826. Only in 1828 was the Observatory buildings
sufficiently completed for the transit instrument to be installed.
The transit circle performed extremely well. [Warner – Astronomers
p.2, p.20, p.65.]
Extra Information:
The Jones
Mural Circle
and Dollond Transit Instrument, which were installed at the Cape
Observatory, were in keeping with the British tradition of using
separate instruments for accurate positional work. On the Continent,
however, a Transit Circle was generally used, which combined the
functions of the two instruments and made it easier to reduce
the results. The Astronomer Royal at Greenwich decided in 1847
to switch to the more practical Continental system and designed
an outstanding Transit Circle. The Cape Observatory received an
exact replica of this instrument, installed in 1855, known as
the Airy
Transit Circle.
The existing Mural Circle (Greenwich) and Dollond Transit Instrument
were dismantled. [Warner – Astronomers, pp.65 - 66.]
Source:
- Laing, J.D. (ed.), and the Royal Observatory at the Cape of Good Hope-1820 - 1970Sesquicentennial Offerings, p. 29.
-Moore, P. & Collins, P., Astronomy in Southern Africa, p.45. (General Source)
-Warner, B., Astronomers at the Royal Observatory Cape of Good Hope.
Description: Jones six-foot Mural Circle
(1828 - 1839)
Location: Cape
Observatory.
Note: There are three Mural Circles involved here. The original
built by Troughton, and based on Troughton's design, two identical
Mural Circles built by Jones. The first one by Jones remained
at Greenwich, and was later, in 1839; send out to the Cape Observatory.
This instrument is known as the Greenwich Mural Circle. The identical
twin Mural Circle by Jones is this specific instrument,
referred to as the Jones Mural Circle. Due to an error
in construction of this Mural Circle it was send back to Greenwich
and replaced by the twin Jones Mural Circle known as the Greenwich
Mural Circle.
History:
At Greenwich Observatory stands the original mural circle that
was designed and built by the foremost instrument builder of England
at the time, Edward Troughton. The British Admiralty decided to
equip the Observatory at the Cape with instruments of the same
calibre. This mural circle is a copy of the original, but the
manufacturing of the circle was contracted out to Thomas Jones.
After Jones constructed the Mural Circle, the Astronomer Royal
liked it so mush that he kept it at Greenwich. Jones manufactured
a second one for the Cape. (There was an error during the manufacturing
process.) The assistant to Fallows,
Captain Ronald, brought the instrument out to the Cape and arrived
on 19 November 1826.
The instrument was dropped during transportation. Only in 1828 was the Observatory buildings sufficiently completed for the mural circle to be installed. (One of the first instruments installed at the Cape Observatory.) Once installed it would not perform satisfactorily. "Consistent readings could not be obtained and only when Fallows took the average of the six microscopes on the around its graduated circle – a laborious task – could he get meaningful results. Despite extensive tests the cause of the trouble could not be found. It was assumed that the damage occurred during transportation. [Warner – Astronomers, p.2, p.20, p.25, p.28.]
When Henderson
took over as director of the Cape Observatory he confirmed the
faulty nature of the instrument but with the help of Lieutenant
Meadows
made many valuable observations. [Warner – Astronomers, p.32.]
Maclear
as the next director also used the instrument, but with the help
of his influential friend, John
Herschel,
managed to get the Jones Mural Circle replaced. At Greenwich there
were two Mural Circles, from which the astronomer John Pond developed
a double-instrument technique. The new Astronomer Royal at Greenwich,
George Airy, stopped using the double-instrument method and on
the recommendation of Herschel agreed to exchange one of the instruments
(its twin Mural Circle also manufactured by Jones, known as the
Greenwich Mural Circle) for the problematic Jones Mural Circle
(in 1839).
The Jones Mural Circle was sent to England and on arrival Airy sent it to the instrument maker William Simms for inspection. "… and after some examination of its large pivot, which was evidently deformed, Mr Simms proceeded under my direction to re-turn it, when, to our great astonishment, the steel collar of the pivot was found quite loose, having been attached merely by soft solder." The usual method of fitting the collar was to make it slightly undersize and then to expand it with heat and then to fit it in place when still hot. When the collar cooled down and contracted it would form a solid joint. The sloppy work, Warner speculates, must be due to an assistant who made a shortcut when Jones' back was turned. This act led to endless frustration for the early astronomers at the Cape. [Warner – Astronomers, pp.55 - 56.]
