As the dying Isambard Kingdom Brunel was carried over the newly finished Saltash Bridge (over which his twenty-five foot high name is emblazoned) in an open wagon, did his mind go back to his first great feat of engineering completed eighteen years earlier, Box Tunnel? Had he signed that in a more subtle way?

By what is generally designated as an amazing coincidence the sun shines right through Box Tunnel annually at dawn on Brunel’s birthday – Snell 1965

It is said that on or about the 9th April the sun is visible from the west end before it rises over Box Hill – Macdermot and Clinker 1964

It was subsequently found that by a slight change in the alignment of the tunnel axis Brunel had contrived to reorientate the Box Tunnel so that the sun floods it with red light at sunrise on 9th April, his birthday – Sandstrom 1963

He treated mathematics and field astronomy as fine arts, and before long it was discovered that on his birthday, April 9th, the rising sun shone through the tunnel from end to end – Hamilton-Ellis 1954

It is remarkable that annually on the morning of April 9th the sun’s rays penetrate through the great Box Tunnel of the Great Western Railway, and on no other day of the year – Daily Telegraph 1859

The Box Tunnel on the Great Western Railway was an object of some interest on Tuesday last, as on that day, at 25 minutes past five, the sun shone through it.  The only other periods that such an event occurs on are the 3rd and 4th of September – Illustrated London News – 1850

 The evolution of a myth – but is it true? I decided to find out.

 First I asked myself was it physically possible?  Could one see the sky through the tunnel? The tunnel runs in a generally east-west direction straight as a die for 2,940 mt, opening at the top into a deep cutting that bends northwards through rock and is spanned by a bridge. A thirty-foot wide portal viewed from two miles away provides a pretty restricted view, and it seemed to me that the curving cutting wall could easily block it, as might the Marlborough Downs in the distance. A section through the alignment dispelled the latter doubt, however a previously unknown original Brunel section through the tunnel and cutting which I discovered in the National Archives allowed me to construct a 3D computer model allowing a ‘virtual’ telescope view through the tunnel – this confirmed that the cutting wall blocked the view of the sky. A long lens photograph through the tunnel from beyond the western portal, though at the limits of resolution, confirms this – you cannot see the sky through the tunnel, and therefore you cannot see the sun even if the alignment is correct.

Box Tunnel west portal, looking east towards the sunrise. The far portal is visible on the original, (6 pixels wide on this scan), but much dimmer than the sky. If the sky was visible it should be seen here as a brilliant white dot.

The view from the east portal, looking east towards the sunrise, through the cutting that curves to the left.

An enlargement of the above showing the predicted view through the east portal from the west portal – note the triangular patch of ivy on the bridge, and the dark vegetation on the right. No sky is visible.

Enlarged telephoto shot from 4.7km through the tunnel to the bridge. The same dark patches are visible. (All photographs Roger J Morgan 1979)

 Second, was it astronomically valid? The mathematics of the sun’s movement are quite complex, but well understood. It rises in the east, slants up to noon in the south, and sets in the west. The noon peak is high in summer and low in winter – this is measured by the angle from the central position, known as the ‘declination’. With the relevant declination, obtainable from an almanac, the position of true sunrise can be calculated.

 Unfortunately the Earth’s atmosphere bends the very oblique light at sunrise in the same way that a stick appears bent when thrust into water, an effect known as refraction. The sun can appear to be above the horizon when it is in fact below it and this effect must be taken into account.

 The sun’s declination on a given day decreases from year to year by about a tenth of a degree because the calendar year is too short; this is adjusted when leap year day is added, causing the declination to overcorrect by about a hundredth of a degree. The accumulated error is finally corrected by having non-leap year centuries, for example 1900. The result of all this is that rather surprisingly it turns out that the sun does not return to the same rising place on the same day each year - it is not actually possible to design a tunnel so the sun shines through it on the same date each and every year. It will only align every fourth year, and even this will eventually stop.  By consulting declination tables I was able to construct a graph giving true sunrise bearing from 1830 to the present for April 6th, 7th, 8th and 9th.

