THE LIGHT AT THE END OF BOX TUNNEL
As the dying Isambard Kingdom Brunel was carried over the newly finished Saltash Bridge (over which his twenty-
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-
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-
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-
Step two was to obtain the exact bearing and elevation of the tunnel. The large-
Putting all this together the alignment is perfect – but for April 7th – two days early!
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.
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-
From personal experience it seems this scene is still enacted in the cold pre-
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 -
So the verdict – a good but not perfect exercise in astro-
He lies now with his father beneath a perfect snow-
Supported by maps, charts, diagrams, photographs and a video!
All Copyright Roger J Morgan 14th February 2006