Extra Information:
The Jones Mural Circle and Dollond Transit Instrument that were
installed at the Cape Observatory were in keeping with the British
tradition of using separate instruments for accurate positional
work. On the Continent, however, a Transit Circle was generally
used, which combined the functions of the two instruments and
made it easier to reduce the results. The Astronomer Royal at
Greenwich decided in 1847 to switch to the more practical Continental
system and designed an outstanding Transit Circle. The Cape Observatory
received an exact replica of this instrument, installed in 1855,
known as the Airy
Transit Circle.
The existing Mural Circle (Greenwich) and Dollond Transit Instrument
were dismantled. [Warner – Astronomers, pp.65 - 66.]
Source:
- Laing, J.D. (ed.), and the Royal Observatory at the Cape of Good Hope-1820 - 1970Sesquicentennial Offerings, p. 29.
-Moore, P. & Collins, P., Astronomy in Southern Africa, p.45. (General Source)
-Warner, B., Astronomers at the Royal Observatory Cape of Good Hope.
Description: Greenwich Mural Circle
Twin to Jones six-foot Mural Circle
(1839 - 1855)
Location: Cape Observatory
Note: There are three Mural Circles involved here. The original
built by Troughton, and based on Troughton's design, two identical
Mural Circles built by Jones. The first one by Jones remained
at Greenwich, and was later, in 1839; send out to the Cape
Observatory.
The first instrument is this specific instrument, known
as the Greenwich Mural Circle. The identical twin Mural
Circle by Jones is referred to as the Jones
Mural Circle.
Due to an error in construction of Jones Mural Circle it was send
back to Greenwich and replaced by the "twin Jones Mural Circle"
(this one), the Greenwich Mural Circle.
History:
At Greenwich Observatory stands the original mural circle that was designed and built by the foremost instrument builder of England at the time, Edward Troughton. The British Admiralty decided to equip the Observatory at the Cape with instruments of the same calibre. The mural circle is a copy of the original, but the manufacturing of the circle was contracted out to Thomas Jones. After Jones constructed this Mural Circle, the Astronomer Royal liked it so mush that he kept it at Greenwich. Jones manufactured a second one for the Cape that arrived in 1826.
The Jones Mural Circle at the Cape was problematic
(there was an error during construction) At Greenwich there were
two Mural Circles, from which the astronomer John Pond developed
a double-instrument technique. The new Astronomer Royal at Greenwich,
George Airy, stopped using the double-instrument method and on
the recommendation of Herschel
agreed to exchange one of the instruments (the Greenwich Mural
Circle) for the problematic Jones Mural Circle. [Warner – Astronomers
p.2, p.20, pp.55 – 56.]
William Mann bought this instrument to the Cape in 1839.
Source:
-Warner, B., Astronomers at the Royal Observatory Cape of Good Hope.
Description: Transit Circle (Airy)
8 inch telescope
(1855 – 1960)
Location: Cape
Observatory
At the Cape Observatory the Admiralty installed in 1828
the Dollond
Transit Instrument
and the Jones Mural Circle. (In 1839 the Jones
Mural Circle
was replaced with the Greenwich Mural Circle due to a flaw.) The
reason why two instruments were installed was because of British
Astronomical tradition. Where most of Europe used one instrument
for accurate positional work i.e. a Transit Circle, Britain used
separate instruments, i.e. a Transit Instrument and a Mural Circle.
Now, in 1847 the British Astronomer Royal, George Airy, decided that a single instrument (as with the European example) was more practical. He designed an outstanding Transit Instrument. In 1850 the Transit Circle was installed at Greenwich and it was an immediate success. A duplicate was made for the Cape Observatory.
On 9 October 1852 William
Mann
went to England to learn the workings of the new instrument. He
returned on 22 December 1853. The instrument destined for the
Cape, known as the Airy Transit Circle, arrived in various
pieces during February and March 1854. Assembly was completed
by 27 January 1855.
