Herschel, Uranus and the Infra-red

 

By Paul Curnow [B.ED]

 

Sir William Herschel (1738-1822) is without doubt one of the greatest observational astronomer’s who ever lived.  Born ‘Friedrich Wilhelm Herschel’ in Hanover, Germany in 1738, he quickly established his reputation as a musician and a keen astronomer. As a musician he composed some twenty-four symphonies, however, it was his keen eye on the cosmic ballet above that would bring him greatest fame. 

 

One of 10 children, his father Isaak was an oboist in the ‘Hannoverian Footguards’. William, who also played the oboe, would visit England while he and his brother Jakob served with the Hannoverian Guards regiment in 1755.  This visit so impressed the young William that in 1757, while still a teenager he moved to England permanently with his brother Jakob.  It was here that he went by the more commonly known Anglicised version of his name ‘Frederick William Herschel’. In 1767, he was appointed as an organist in a church in Bath. Then later in 1772, his sister Caroline Herschel (1750-1848) who would become a renowned astronomer in her own right, moved to England to be with her brother. 

 

It was around this time that Herschel was developing a keen interest in astronomy. He would become well known as one of the finest telescope makers of his time, often spending up to 16 hours a day grinding and polishing the mirrors for his telescopes. He would also develop into an excellent observer, and as a result of his observations, a number of catalogues featuring deep sky objects and stars were eventually published. However, it was the discovery of the planet Uranus in the constellation Gemini on the 13th of March, 1781 that would cement his name into the history books.  Uranus, named after the Greek god of the sky ‘Ouranos’, was the first new planet found since antiquity. This had a huge impact on the general public because all the other planets had been known since prehistoric times. Herschel had discovered it while observing from his back garden in Bath, England while using his 15.2 cm (6 inch) reflecting telescope.

 

At first he thought the planet was a comet, however, he continued making observations of its movement and was able to determine that it had a rather circular orbit, which inferred that it must be a planetary body orbiting beyond the planet Saturn. Herschel named his new planet Georgium Sidus (George’s Star) after King George III, but his attempt to pay homage to the king didn’t sit well with the French and other nations. As a result, the French refused to make any reference to the British monarch and would call the planet ‘Herschel’ instead until the name Uranus was later accepted.  The name Uranus would eventually be adopted after a suggestion by the German astronomer Johann Elert Bode (1747-1846).

 

Nevertheless, Herschel’s attempt to pay King George III reverence put him in good standing with the monarchy. In 1782, he was made "The King’s Astronomer" and given an initial salary of £200 per year. Around the same time he and his sister Caroline would move to a village named Datchet, near Windsor to continue his observations and telescope making, although, in 1786 he moved to Slough where he would remain for the rest of his life.

 

Consequently, he was now an established telescope maker and sold many of his finely built telescopes to fellow astronomers. In fact he would go on to sell his telescopes to the king of Spain and the Russian emperor. Moreover in 1783, he gave his sister Caroline a telescope in order for her to make her own observations. Caroline would go on to discover 8 comets and make many cataloguing contributions to astronomy and as a result, would later be officially honoured by the Royal Astronomical Academy. In addition, much of the work that William Herschel did – he would have not been able to accomplish without the lifelong assistance of Caroline.

 

Also, Herschel would later go on to discover a number of our solar system’s moons. On the 11th of January, 1787 he discovered the moons Titania (pictured) and Oberon orbiting the planet Uranus. Then on the 28th of August, 1789 he discovered the Saturnian moon Enceladus. Furthermore, on the 17th of September, 1789 he discovered another Saturnian moon Mimas. These moons would later be named posthumously by his son John Herschel (1792-1871) who also went on to become a famous astronomer. In addition, William Herschel was the person that coined the term ‘asteroid’, taken from the Ancient Greek meaning “star-like.” Furthermore, Herschel catalogued around 800 double stars and listed some 2,000 nebulae, building upon the earlier work of the French astronomer Charles Messier (1730-1817).

 

Herschel built some of the largest telescopes of the time culminating with his 12-metre long (40 foot) reflector. This telescope had an aperture of 1.2 metres (4 foot) and was completed in 1787, though; soon after he would replace the original mirror in 1789. King George III who was keen on astronomy financed the cost of building this £4000 telescope and the £200 required each year for its maintenance. However, there was some risk involved in climbing up in the darkness to view through the eyepiece of this 12-metre long telescope. In fact, the unfortunate Italian astronomer Giuseppe Piazzi (1746-1826) fell from the telescope one night and broke his arm.

