A mysterious spectacle which appears to honour two significant days in the solar calendar has been discovered in a Suffolk church. This year's spring equinox - when the day and night are of equal length - will shed light on a religious statue and with it an extraordinary secret that remained hidden for hundreds of years.

Every year thousands of revellers flock to watch dawn break on the summer and winter solstices at Stonehenge, which Druids believe is a temple to the alignment of the sun.

Although no-one knows the reason behind the prehistoric stone circle in Wiltshire, it is widely accepted its design was such that it would mark the longest and shortest days of the year.

Now a mysterious spectacle which appears to honour the two other most significant days in the solar calendar has been discovered in a Suffolk church. This year's spring equinox - when the day and night are of equal length - will shed light on a religious statue and with it an extraordinary secret that remained hidden for hundreds of years.

Weather permitting, for just four minutes on March 20 a beam of evening sunlight will filter through a small window at Barsham Church, near Beccles, and bathe a sculpture of Christ on the Cross in a golden light.

It is a phenomenon that was only recently rediscovered by a vicar after being hidden for centuries because the sculpture, also known as the rood, was taken down. It may date back as far as the 1300s when the window was built.

The Rev John Buchanan discovered it in the early 1990s, but has only recently been able to make sense of it after recording its occurrence over a number of years.

This year he predicts that the entire rood will be directly lit on March 20, while the preceding day and the following day will see the figure partially lit. The phenomenon is repeated at the autumn equinox in September.

'To actually see it was a matter of luck,' said Mr Buchanan. 'You have to go in there at the right time and there's got to be no cloud. That's why it's taken myself and others such a long time to find it.

'I'm a curious sort of guy and my immediate thought was that it wasn't by chance - I thought, 'what sort of event does it mark?' So I went through the church calendar, and it got terribly complicated because in the 1600s they shifted it from Julian to Gregorian. But I realised that the one thing that would stay the same is the equinox.'

He added: 'It's a sort of camera obscura effect like at Stonehenge.'

There is nothing in the record books about the phenomenon, although Mr Buchanan believes that the window and statue were positioned by design because the window is conspicuously off-centre.

'I've often wondered whether a cunning priest was trying to do a bit of magic,' he said. 'But I think it was for an agricultural purpose, to mark the time between winter and summer. I think it was a fixed point in time to say when the cattle can go back on the marshes.'

The rood was removed in accordance with Henry VIII's wishes in the 16th century, and even after it was remade in 1870 there was only a period of five years before the window was covered by a large painting, again hiding the church's secret. In 1979, when a fire at the church destroyed the entire nave roof, the painting was removed.

Visitors are invited to see the spectacle at about 5.20pm on March 20, or on March 19 or 21. 'It's only four minutes, so it's one of these events where you either drink it in or you pay no attention to what's going on and take your picture,' Mr Buchanan said. 'It is a magical moment.'