Chris,
Thanks for the response and continued interest
I do need to make a couple of corrections,
Firstly, the New Civil Engineer investigation was the 4th April 1985 not 1984
Secondly, my photo was not the cover photo but the article photo on page 31
I looked for the photographs today, and failed to find them but I did find the negatives, herewith a link to a previous posting of my photograph
https://www.flickr.com/photos/28512889@N05/24777797845/
or put in to Google “David Needham Box Tunnel”
I do have a copy of the 4th April 1985 New Civil Engineer though posting it would and could infringe their copyright and I would need express permission from Metropolis International Ltd the current owners of the title.
Looking at the web photo which is nowhere near as clear as the actual Number 13 picture that was originally printed, it is today clear to me that the sun was close but not perfectly aligned on the 9th of April, it was just a very clear a day and a clean tunnel which makes my photograph definitely the best April the 9th picture and probably as good a picture as has ever been taken from the western end of the sun at the eastern end.
My view always was that if you can see the sky from the western end looking east at some point the sun’s travels across our sky must pass the point of the tunnel alignment.
Today's issue is the sky is not visible through the tunnel as per my previous post.

Please can I add some information to your discussion
The sun will not shine through the IKB^▸ (Isambard Kingdom Brunel)’s Box tunnel not on the 5th, 6th, 7th, 8th, 9th or 10th of April or the September equivalent not at the moment anyway.
Several things conspire to stop this from happening.
The 6.00am train from Bristol fills the tunnel with diesel fumes before sunrise, recent electrification might soon resolve this first problem.
The south embankment does cover a portion of the tunnel’s eastern portal, when viewed from the western side, it obstructs from the bottom left corner diagonally up at about 45° to the point where the roof curve meets the top of the right hand tunnel wall.
At the moment there is also very tall shrubs and trees on this embankment which blocks the rest of the tunnel’s alignment out completely.
So recent investigations by the GWR^▸ (Great Western Railway) engineers in 2017 was always going to fail to prove this so called “myth” really works.
The next problem is the English morning weather, how many April days have zero cloud or mist between Bath and Reading at 06.30 am ?
I took a picture on the 9th of April 1982 of the sun shining down the tunnel or at least as good as it ever will on the 9th of April. This photo was the cover photo for the New Civil Engineers investigation 4th April 1984 p. 29 - 31
Why did I get a picture of the sun illuminating the tunnel brightly and the the reflection bouncing off all four rails, here is the reasons…
Recently before 1982 (don’t know the exact year) the eastern south embankment was cleared of all shrubbery and trees.
1982 was a Bank Holiday so there was no 06.00 am train out of Bristol, the best image was taken at about 06.44 am, in those days you were limited to 24 colour shots using a Practica SLR 35 camera.
It was the clearest night I ever saw when driving to Box, which I did from 1977 until 1985. After which I realised I would never see the sun shine through it again so stopped visiting the area.
Also it was different times when it comes to safety, annually up to 20 people would be on the tracks with genuine interest and so would GWR staff assisting us and allowing us to be there, after 1983 the rail staff were only there to keep us out of railway property.
By the way only me and a guy from Birmingham driving a 2CV were there on the 9th April in 1982.
I do believe that a better date is the 6th or 7th April (which all mathematical calculations confirm) and why I think this is,  there is a picture taken by Jim Barnes in 1985 a year where the shrubbery was still low on the east south embankment allowing you to see what is clearly the sun within the Eastern portal. If the 6.00 am train from Bristol had not filled the tunnel with fumes 10 minutes before he took that picture that would be the most stunning proof and I doubt if you would have been able to look up the tunnel at the sun with the naked eye without hurting your eyes. That sun is penetrating through 2 miles of confined diesel fumes.
So don’t rush off trying to see this phenomenon, not until the trees are cleared, 100% of trains are electric using the line or it has to be a Bank Holiday, you need one of the best clear nights prior to “the 6th or 7th of April” and permission from Network Rail or GWR, and a key to the Western access gate.
Sorry to disappoint you all and unless you are very very young now you have no chance of seeing IKB’s genius creation as intended, they will cut the trees again but not until they are big and old enough to cause a danger to the trains.

Regards   david@needhams.uk.com, Chesterfield