Sir William Herschel

1738-1822

The Herschel Space Observatory is named after Sir William Herschel, but who was Herschel and how does he qualify for this singular honour?

Born Friedrich Wilhelm Herschel on November 15^th 1738, it is often forgotten that Herschel was actually a German, born in Hanover (which, at the time, belonged to the British crown under the Hanovarian kings) and who only became a naturalised British citizen quite late in his life, becoming known as William Herschel. He was knighted in 1816, near the end of his long life, by the Prince Regent giving him the title by which he is remembered today.

Herschel was evidently an extremely talented person who shone in many facets of life. He made many important astronomical discoveries despite the fact that he was never a professional astronomer although, after discovering Uranus, he was given the title the King’s Astronomer and a quite generous annual pension of 200 Pounds.

His father was a musician in the regimental band of the Hanovarian Foot Guards and like all his brothers and sisters (Wilhelm Herschel was one of six children in the family, while another four died in infancy) he was expected to learn a musical instrument. Although he learned the violin, it was as an oboist that he shone initially, joining the band with his father at the age of 14. Although a musician, Wilhelm Herschel saw action in the battle of Hastenbeck in 1757. Having had a short posting with the band in England in 1755 Herschel had learned English during his stay; Wilhelm and his brother Jacob, who was also in the band, decided that a musical career in England was preferable to military life. In 1759 Wilhelm and Jacob Herschel moved to England, this time to stay, with the former changing his given name to William.

Initially William Herschel went to the north and earned a living teaching and playing music in Yorkshire. He lived and worked in Richmond and Leeds, before becoming an organist for a short time in Halifax in 1765. It was as an organist that he is best remembered now. This was followed by a move to Bath in early December 1766 where he became an organist and conductor at the Octagon Chapel at the Pump Rooms in Bath. Herschel was to live in Bath for over 15 years and visitors to the city can still find the beautifully kept Herschel Museum there in one of the houses that he lived in during this time.

Herschel composed 23 symphonies, plus many concertos and other works. Although largely forgotten today examples of his compositions have been published and are available on CD. A search of on-line book and music stores will find various examples of Herschel’s music for sale, often played by well-known orchestras.

William Herschel was interested in mathematics, which he found had a natural connection to music. This led him to an interest in astronomy. His progress from beginner to researcher was astonishingly rapid. In May 1773 he bought a popular astronomy book of the day to read and, in less than a year, had started active observing and had become a skilled mirror-maker, making telescopes both for himself and for astronomers in other countries.

His first work was on the height of lunar mountains. He measured around one hundred and the measures were duly presented to the Royal Society. Another of his early tasks was compiling a catalogue of double stars that was to keep him occupied intermittently almost to his death.

His first, and most famous, discovery was made as a result of a major sky survey that he had decided to undertake. In the late 1770s Herschel planned to carry a “review of the heavens” that he would finally start in earnest in 1783. This was a low-tech version of the Palomar Sky Survey: Herschel would scan the sky and his sister Caroline would take down his observations so that he would not need to be distracted from the telescope. On March 13^th 1781, during his scans of the sky in the south of Gemini he noticed a star near H Geminorum that appeared to show a small disk. Observations on the next few nights showed that the object moved from night to night.

A 1 degree field around the position of Uranus on the night of its discovery. The brightest star is HIP 27442, which is magnitude 8.25.

Initially he thought that he had located a new comet and announced the object as such to the Royal Society in his paper “Account of a Comet” but, already astronomers, including his friend the Astronomer Royal, Nevil Maskelyne, suspected that the object was a new and distant planet, something that the new body’s orbit confirmed rapidly. Herschel was modest about his discovery and commented that the object’s appearance was so unlike a star that, if he had not found it on that night, he would surely have found it on the next [despite this, resolving the tiny, 3 arcsecond disk, was a major challenge to other astronomers, even knowing beforehand that the object was a planet and not a star].

This discovery brought him fame and he was elected to the Royal Society in November 1781. The award of the title of King’s Astronomer and life pension that came with it enabled Herschel to give up music and dedicate himself full-time to astronomy.

In March 1783 he started his review of the heavens in earnest using a 47.5-cm (18.7-inch) reflector. Such was his dedication that if a 6^th magnitude star approached the edge of the field he would avert his gaze to avoid losing perfect night vision. One result was his map of the Galaxy (below) based in his star counts around the sky. The shape is substantially correct with the bulge and disk and the Sun in the Galactic Plane; his only significant error was to place the Sun (the large white dot) too close to the centre, although even then he realised that it was off-set from the centre.

In an age when sky surveys are carried out with large telescopes with high-speed computers and astrometry to measure Galactic structure is increasingly moving into space (i.e. Hipparcos and Gaia), it is salutary to remember that the first such studies were purely visual ones carried out by Herschel from a site in the middle of an important English town! Herschel even carried out the first study of Galactic motion by determining, with surprising accuracy, the direction of the apex of solar motion, finding that the point to which the Sun was moving was towards the star Lambda Herculis.

Herschel and the discovery of infrared radiation

Herschel’s interests were wide and varied. He became interested in the Sun and this led to him to start to experiment with spectra. In 1798 Herschel started to examine stellar spectra using a simple prism and was the first person to describe the properties of stellar spectra in terms of the difference in distribution of colours.

Around this time, the dark lines that would later become known as Fraunhofer lines were detected in the solar spectrum. Herschel started to experiment with the solar spectrum, leading to the discovery that is marked by the naming of the Herschel Space Observatory after him. Herschel used a prism to split the Sun’s light into its component colours. He then placed a thermometer in the beam of light at different points and noticed that the temperature rose as sunlight heated the bulb of the thermometer.

As the image below shows, he then, famously, in a flash of inspiration, withdrew the thermometer completely from the beam of light. To the blue end of the spectrum there was no effect but, to the red end, we can imagine that it was to his considerable surprise, he found that the thermometer still registered a rise in temperature even though it was well beyond the limit of the red in the projected spectrum.

Herschel realised that the Sun must emit invisible light beyond the red and termed this radiation “infrared radiation” (inside the red).

With this simple experiment, William Herschel had discovered infrared radiation, giving the first hint that there was a whole range of types of light beyond the visible spectrum.

His thermometers were, effectively, a simple type of bolometer: a modern type of detector that relies on tiny temperature changes to detect faint infrared radiation. The Herschel Space Observatory uses a state of the art version of bolometers in its instruments hence it is apt that Herschel should carry the name of the scientist who carried out the first simple experiments using bolometer technology to detect infrared radiation.