William Herschel

William Herschel

William Herschel lived at a time when interest in astronomy was running high. There was still very much to be learned about our vast universe, the positions and behaviour of its many stars, planets etc. and many astronomers were busily amassing information.

At the time, there were available several excellent designs of telescopes, all theoretically good. But the problem lay in converting theory to practice. It was not easy to make a good telescope with the simple tools and techniques available. William Herschel’s name takes its place in history as the man who made some of the finest telescopes of his age and plotted large sections of the sky using them.

For Herschel, astronomy was a hobby. It grew into an obsession and in the end completely dominated his life. He was not a scientist by training, but a musician, Director of Music at the City of Bath, engaging artists for concerts and composing music to be sung and played.

In 1773, at the age of 35 when his last students had packed up and gone home for summer, Herschel made preparations to look at the sky. He bought a set of lenses and used them to make a refracting telescope. But to get adequate magnification with a refracting telescope, it has to be unmanageably long. Herschel realized that a shorter refracting telescope would suit his purpose better.

        Glass mirrors were laboriously hand ground from pieces of hard metal. And because of this, the few on the market were far too expensive for him, so instead he brought some mirror-making apparatus and started to make his own. The results were so encouraging that he began making bigger and better telescopes with several objective mirrors. To the horror of his sister, the house began to turn into a workshop. A lathe was erected in the bedroom and the drawing room became the carpentry room. Several times the Herschels changed house so that they could have more space for telescopes and plots of land on which they could have yet more space for telescopes.

For nine years Herschel stood up to the strain of being musician by day and astronomer by night, during which time he tried to use every available hour of starlight for viewing. When the weather was unsuitable he devoted his time to mirror making, sitting hurs at a stretch patiently rubbing abrasive across the surfaces of his metal mirrors.

In 1782 King George III appointed him Royal Astronomer after his telescopes had proved to be far superior to those in the Royal Observatory. The new appointment meant he could give up his post as musician.

He spent hours ‘sweeping the heavens’ subjecting selected areas of the sky to intensive examinations. Often he examined 400 stars in a night. A workman moved the telescope up and down and Herschel dictated his results to his sister Caroline. From his many star counts he concluded that the sideral system was flattened like a grindstone.

In his spare time he made yet more telescopes including one giant 40-foot instrument with which he was the first person to detect a seventh satellite of Saturn. He discovered two satellites of Uranus and two of Saturn.

Herschel was also interested in sun-spots but found them difficult to examine because of the burning effect the image had on the eye. He experimented using various filters, and found a dark smoky green to be the best, and red filters to be useless. Many of his coloured eye pieces cracked and exploded with the heat, luckily not injuring him. Wondering why green was the best, he produced a spectrum and placed a thermometer in each colour band. The red end was hottest, but the highest reading of all was the invisible region just beyond the red. Later he showed that the invisible heat rays could be reflected and refracted just as light rays could, but it never dawned on him that the two were virtually the same.

He died in 1822 after a life that could never be called restful.