William Herschel's astronomical discoveries

William Herschel made a large number of discoveries of nebulae and double stars and there is no way we could list all of these here. However, we present a selection of his discoveries and papers to give an impression of his remarkable contributions. It is fair to say that Herschel was the first cosmologist:

17 August 1779.
When viewing Polaris, the Pole Star, Herschel resolved it into two stars showing for the first time that this was a double star. They have very different magnitudes, Polaris A being about magnitude 2 and Polaris B, which Herschel saw for the first time, magnitude 9.

13 March 1781.
Herschel discovered the planet Uranus. At first Herschel was unsure whether he had discovered a planet or a comet. He communicated his discovery to the Royal Society in the paper Account of a Comet read on 26 April 1781. The paper begins: "On Tuesday the 13th of March, between ten and eleven in the evening, while I was examining the small stars in the neighbourhood of H Geminorum, I perceived one that appeared visibly larger than the rest; being struck with its uncommon magnitude, I compared it to H Geminorum and the small star in the quartile between Auriga and Gemini, and finding it so much larger than either of them, suspected it to be a comet. I was then engaged in a series of observations on the parallax of the fixed stars, which I hope soon to have the honour of laying before the Royal Society; and those observations requiring very high powers, I had ready at hand the several magnifiers of 227, 460, 932, 1536, 2010, &c. all which I have successfully used upon that occasion. The power I had on when I first saw the comet was 227. From experience I knew that the diameters of the fixed stars are not proportionally magnified with higher powers, as the planets are; therefore I now put on the powers of 460 and 932, and found the diameter of the comet increased in proportion to the power, as it ought to be, on a supposition of its not being a fixed star, while the diameters of the stars to which I compared it were not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars preserved that lustre and distinctness which from many thousand observations I knew they would retain. The sequel has shown that my surmises were well founded, this proving to be the Comet we have lately observed." Although he called it a comet in the paper, Herschel was unsure whether it was a comet or a planet. Most astronomers thought that it was a planet and this was confirmed by Anders Lexell who computed its orbit, finding its distance from the sun to be 16 times the distance of the earth from the sun. Pierre-Simon Laplace independently obtained a similar result from his calculation of the orbit.

7 September 1782.
Herschel discovered the 'Saturn Nebula', a planetary nebular which looks remarkably like the planet Saturn with the rings seen edge on. Its proper name is NGC 7009. William Herschel's catalogue was named the General Catalogue of Nebulae. This was later enlarged as the New General Catalogue and a new numbering system adopted. This is the NGC number we have given for the 'Saturn Nebula' and will give for other objects discovered by Herschel.

By observing double stars over a period of years Herschel discovered that some were not chance alignments but were binary stars orbiting their common centre of gravity. This was the first observation that showed Newton's laws of gravity applied outside the solar system.

4 March 1783.
Herschel discovered the Tau Canis Major star cluster now designated NGC 2362. In fact his sister Caroline had observed the star cluster some days earlier but the two discoveries were independent.

28 October 1783.
Herschel discovered the 11.2 magnitude galaxy now called NGC 7184 in Aquarius.

Herschel discovered the Spiral Galaxy in Leo now known as NGC 3274. He did not recognise the spiral structure.

Herschel discovered the motion of the sun. Comparing the positions of the stars he was observing with the positions given by John Flamsteed 100 years earlier, he showed that the solar system is moving relative to nearby stars towards the star Lambda Herculis. He coined the term Solar Apex for this direction. His finding, although based on the incorrect assumption that all stars have equal brightness, is close to the correct direction of travel. Although, of course, Herschel was not away of it, the motion of the solar system relative to nearby stars is a result of all stars rotating about the centre of the Milky Way Galaxy. He published his results in the paper 'On the proper motion of the Sun and solar system; with an account of several changes that have happened among the fixed stars since the time of Mr Flamsteed', Philosophical Transactions of the Royal Society 73 (1783), 247-283.

Herschel discovered and catalogued 2500 new nebulae and star clusters although he was unable to distinguish between galaxies and planetary nebulae. He made the first 1000 discoveries in 18 months to April 1785 but later his rate of discovery slowed as his family and other commitments increased. His second 1000 were catalogued by 1788 and published in the following year. The final 500 were published in 1802. A further 8 remained unpublished until his son John Herschel published them in 1847.

The globular cluster M56 in Lyra had been discovered by Charles Messier on 23 January 1779. Messier, who was recording things which might be confused with comets, only saw it an a nebulous patch. However in 1784 Herschel resolved M56 into stars. He writes that it is "a globular cluster of very compressed and very small stars. They are gradually more compressed towards the centre."