 

Nevertheless, the significance of the discovery of Uranus and a number of satellites notwithstanding, many of his other discoveries were overshadowed. To my mind one of the greatest contributions to the understandings of science that Herschel made was his serendipitous discovery of the infrared on the 11th of February, 1800. He was building upon some of the pioneering work of Isaac Newton’s experiments with prisms, because scientific discoveries rarely exist in isolation. He was experimenting with colour filters in order to find out how heat was distributed through each filter which he was using to observe sunlight.

 

Consequently, he was able to determine that filters of different colours seemed to radiate different amounts of heat. Herschel had contemplated that the colours themselves might be of varying temperatures; therefore, he needed to devise an experiment to confirm his hypothesis. Earlier, Isaac Newton (1643-1727) had found that when shining sunlight through a prism the light is broken into its constituent colours – red, orange, yellow, green, blue, indigo and violet. Herschel had placed a thermometer in each of the colours radiating away from his prism, and had placed a thermometer just outside of the red part of the visible spectrum in order to establish the ambient room temperature. He had noticed that the temperature increased from the blue to the red part of the spectrum. Though, to his astonishment, and I would imagine excitement, the temperature just beyond the red part of the visible spectrum was higher! Herschel had just discovered the infrared part of the electromagnetic spectrum, in the invisible. How astounded and amazed must have the 62 year old Herschel felt on that day.

 

The word ‘infra’ is taken from Latin and means “below,” referring to red being the colour with the longest wavelength of visible light, and ‘infrared’ being below this. Although, Herschel had called them "Calorific Rays," the term ‘infrared’ was adopted in the 19th century. I’m sure many people of the day wouldn’t have realised the implications that Herschel’s discovery would make in our future understandings of the cosmos.  Today the ‘infrared’ now reveals to us hidden parts of the universe and grants us a deeper comprehension of the cosmos in which we live.  

 

Consequently, other astronomers would begin to use the infrared to learn more about the celestial waltz taking place above. In the 1850’s, the somewhat eccentric astronomer Charles Piazzi Smyth (1819-1900) would be the first to detect infrared radiation coming from the moon. By 1870, the 4th Earl of Rosse would measure the infrared coming from the Moon in order to estimate its surface temperature, and by the 1960’s, balloons were being launched with telescopes onboard in order to study the infrared from celestial objects.

 

In 2003, NASA launched the Spitzer Space Telescope named in honour of the astronomer and physicist Lyman Spitzer (1914-1997). This orbiting space telescope would be able to sit above the obscuring effects of Earth's atmosphere and analyse the infrared signatures of various stellar and non-stellar objects. The Earth’s atmosphere blocks most incoming infrared radiation, therefore, is difficult to view from ground based observatories. At 0.85 metres (33.5 inches), Spitzer is the largest infrared telescope ever launched into space. 

 

The Spitzer Space Telescope has revealed to us parts of the universe that had previously remained hidden from view. Much of our universe consists of dense and vast clouds of dust which radiation at infrared wavelengths is unable to penetrate. However, Spitzer has allowed us to peek into stellar nurseries, the centre of galaxies and help us detect the presence of organic molecules in space. If Herschel were alive today, how he would marvel at what an understanding of the infrared has revealed to us about the cosmos in which we live. Consequently, in 1981, the go ahead was given to build the William Herschel Telescope - 200 years after the discovery of the planet Uranus. This telescope is located at La Palma on the Canary Islands and was fittingly named in honour of one of the greatest observational astronomers who ever lived.

 

References

Aughton, Peter, 2008, The Story of Astronomy: from Babylonian Stargazers to the Search for the Big Bang, Quercus Book Company, London.

Daintith, John, & Gjertson, Derek (Editors) et al, 1999, A Dictionary of Scientists, Oxford University Press, Oxford.

Herrmann, Dieter, B., 1984, The History of Astronomy from Herschel to Hertzsprung, Cambridge University Press, Cambridge.  

URL: http://www.spitzer.caltech.edu/about/index.shtml

URL: http://en.wikipedia.org/wiki/William_Herschel

 

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Updated 25th of March 2011