April 1, 2020

Review: Discoverers of the Universe by Michael Hoskin.

Few realize that we owe much of our knowledge to an astronomical dynasty of the 18th-19th century. This week, we review Discoverers of the Universe by Michael Hoskin. This fascinating book covers the life and times of astronomers William and Caroline Herschel and the eventual hand off of the mantle of British astronomy to William’s son John. Much has been written about the pursuits of the Herschels, but Discoverers gives it to you in the kind of detail that we observational astronomers love.

Like many an amateur and professional astronomer before or hence, William was bit hard by “aperture fever” and built larger and larger scopes to this end, culminating with the enormous 40 foot reflector installed at Slough. Don’t forget, they measured ‘em not by aperture but by focal length in those far gone days. The technique of figuring a fast focal length mirror was an impossible task for those early opticians, and it tended to result in instruments with extreme focal lengths to compensate for this fact.

But what I really admire about William Herschel is how he got his “hands dirty,” figuring the mirrors himself, often with Caroline reading to him or even feeding him as he worked. Trained as his assistant, Caroline herself became an expert observer, discovering a total of 8 comets. A controversial 9th comet is noted in her log books… is this a comet awaiting a future recovery and connection?

The discovery of Uranus in the year 1781 made Herschel’s superstardom possible. Thankfully, the name “George” suggested to honor his financial benefactor didn’t stick. William’s obsession for ever larger optics makes for a remarkable read, as does just how those astronomers of yore operated. Remember, speculum metal was the technology of the day, although William experimented with other more exotic surfaces, including a polished iron mirror. These were far less reflective than the aluminized or silvered glass mirrors we have today, and William found it necessary to tip the primary mirror on the 40-foot to prevent obstruction from the secondary (what we today would call a “Herschelian”) to gather as much light as possible. Large instruments were only partially steerable, and viewers had to resign themselves to sweeping the local meridian, the telescope being raised and lowered via a system of pulleys and workers. William even insisted on placing the 49.5”  primary back in the telescope each night to check the progress made in the finishing of the mirror!

In the end, William found use of the smaller 20-foot scope more to his liking, and it’s interesting to note that Caroline was very prolific using the more versatile seven foot “comet sweeper” Newtonian. Caroline was an astute note taker and recorder while William was at the eyepiece, and took this intimate knowledge of the skies and applied it to her own landmark discoveries. One of the most poignant final acts of their careers is recounted in the closing chapter of the book, as William and Caroline resume their familiar positions at the 20 foot telescope and instruct young John Herschel on the art of viewing, sweeping and recording. John would eventually continue the work of his father and extend it to surveys of the southern hemisphere skies from South Africa.  It is also worth noting that William was also known for another fundamental discovery that would eventually be applied to astronomy today; that of infra-red radiation.

Discoverers of the Universe is a very highly readable account of the Golden Age of British astronomy, and I would recommend it to anyone interested in getting a look at just how those astronomers of yore operated. An interesting side note: one of Williams’s 20-foot instruments was gifted to the King of Spain in 1802, only to be destroyed by Napoleons troops (perhaps they thought it was a canon?) The mirror was hidden and survived; the telescope has been reconstructed and refurbished in the north of Spain. Do I sense an Astroguyz road trip coming on?


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