11 March 1784.
Herschel published On the Remarkable Appearances at the Polar Regions of the Planet Mars, the Inclination of Its Axis, the Position of Its Poles, and Its Spheroidical Figure; With a Few Hints Relating to Its Real Diameter and Atmosphere, in the Philosophical Transactions of the Royal Society 74 (1784), which described the Martian polar caps for the first time. He writes, "When I found that the poles of Mars were distinguished with remarkable luminous spots, it occurred to me, that we might obtain a good theory for settling the inclination and the nodes of that planet's axis, by measures taken of the situation of those spots." He also writes in this paper, "I have often noticed occasional changes of partial bright belts ... and also once a darkish one, in a pretty high latitude ... and these alterations we can hardly ascribe to any other cause than the variable disposition of clouds and vapours floating in the atmosphere of the planet."

12 March 1784.
Herschel discovered the spiral galaxy in Virgo now known as NGC 3190. We note that Herschel did not recognise the spiral structure.

21 March 1784.
Herschel discovered the dwarf elliptical galaxy in Coma Berenices now known as NGC 4489.

12 April 1784.
Herschel discovered the elliptical galaxy in Virgo now known as NGC 4458.

17 June 1784.
Herschel's first paper on the 'construction of the heavens', Account of Some Observations Tending to Investigate the Construction of the Heavens in which he said he believed the universe consisted of "nebulous and sidereal strata (to borrow a term from the natural historian." He could "as yet give a few outlines, or rather hints. As an apology, however, for this prematurity, it may be said that the end of all discoveries being communication, we can never be too ready in giving facts and observations, whatever we may be in reasoning upon them."

7 August 1784.
Herschel discovered the globular cluster in Sagittarius now known as NGC 6717. Herschel described it as "a faint round nebula."

6 October 1784.
Herschel discovered the edge on spiral galaxy in Andromeda now known as NGC 891. We note that Herschel did not recognise the spiral structure.

18 October 1784.
Herschel discovered the barred spiral galaxy in Pegasus now known as NGC 7479. We note that Herschel did not recognise the spiral structure.

16 October 1784.
Herschel discovered the reflection nebula in Monoceros now known as NGC 2170.

3 February 1785.
Herschel published On the Construction of the Heavens. He wrote the following, "if we would hope to make any progress in an investigation of this delicate nature, we ought to avoid two opposite extremes, of which I can hardly say which is the most dangerous. If we indulge a fanciful imagination and build worlds of our own, we must not wonder at our going wide from the path of truth and nature; but these will vanish like the Cartesian vortices, that soon gave way when better theories were offered. On the other hand, if we add observations to observation, without attempting to draw out not only certain conclusions, but also conjectured views of them, we offend against the very end for which only observations ought to be made; I will endeavour to keep a proper medium." The paper contains the first accurate description of the shape of the Milky Way galaxy.

7 February 1785.
Herschel discovered the 'Antennae Galaxies' in Corvus, now with New General Catalogue numbers NGC 4038 and NGC 4039.

17 March 1785.
Herschel coined the term 'planetary nebula' because of the disk-like appearance of these objects. Herschel wrote to Jerôme Lalande on 17 March 1785. What follows is an English translation from Herschel's French text. "At the close of the paper I give the location of half-a-dozen planetary nebulae, as I call them. These are celestial bodies of which as yet we have no clear idea and which are perhaps of a type quite different from those that we are familiar with in the heavens. I have already found four that have a visible diameter of between 15 and 30 seconds. These bodies appear to have a disk that is rather like a planet, that is to say, of equal brightness all over, round or somewhat oval, and about as well-defined in outline as the disk of the planets, of a light strong enough to be visible with an ordinary telescope of only one foot, yet they have only the appearance of a star of about ninth magnitude."

10 September 1785.
Herschel discovered the spiral galaxy in Cetus now known as NGC 0895. We note that Herschel did not recognise the spiral structure.

26 December 1785.
Herschel discovered the 'Cone Nebula', a dark nebula associated with the star cluster known as the Christmas tree Cluster. It is NGC 2264.

15 February 1786.
Herschel discovered the 'Cat's Eye Nebula' in Draco, now with New General Catalogue number NGC 6543.

24 October 1786.
Herschel discovered the 'North American Nebula', now known as NGC 7000, an enormous emission nebula in Cygnus. Herschel described it as "faint, extremely large, diffuse nebulosity."

11 January 1787.
Herschel discovered the moons Titania and Oberon of Uranus. In fact he discovered Oberon before spotting Titania.

