Considered the father of modern Astronomy, discoverer of planet Uranus and of infrared radiation, Sir William Herschel (1738-1822) arrived to England from his homeland, Germany, as a multifaceted musician and composer. A virtuous of the oboe, the harpsichord, the organ, the violin and the cello and author of 24 symphonies and many concertos, Herschel was the head of the Bath orchestra, first organist at the St John the Baptist church in Halifax, first violin of the Newcastle orchestra and head of the Durham Militia band.

As an astronomer, Herschel, that used to make his own lenses and telescopes, discovered Uranus and two of its moons, as well as two moons of Saturn. He also demonstrated that double stars are not an optical illusion, as it was believed. He also studied the Milky Way, deducing it is disc-shaped, and co-discovered Mars’ ice caps and their seasonal changes.

Researching on the calorific power of sunlight, Herschel used a prism to disperse a sunlight beam and thermometers to check the heating produced by each color. Observing that thermometers got hotter going towards the red, Herschel was bold enough to place a thermometer beyond that color (despite statements from different sources, Herschel papers do not tell that he was measuring by chance the temperature of the room in that region, but rather that his curiosity and boldness made him check it). Seeing that beyond the red and the visible spectrum («prismatic spectrum») there was an enhanced heating, Herschel deduced there had to be a form of invisible light with a large heating power (his «calorific rays» that we now call «infrared radiation» and that Herschel distinguished from the «colourific rays»).

In what follows, sentences in boldface reflect the astonishment of this entry’s author:
“…it is evident that radiant heat is subject to the laws of refraction, and also to those of the different refrangibility of light. May not this lead us to surmise, that radiant heat consists of particles of light of a certain range of momenta, and which range may extend a little farther, on each side of refrangibility, than that of light? We have shewn, that in a gradual exposure of the thermometer to the rays of the prismatic spectrum, beginning from the violet, we come to the maximum of light, long before we come to that of heat, which lies at the other extreme. By several experiments, which time will not allow me now to report, it appears’ that the maximum of illumination has little more than half the heat of the full red rays; and, from other experiments, I likewise conclude, that the full red falls still short of the maximum of heat; which perhaps lies even a little beyond visible refraction. In this case, radiant heat will at least partly, if not chiefly, consist, if I may be permitted the expression, of invisible light“^[1]

And would these rays be of a different nature from that of visible light? Herschel sensibly uses Okham’s Razor: “if we call light, those rays which illuminate objects, and radiant heat, those which heat bodies, it may be inquired, whether light be essentially different from radiant heat? In answer to which I would suggest, that we are not allowed, by the rules of philosophizing, to admit two different causes to explain certain effects, if they may be accounted for by one.”^[2] For Herschel both types of rays have the same nature and their difference comes from their distinct perception due to the different interaction with the visual organs: eye «admits» sunlight beams contained in the «prismatic spectrum» appearing as light and colors, while aqueous humour and the eye’s coatings, as other parts of the body, absorb the rays contained in the «thermometric spectrum» beyond the «prismatic» one, being thus not visible, but providing a heat feeling.

After that, we can only take our hat off, with astonishment and admiration, to the genius and boldness of this great musician-scientist.

Herschel was a member of the Royal Society and Astronomer of the King. Among many tributes (honoring him and his sister and collaborator, the soprano and astronomer Caroline Herschel), the European Space Agency named after him the Herschel Space Observatory, an infrared and submillimetre telescope with the largest single mirror ever built for a space telescope (3.5 m).

Considerado el padre de la astronomía moderna, descubridor del planeta Urano y de la radiación infrarroja, Sir William Herschel (1738-1822) llegó a Inglaterra desde su Alemania natal como músico multinstrumentista y compositor. Un virtuoso del oboe, clavicordio, órgano, violín y chelo y autor de 24 sinfonías y numerosos conciertos, Herschel fue director de la orquesta de Bath, primer organista de la iglesia de San Juan Bautista de Halifax, primer violin de la orquesta de Newcastle y director de la banda militar de Durham.