The instrument was designed by Airy, and built be Troughton and Simms. The Dollond Transit Instrument and the Jones Mural Circle were now obsolete and were removed from the main building, and the Airy Circle was installed in the room previously occupied be the Mural Circle. As the instrument was larger than the Mural Circle the room had to be lengthened from 24 ft (7.3 m) to 36 ft (10.9 m). [Warner – Astronomers pp.65 – 66]
Jones
3 ½ -inch
telescope, an auxiliary instrument, was eventually used as a collimator
for the Airy Transit Circle. [Warner - Astronomers, p.63.]
It was dismantled in 1960. [Laing]
Drawing Moore, P. & Collins, P., Astronomy in Southern Africa, p69.
Source:
- Laing, J.D. (ed.), The Royal Observatory at the Cape of Good Hope 1820 - 1970 Sesquicentennial Offerings, p.11; p.29.
-Moore, P. & Collins, P., Astronomy in Southern Africa, p.70.
-Warner, B., Astronomers at the Royal Observatory Cape of Good Hope.
Fallows Circle and Transit Instrument
1821 – 1828
Location: Cape
Observatory.
Two instruments are discussed here. The one instrument is a Circle, probably a Mural Circle. The other is a Transit Instrument. Very little is known about these two instruments. The word "Fallows" was arbitrarily selected in order to name the instruments.
When Fearon
Fallows
came to Cape Town he was tasked with finding a site and establish
the Royal
Observatory at the Cape of Good Hope.
He brought with him the two instruments and a clock as portable
instruments.
Only once the construction of the Observatory was far enough
to install the permanent instruments did Fallows stop using the
portable instruments. The initial permanent instruments made for
the observatory were Jones
Mural Circle
and Dollond
Transit Circle,
as well as a clock made by Harrison.
The clock mentioned above that Fallows used with the portable
instruments were probably the Harrison Clock.
What is known about the portable instruments is that they came out on the same boat that Fallows came on. They had problems in unloading and storing the instruments. Fallows was supplied with a prefabricated hut (intended for the 1820 Settlers) which was erected at his home, 13 Kloof Street, Gardens, in order to serve as a weather shelter for the temporary instruments. He used the instruments to prepare a preliminary catalogue of the positions of bright stars in the Southern Hemisphere. Fallows moved to a different dwelling, called Garden Zorg-en-Lust, where he completed the catalogue, and also used the instruments in order to establish a time service. The instruments were also used to align the construction of the Observatory with the pillars for the permanent instruments on accurate meridian line. The 1820 Settlers hut was moved to the site of the Cape Observatory to house the instruments as well as a pendulum clock in order to execute experiments on the gravity and shape of the Earth. [Warner – Astronomers pp.6 - 10, p.16, p.21.] This is all the information available at present.
Source:
-Warner, B., Astronomers at the Royal Observatory Cape of Good Hope.
Description: Bradley Zenith Sector
1837 – 1850
Location: Greenwich Observatory Museum.
This instrument was on loan to the Cape
Observatory.
It was already a historical instrument when it arrived
at the Cape as James Bradley used it in 1727 for observations
that resulted in the discovery for the aberration of light. As
can be imagined the Astronomer Royal for England, George Airy,
was reluctant to release this instrument to Maclear,
but on the assurances of Herschel
he released the instrument to Maclear in 1837. It was returned
unharmed to England in 1850.
The reason the instrument was needed at the Cape was in
order for Maclear to re-measure the Arc
of the Meridian.
When De
La Caille
measured the Arc in 1751 –53 he made a mistake, which was that
in taking his plumb lines to get absolute vertical at Table
Mountain and Piketberg, the gravity exerted
by the mountains pulled the plumb lines skew. With the vertical
incorrect as a variable his calculations showed that the Earth
was pear shaped, not orange shaped as scientist expected it to
be. The Zenith Sector allowed Maclear to determine the
correct vertical angle in the presence of gravitational forces
that would confound instruments like plumb lines.
The Bradley sector was a fragile and ungainly instrument 12 ½ feet (3.81 meter) long and mounted in the vertical position. Maclear moved it to a few sites. High winds were always a problem. In the field the instrument was housed in a tent, 13 feet high, specially made for the purpose. [Warner – Astronomers, pp.56 – 57.]
Source;
-Warner, B., Astronomers at the Royal Observatory Cape of Good Hope.
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