William Parsons Biography

Thomas Pring

DE Astronomy, 2

Mr. Percival

1/10/2013

William Parsons

            William Parsons was on born June 17^th, 1800 in York England. He was also known by the title of Earl Rosse the Third, as he was the eldest son of Earl Rosse the Second, of Birr Castle near Parsonstown, King’s County in Ireland. He got his education at Trinity College in Dublin, and Oxford University’s Magdalen College. He graduated with first-class honors in Mathematics in 1822. He took over for this father as Earl in 1841 at the age of 41.

            He decided that he wanted to follow the recent uprising in interest of the stars, and designed a 72 inch diameter mirror reflecting telescope. A telescope of this size would be the largest reflecting telescope in the world during this time period. He began by spending 5 years working on a suitable alloy for a half scale test model of the mirror. He finally chose a mixture of 1 part tin and 2 parts copper. The greater amount of copper made the mirror less brittle and added stability with the weight it had to support.  However, the copper also allowed for more surface cracks during the cooling process, and gave it a less white color.

            The first mirror measured 36 inches in diameter, and was created by soldering 16 thin plates of the alloy to a brass framework. This mirror had imperfections, but worked well enough that he believed he could cast a solid 36 inch mirror. This mirror was a success, and prompted his work on his goal, the 72 inch diameter mirror. He began working on it in 1842, and finished 3 years later in 1845. The final disk weighed a grand total of 4 tons.

            The 72 inch disk was mounted at Birr Castle estate in Ireland, to a 54 foot mounting tube. It was used to observe nebulae, but could only be used when England’s cloudy whether permitted (which isn't often at all). These telescopes allowed him to make observations on what he thought were nebulae, but were mostly actually galaxies. He made the drawing of the spiral galaxy (M51), which is considered a classic work of mid-19^th century astronomy. He named the Crab Nebula, and also made detailed observations of the Orion Nebula. Unfortunately, his telescope was dismantled in 1908, and was not reconstructed until 1917, when the 100 inch diameter mirror was created and mounted in California (the now largest reflecting telescope in the world).