It was reported in 1842 that twice a year the sun shines through the tunnel.
It has been assumed that Brunel aligned the tunnel so that the rising sun is visible through it on his birthday, April 9.

Angus Buchanan (2002, p. 269) writes:
The alignment of the Box Tunnel has been the subject of serious discussion in the New Civil Engineer and elsewhere.
I am grateful to my friend James Richard for making calculations which convinced me that the alignment on 9 April would permit the sun to be visible through the tunnel soon after dawn on a fine day.

It has been asserted that it is impossible to guarantee the effect on a particular calendar day, because the angle at which the sun rises on a given date varies with the cycle of leap years.
However, the sun subtends an angle of about half a degree, which is more than the year-to-year variation and more than the field of view through the tunnel, so it quite possibly seems to fill the tunnel every year.
It is also asserted that Brunel failed to account for atmospheric refraction, and the effect is visible a few days too early.

Buchanan (p. 226) concludes:
…I have found no documentary evidence for the often-repeated story that Brunel aligned the Box Tunnel so that the rising sun shone through it on his birthday, even though careful examination shows that it could indeed do so, and it is certainly a good story.

A mathematical exploration of the possibility of the phenomenon occurring on Brunel's birthday has been undertaken by C.P. Atkins.
Atkins concludes that, taking atmospheric refraction into account:
…full illumination [of the tunnel profile] can occur on April 7 in three (non-Leap) years out of four, whereas in a Leap-Year this should occur on April 6. …
partial illumination is not possible on April 8, let alone the significant April 9.

The author suggests that reports of the sun shining through Box Tunnel on April 9, or even on April 15 and 16, as reported by "the local head ganger … suggests some reflection effect and is, in itself, worthy of further investigation".

On Sunday 9 April 2017, when the line was closed for electrification and upgrades at Bath Spa railway station and the weather was favourable, a team from the GWR franchise tested the theory, finding that the sunrise did not shine fully through the tunnel.
Images released show that the sun is well aligned to the tunnel.

During December 2016, Genealogists’ Magazine published an analysis confirming two previous analyses, that the sun does indeed shine through the tunnel, but a few days before Brunel's birthday.
On 6 April through the 1830s, complete solar penetration occurred on three years out of four in the leap year cycle.
The author of the article was able to show that the 6th of April was the birthday of Brunel's sister, Emma Joan Brunel, and that therefore the alignment on this day may have been deliberate.
It was reported in 1842 that twice a year the sun shines through the tunnel. It has been assumed that Brunel aligned the tunnel so that the rising sun is visible through it on his birthday, April 9.
Angus Buchanan (2002, p. 269) writes:
The alignment of the Box Tunnel has been the subject of serious discussion in the New Civil Engineer and elsewhere. I am grateful to my friend James Richard for making calculations which convinced me that the alignment on 9 April would permit the sun to be visible through the tunnel soon after dawn on a fine day.
It has been asserted that it is impossible to guarantee the effect on a particular calendar day, because the angle at which the sun rises on a given date varies with the cycle of leap years. However, the sun subtends an angle of about half a degree, which is more than the year-to-year variation and more than the field of view through the tunnel, so it quite possibly seems to fill the tunnel every year. It is also asserted that Brunel failed to account for atmospheric refraction, and the effect is visible a few days too early.
Buchanan (p. 226) concludes:
…I have found no documentary evidence for the often-repeated story that Brunel aligned the Box Tunnel so that the rising sun shone through it on his birthday, even though careful examination shows that it could indeed do so, and it is certainly a good story.
A mathematical exploration of the possibility of the phenomenon occurring on Brunel's birthday has been undertaken by C.P. Atkins. Atkins concludes that, taking atmospheric refraction into account:
…full illumination [of the tunnel profile] can occur on April 7 in three (non-Leap) years out of four, whereas in a Leap-Year this should occur on April 6. …partial illumination is not possible on April 8, let alone the significant April 9.
The author suggests that reports of the sun shining through Box Tunnel on April 9, or even on April 15 and 16, as reported by "the local head ganger … suggests some reflection effect and is, in itself, worthy of further investigation".
On Sunday 9 April 2017, when the line was closed for electrification and upgrades at Bath Spa railway station and the weather was favourable, a team from the GWR franchise tested the theory, finding that the sunrise did not shine fully through the tunnel. Images released show that the sun is well aligned to the tunnel.
During December 2016, Genealogists’ Magazine published an analysis confirming two previous analyses, that the sun does indeed shine through the tunnel, but a few days before Brunel's birthday. On 6 April through the 1830s, complete solar penetration occurred on three years out of four in the leap year cycle. The author of the article was able to show that the 6th of April was the birthday of Brunel's sister, Emma Joan Brunel, and that therefore the alignment on this day may have been deliberate.
Box Tunnel