17 January 1787.
Herschel discovered the 'Eskimo Nebula', so called because it resembles a face with a fur hood. This faint 10.1 magnitude nebula is now known as NGC 2392.

3 November 1787.
Herschel discovered the 'Bubble Nebula' in Cassiopeia now known as NGC 7635.

Herschel discovered that Mars has a very thin atmosphere. He deduced this by observing the occultation of a star by Mars. The light of the star was unaffected until it came extremely close to Mars's limb and only at the very last moment before Mars passed in front of the star did it flicker and fade. There would have been a much greater affect on the star's light if Mars had a substantial atmosphere but no flickering if Mars had lacked an atmosphere.

Herschel suggested that his nebulae were objects in different stages of development. He writes, "This method of viewing the heavens seems to throw them into a new kind of light. They are now seen to resemble a luxuriant garden, which contains the greatest variety of productions, in different flourishing beds; and one advantage we may at least reap from it is that we can, as it were, extend the range of our experience to an immense duration. For, to continue the simile I have borrowed from the vegetable kingdom, is it not almost the same thing whether we live successively to witness the germination, blooming, foliage, fecundity, fading, withering and corruption of a plant, or whether a vast number of specimens, selected from every stage through which the plant passes in the course of its existence, be brought at once to our view?" Although the different nebulae that Herschel was recording were not all similar objects at different stages of development, his basic idea is correct.

28 August 1789.
Herschel discovered the moon Enceladus of Saturn. He had spent two years building his 48 inch reflector, then and for many years the largest telescope in the world, and he discovered Enceladus on the first night when he observed with his 48 inch.

17 September 1789.
Herschel discovered the moon Mimas of Saturn also with his 48 inch reflector. This moon is now seen to have one huge crater, much larger than all other craters ob the moon, and this had been named 'Herschel'.

19 June 1790.
Herschel discovered the elliptical galaxy in Virgo now known as IC 1101.

Herschel proposed that the nebulae he had found were of different types. One type seemed associated with the Milky Way while the other type seemed more evenly distributed but were more frequent near the poles of the galactic circle. Although Herschel did not know it, he was basically correct in that his second type were external galaxies which are more visible near the poles of the galactic circle since there they are least obscured by the dust and gas in the Milky Way.

Herschel began to examine stellar spectra using a simple prism in 1798. In 1814 he published the first description of the properties of stellar spectra of different types.

9 September 1798.
Herschel discovered the spiral galaxy now known as the 'Fireworks Galaxy' in Cephus and Cygnus now known as NGC 6946. Herschel did not recognise the spiral structure.

Herschel discovered infrared radiation. In observing the sun he had to use coloured glass filters to reduce the brightness. He realised that in reducing the brightness with a filter, the heat measure by a thermometer was unchanged. He was surprised by this and so used a prism to break the sun's light into a spectrum. He found that the heating was greatest at the red end of the spectrum. However, leaving a thermometer sitting away from the red end of the spectrum he noticed that this was recording an even greater heat than at the red end. He described this in his 1800 Royal Society paper On the rays which occasion heat. The term 'infrared' is due to Herschel. Many consider this to be Herschel's greatest discovery.

May 1802.
Herschel coined the term 'asteroid' for the Minor Planets. Today the word is used with a somewhat different meaning. Herschel first observed Ceres on 7 February 1802 (it had been discovered on 1 January 1801 by Guiseppe Piazzi at the Palermo Observatory in Sicily), and Pallas on 21 April 1802 (which had been discovered by Wilhelm Olbers at Bremen on 28 March 1802). In a paper in 1802 'Observations on the two lately discovered celestial Bodies' Herschel tried to put these discoveries into a bigger picture.

1803, 1804.
Herschel published two papers in the Philosophical Transactions which give examples of three binary systems where, using data from James Bradley observations in 1759, Herschel was able to show that the stars orbited their common centre of gravity. One of these stars is Castor in Gemini which has two bright components, Castor A with magnitude 2 and Castor B with magnitude 2.9. This is the first experimental evidence that Newton's laws of gravitation hold outside the solar system. [Note. In fact there is a third star Castor C in the system and each of the three components is a close binary making a system of six stars. Herschel was only aware of the two components, Castor A and Caster B.]

Herschel published the final supplement to his catalogue of double stars. Herschel's final list contains 145 double stars. His three catalogues of double stars are Catalogue of Double Stars (1782) with 269 entries, Catalogue of Double Stars (1784) with 484 entries, and On the Places of 145 New Double Stars (1821).

JOC/EFR November 2017

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