Arcadian Times

 

              
 
 
 
 
       

 

The King's Observatory at Kew & The Transit of Venus 1769
                                                               
   
 

If you walk along the Thames towpath between Richmond and Kew you will come across a line of meridian.Glancing through the trees you will see an obelisk and the King's Observatory Kew, completed in 1769 ostensibly for the observation of the transit of Venus that occurred on June 3rd 1769.

" His Majesty the King who made his observation with a shorter reflecting telescope, magnifying Diameter 170 Times was the first to view the Penumbra of Venus touching the Edge of the Sun's Disk. The exact mean time (according to civil Reckoning) was attended to by Stephen Demainbray, appointed to take exact time by Shelton's Regulator, previously regulated by several astronomical observations."1

Transits of Venus

Transits of Venus across the disk of the Sun are among the rarest of planetary alignments. They come in pairs, 8 years apart, separated by approximately 120 years. Prior to the 18th Century and since the invention of the telescope they had occurred in 1631 and 1639. The first recorded transit of Venus in this country had been in 1639 and no measurements had been taken.

One of the major questions of 18th century science was the size of the solar system. Six planets were known that orbited the Sun; Mercury, Venus, Earth, Mars, Saturn and Jupiter, but the size of the solar system in a quantifiable measurement was unknown. Astronomers knew the relative spacing of the known planets but not the absolute distances.

Edmund Halley (1656-1742) calculated in 1716 that during a transit of Venus, astronomers could calculate the distance to Venus using the principles of parallax. If measurements of the start and stop times of the transit of Venus were taken from other parts of the world they could determine the distance of the earth from the Sun. This would determine the measurement of the astronomical unit and from this then deduct the dimensions of the solar system.

Present on that day included George III, Queen Charlotte, Doctor Stephen C.T. Demainbray, Stephen Rigaud, Justin Vulliamy, Ben Vulliamy. On the day of the transit the weather had been rainy and overcast probably due to the prevailing westerly wind. It cleared at about 4.00pm and the transit began just after 7.00pm.

  
   
   
   
    Above: Manuscript of Dr. Demainbray's notebook of the Transit of Venus 1769                        
                                             
         
   
The next Transits of Venus were due in 1761 and 1769. Halley himself never witnessed a Transit of Venus, but for the 1761 transit the scientific world was prepared with expeditions to various corners of the world. The results in 1761 were disappointing as a phenomenon known as 'black drop' probably caused by the atmosphere of Venus, created a protrusion in the shape of Venus as it crossed the Sun and thus compromised the timing of the transit. This disappointment in the international effort meant that the 1769 Transit became more important as the next transit was not until 1874 and the scientists would not be able to witness it.

 
                 
  The Path of a Transit of Venus across the Sun
 

Above - Painting of the Observatory by unknown artist

  

The Observatory

Augusta Princess of Wales, the mother of George III lived at Kew Palace. There was an observatory attached to it but it had greatly deteriorated and did not have the facilities to view the transit of Venus. George III therefore ordered the building of a new observatory most likely advised by Dr. Stephen C.T. Demainbray.

The Kew Observatory stands on the borders between Richmond and Kew in the Old Deer Park, built of Portland stone it has seven acres of grounds. It was originally called the "King's Observatory at Richmond". Sometime in the early nineteenth century it became known as Kew Observatory. The Observatory belonged to the Kew Estate and probably this was the reason why it became known as Kew Observatory, though it was in the Parish of Richmond.

   
         
 

Sir William Chambers (1726 - 1796) was the architect, he also designed the Pagoda in Kew Gardens and Somerset House in London. It was built where a Carthusian Priory "The House of Jesus of Bethlehem at Shene " had formally stood. King Henry V had founded the priory in 1414, it declined during the Reformation and was eventually demolished when the building of the observatory began. It is designed as a villa with a moveable dome observatory mounted on the top. The building is in its original design state apart from some raising of side roofs to the level of the observing dome that happened in the 1880's.

       
  Front and rear of the observatory  
       
 
 

The main telescopes were in the movable dome that had an opening roof. The west of the observatory was used for the day to day transit observations. The room on the east side was used for quadrant observations. There are three obelisks built in 1769 of Portland stone to aid the alignment of the instruments. One is positioned to the North of the centre of the observatory. The two others mark south from the transit and quadrant rooms. All three obelisks can be seen walking along the towpath due to the meander of the Thames. When the building of the Observatory had been completed in 1769, Dr. Demainbray adjusted the instruments there ready for the transit of Venus.

