The alignment will occur for a few days this week

The 20-minute alignment of the sun will occur on only a few days this week at the Bryn Celli Ddu chamber on Anglesey.

The video of the summer solstice at the 5,000-year-old chamber will form part of a museum exhibition in Cardiff.

Archaeologist Steve Burrow made the discovery after reading a book by Sir Norman Lockyear published almost 100 years ago.

When the sun shines through, you feel the site around you - it is an extremely elating moment Dr Steve Burrow

Mr Burrow said: "The sun doesn't move that much on the days around the solstice.

"Last year I spent six days going up there, on the last day we managed to get it - the weather was a bit dodgy.

"It was absolutely incredible," he said.

Sir Norman - the man who discovered helium - had travelled to the site, otherwise known as the Hill of Black Grove, and measured the alignment of the sun at Easter.

"I came across this reference in a book dating back to 1908 but nobody had checked it, nobody had gone and verified it in person," he said.

"The anticipation on my part was incredible, I wasn't sure until it actually happened. It's definitely a very personal thing, I think.

Light shimmer

"There is a very tall pillar stone within the mound which is never illuminated, it is like having a companion with you.

"When the sun shines through, you feel the site around you. It is an extremely elating moment.

"5,000 years ago they had the sophistication to build this monument," he added.

The solstice starts with a blink of light which shimmers and grows more intense until it shines through the burial chamber.

Mr Burrow said human remains, which would have been placed in the chamber, would have been illuminated by the sun.

"The bones would have been almost warmed by the sun," he said.

Mr Burrow, a curator of Neolithic archaeology at the National Museum of Wales, delayed his book by a year to test the theory.

His book Tomb Builders, published on Wednesday, explores the megalithic tombs which remain in Wales.