SOCIAL SCIENCE: This passage is adapted from the book The Age of Wonder by Richard Holmes (©2008 by Richard Holmes).
In the summer of 1785 astronomer William Herschel embarked on his revolutionary new project to observe and resolve the heavens with a telescope more powerful than ever previously attempted.
What. he intended to build1 was a telescope of the Newtonian form, with an octagon tube 40 foot long and five feet in diameter; the specula [mirrors] of which it would be necessary to have at least two, or perhaps three. The telescope would have to be mounted in an enormous wooden gantry, capable of being turned safely on its axis by just two workmen, but also susceptible to the finest fingertip adjustments by the observing astronomer.
The forty-foot would be higher than a house. The astronomer (William) would be required to climb a2 series of ladders to a special viewing platform perched at the mouth of the telescope. The assistant (William's sister, Caroline) would have to be shut in a special booth below to avoid light pollution3, where she would have her desk and lamp, celestial clocks, and observation journals. Astronomer and assistant would be invisible to each other for hours on end, shouting commands and replies, although eventually connected by a metal speaking-tube.
William had decided that his grand project required a new house with larger grounds for constructing and erecting the telescope. On 3 April 1786 they moved to 'The Grove', a quite small and rather dilapidated country house on the edge of the tiny village of Slough, England.
The house itself was not large, but it had sheds and stables which were gradually converted into workshops and laboratories. Above the stables were a series of haylofts which could be converted into a separate apartment. Caroline claimed these for her own. A small outside staircase led up to a flat roof from which she hoped to carry out her comet 'sweeps' in security and independently. She would check over the calculations of William's nebulae by day, and make her own sweeps up on the roof by night.4
William had built Caroline a special two-foot Newtonian reflector. Because of its large aperture, its tube appeared much fatter, heavier and stubbier5 than normal reflectors of this type. Suspended from a pivot at the top of the box-frame, the telescope could be precisely raised or lowered by a system of pulleys operated by a winding handle. These adjustments were easy to make, and extremely fine.
This beautiful instrument was designed specifically for its huge light-gathering power and its wide angle of vision. The magnification was comparatively low at twenty-four times. As with modern binoculars, this combination of low power with a large viewing field allowed the observer to see faint stellar objects very brightly, while placing them within a comparatively wide context of surrounding stars. The telescope was perfectly designed to spot any strange or unknown object moving through the familiar field of 'fixed stars'. In other words, to catch new planets or new comets.
On 1 August 1786, only two nights after starting her new sweeps, Caroline thought she had spotted an unknown stellar object moving through Ursa Major (the Great Bear constellation). It appeared to be descending, but barely perceptibly, towards a triangulation of stars in the beautifully named constellation Coma Berenices. To find something so quickly, and in such a familiar place (the Great Bear or Big Dipper being the first stop of every amateur stargazer wanting to locate the Pole Star), seemed wildly unlikely.6 Caroline's Observation Book conveys meticulous caution, but also remarkable certainty.
Unable to calculate the mathematical coordinates of the object, she accompanied her observations with a series of three neat drawings or 'figures', over an eighty-minute time lapse. These showed the circular viewing field of her telescope, with an asterisk shape very slightly changing position relative to three known fixed stars. The account written into her 'Book of Work Done' catches something of her growing excitement
August 1st. I have calculated 100 nebulae today, and this evening I saw an object which I believe will prove tomorrow night to be a Comet. August 2nd. 1 o'clock. the object of last night IS A COMET.7 August 3rd. I did not go to rest till I had written to Dr Blagden [at the Royal Society] and Mr Aubert to announce the Comet.
The verification of Caroline's comet was achieved much more rapidly than William's discovery of the planet Uranus had been. Its movement through Coma Berenices was relatively easy to ascertain, and its fine hazy tail or coma was unmistakable.8