Como astrónomo, Herschel, que fabricaba sus propias lentes y telescopios, descubrió Urano y dos de sus lunas, así como dos de las lunas de Saturno. Demostró, además, que las estrellas dobles no son un efecto óptico, como se creía. También estudió la Vía Láctea, concluyendo su forma de disco, y codescubrió los casquetes polares de Marte y sus variaciones estacionales.

Investigando sobre el poder calórico de la luz solar, Herschel usó un prisma para descomponer un rayo de sol y termómetros para observar el calentamiento que producía cada color. Al observar que los termómetros se calentaban más al ir hacia el rojo, Herschel fue suficientemente atrevido como para colocar un termómetro aún más allá (pese a lo que se dice en muchas fuentes, los artículos de Herschel, ver abajo, no relatan que estuviera midiendo por casualidad la temperatura de la habitación en esa zona, sino que su curiosidad y atrevimiento le llevaron a hacer la comprobación). Al observar que más allá del rojo y del espectro visible («espectro prismático») había un calentamiento mayor aún, Herschel concluyó que debía haber una forma de luz invisible y con gran poder calórico (los «rayos caloríficos» que hoy llamamos «radiación  infrarroja», y que Herschel distinguió de los «rayos coloríficos»).

En lo que sigue, las negritas son del estupefacto redactor de esta entrada:

«…it is evident that radiant heat is subject to the laws of refraction, and also to those of the different refrangibility of light. May not this lead us to surmise, that radiant heat consists of particles of light of a certain range of momenta, and which range may extend a little farther, on each side of refrangibility, than that of light? We have shewn, that in a gradual exposure of the thermometer to the rays of the prismatic spectrum, beginning from the violet, we come to the maximum of light, long before we come to that of heat, which lies at the other extreme. By several experiments, which time will not allow me now to report, it appears’ that the maximum of illumination has little more than half the heat of the full red rays; and, from other experiments, I likewise conclude, that the full red falls still short of the maximum of heat; which perhaps lies even a little beyond visible refraction. In this case, radiant heat will at least partly, if not chiefly, consist, if I may be permitted the expression, of invisible light«^[1]

¿Y serán esos rayos de naturaleza distinta a la de la luz visible? Herschel usa muy atinadamente la navaja de Okham: «if we call light, those rays which illuminate objects, and radiant heat, those which heat bodies, it may be inquired, whether light be essentially different from radiant heat? In answer to which I would suggest, that we are not allowed, by the rules of philosophizing, to admit two different causes to explain certain effects, if they may be accounted for by one.»^[2] Para Herschel ambos tipos de rayos poseen la misma naturaleza y lo que cambia es su percepción debido a su distinta interacción con los órganos de la visión: el ojo «admite» los rayos del sol contenidos en el «espectro prismático» bajo la apariencia de luz y colores, mientras que el humor acuoso y los recubrimientos del ojo, como otras partes del cuerpo, absorben los rayos contenidos en el «espectro termométrico» más allá del «prismático», no siendo así visibles, pero dando sensación de calor.

Después de lo cual no queda más que descubrirse, con asombro y admiración, ante el genio y atrevimiento de este gran músico-científico.

Herschel fué miembro de la Royal Society y Astronomer of the King. Entre otros mucho homenajes (a él y a su hermana y colaboradora, la soprano y astrónoma Caroline Herschel), la Agencia Espacial Europea dió el nombre de este astrónomo al Observatorio Espacial Herschel, un telescopio de infrarrojos y longitudes de ondas submilimétricas con el mayor espejo construido para un telescopio espacial (3,5 m).

[1] W. Herschel, Philosophical Transactions of the Royal Society of London, Vol. 90 (1800), pp. 255-283
[2] W. Herschel, Philosophical Transactions of the Royal Society of London, Vol. 90 (1800), pp. 284-292