The aperture for observing
     
        Above: The Obelisks marking South and Magnetic South from the Observatory        
                       
       
     
Above: Shelton's Regulator used for the timings of the 1769 Transit at Kew  

Please click above to see a movie of inside the dome

This is used to turn the dome

  
          The Observatory from the side      
                                               
 

The 1769 Transit of Venus

In 1769 enthusiasm was high, as advances had been made in telescopes and observation methods. As well as Kew, observations were made at Greenwich where Nevil Maskelyne (1732-1811) the Astronomer Royal for England for most of the second half of the 18th century observed and wrote his account of the transit.

In the South Seas Captain James Cook in His Majesty's Bark Endeavour had set sail on August 12, 1768, from Plymouth bound for Tahiti specifically for the purpose of observing this transit. The worldwide results of the 1769 transit were on the whole disappointing. The results from the 76 points around the globe were not precise enough to set the scale of the solar system.

Dr. Demainbray noted that the observers at Kew were in agreement of their timings within one second however they were 60 1/2 seconds out with the timings at Greenwich. This would be reflected in other observations around the world. The Greenwich observations were recorded and used in the scientific calculations that were made after the transit, as were the timings of Captain James Cook.

The Kew observation timings were not published nor used in the worldwide calculations. The observations of the 1769 transit were again compromised by the 'black drop' phenomenon and Dr. Demainbray briefly mentioned this in his notes. Maskelyne at Greenwich however was far more concerned with the 'black drop' phenomenon as his notes emphasised this fact more so than Dr. Demainbray's.

Incidentally astronomers didn't manage an accurate calculation of The Transit of Venus until the 19th century when they used photography to record the next pair of transits. It was not until the 20th Century with the advent of radio astronomy that the astronomical unit was accurately measured and refined to the consistent value it is today.

   
 

 

The Transits of Venus 2004 & 2012

The last Transit of Venus took place on June 8th 2004. It was the most widely photographed and observed transit there has ever been. The entire transit was visible from Europe, Africa (except western parts), Middle East, and most of Asia (except eastern parts). The Sun set during the transit in Australia, Indonesia, Japan, Philippines, Korea, easternmost China and Southeast Asia. The Sun rose with the transit already in progress for observers in western Africa, eastern North America, the Caribbean and most of South America. The transit was not visible from southern Chile or Argentina, western North America, Hawaii or New Zealand.

 
 
Above: Drawing by Captain Cook of the Transit of Venus, showing the stages of the Transit.
   
       

The next transit of Venus will be on June 6th 2012. The entire transit will be visible from north western North America, Hawaii, the western Pacific, northern Asia, Japan, Korea, eastern China, Philippines, eastern Australia, and New Zealand. The Sun will set during the transit in most of North America, the Caribbean, and north west South America. The Sun will rise during the transit for observers in central Asia, the Middle East, Europe, and eastern Africa. The transit will not be visible from Portugal or southern Spain, western Africa, and most of the south eastern area of South America. The next pairs of transits will not occur until December 2117, December 2125, June 2247, June 2255, December 2360 and December 2368.

 
   
     
Wherever you are for the next transit in 2012, remember it will be your last opportunity to witness this most rare of planetary alignments. Perhaps also spare a thought for George III, Stephen Demainbray, Stephen Rigaud, Justin Vulliamy, Benjamin Vulliamy and Queen Charlotte who witnessed the 1769 transit and their attempts to contribute to our further understanding of our solar system. The Kew observatory stands today as a monument to man's never-ending fascination with the movement of the planets and our efforts to find our place in the cosmos.
 
 
Map showing the path of the 2012 transit
 
                                                           
   

References 1 - Taken from the manuscript of Dr. Demainbray. This is in the Library of King's College London.

John Harris in his biography of Sir William Chambers Other references used: The Meteorological Magazine 98 1969 , Articles by L. Jacobs " The 200 Years story of Kew Observatory" History of the Kew Observatory - Robert H. Scott Websites: National Maritime Museum Greenwich, NASA, The Armagh Observatory, The Science Museum, European Southern Observatory. Local Studies Unit, Richmond Library. Manuscript of Dr. Demainbray's notebook of the Transit of Venus 1769, From King's College London, 'The Observatory: A Monthly Review of Astronomy' (1882) called 'Dr Demainbray and the King's Observatory at Kew'