 Step two was to obtain the exact bearing and elevation of the tunnel. The large-scale Ordnance Survey map gives the bearing, and the Victorian accounts of the construction give its gradient as 1:100, a figure that could not be confirmed by Railtrack as the Official Secrets Act apparently covered it! The construction accounts also give the tunnel profile at its most constricted point; this is much flatter than the west portal, designed for magnificence; and the unlined portion, cut as a gothic arch through the Bath Stone, had proved unstable in frost and has been lined. From the tunnel length the angular size of the east portal from the western can be calculated – and it turns out to be much smaller than the sun, so there is no margin for error!

 Putting all this together the alignment is perfect – but for April 7th – two days early!

Sunrise allowing for Atmospheric Refraction

How the Position of Sunrise Varies and Its Attempted Correction by Leap Year, with Alignment for 6th – 9th April Showing 1835 As the Most Likely Date.

 Thirdly, could it be accidental, and if not how much of a luxury was it? The whole rationale of the Great Western Railway was to obtain maximum speed – hence the broad gauge, easy curves and shallow gradients for as far as possible – ‘Brunel’s Billiard Table’. The main drop was bunched up and taken all in one go in penetrating the Box ridge between an oxbow of the Avon. An examination of the contour map shows the chosen route and alignment of the tunnel to not look unreasonable, utilising as it does the longest finger of valley pointing directly down to Bath. Similarly, the gradient profile through the tunnel and line does not seem unduly contorted from optimum.  It seems evident that given the constraints, and the necessity for minimum deviation from the direct line, any other engineer with sufficient daring would have constructed a tunnel very like Box. This is an important point when over 100 lives were lost in what otherwise might be regarded as an egotistical conceit.

Sketch Contour map showing GWR utilising river valleys to penetrate the Box Ridge

 How then did the alignment come to be known?  This could give an insight into its validity. I tracked it back through the British Newspaper Library until I arrived at the source – the Devizes Gazette for Thursday April 14th 1842.

On Saturday the Box Tunnel presented a most splendid though singular appearance, caused by the shining of the sun directly through it, and giving the walls a brilliancy, to use the expression of an eye witness, “as though the whole tunnel had been gilt.” This circumstance will occur again in the fall of the year (during the month of October). At this time of the year it appears about 5 o’clock in the morning; in the fall, of course, not so early.

 This was the first possible occurrence – the tunnel was opened in June 1841! I never dreamed that effect was widely known at the time – this admits of only two possibilities; the dawn rays had penetrated the rapidly advancing heading in April 1841, as Brunel personally screwed the last effort out of the navvies on an overdue contract, revealing to all present the effect; or Brunel had deliberately calculated the alignment and either returned with friends to observe his handiwork, or had boasted of it to others.  It is supposed that his Journal covering this period, known to have existed from the biography of his eldest son, was destroyed on his death as being too self-revelatory – perhaps this was the terrible secret? In April 1842 he was setting out Swindon New Town, and then immediately left for Sardinia to advise the King – could the timing of these activities have been designed so that he could slip away to Box? We may never know the truth, but it is not inconceivable that the Journal exists to this day and will eventually confirm or deny.

 From personal experience it seems this scene is still enacted in the cold pre-dawn of April 9th, as an expectant gathering huddles under the A4 bridge in the hope of a clear sun-rise. Some claim to have seen the golden tunnel, others do not, and yet others swear by some other day.  As I have shown the sun cannot in any case ever be seen, and if it could be it would be on April 7th! Apart from this I think the problem is that clear sunrises in April are pretty rare to begin with; 2 miles of mist often obscure the interior of the tunnel; and the morning light will penetrate some way down the tunnel either side the correct day, providing brightness which may be taken as the sun itself.

 We are left with the problem of why the alignment is not perfect, being two days early?  It is evident Brunel was utilising the true sun and not allowing for atmospheric refraction - Bessel’s Refraction Tables would not be readily available in the 1830’s. My sunrise graph shows that the most likely declination used was 1835 – this corresponds well with the facts, as Brunel surveyed the route in 1833 and the shafts for the tunnel were sunk (this fixing the alignment) in spring 1836.

 So the verdict – a good but not perfect exercise in astro-surveying using inadequate knowledge by a 28 year old egocentric, who strangely apparently forgot the cutting would block the view of the sky.

 He lies now with his father beneath a perfect snow-white cube in All Soul’s Cemetery, Kensal Green, London.

Supported by maps, charts, diagrams, photographs and a video!

All Copyright Roger J Morgan 14th February 2006