Transit Instruments

Index:
Galpin; Jones; Dolland; Greenwich; Airy; Fallows Circle; Fallows Transit; 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.]

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.

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.

ASSA

Astronomical Society of Southern Africa

transit_instr