A GBP900,000 mirror sculpture destined for a square in Nottingham, UK, will have to be shielded to prevent it focusing the Sun's rays and barbecuing passing birds.
Anish Kapoor's highly polished concave steel mirror is six metres in diameter. Direct sunlight hitting the mirror would be focused into a narrow beam of light as hot as the surface of the Sun, says astronomer Michael Merrifield of Nottingham University.
"The beam would be only a few centimetres across, but you're talking about concentrating a huge amount of power - tens of kilowatts - into that tiny area, so it's not something to mess with," he told New Scientist.
The beam could blind people, set fire to birds, or ignite trees, Merrifield says. But the Sky Mirror's angle and position have been carefully chosen to minimise the length of time that direct sunlight would strike.
The potential danger spots are two hours each morning for four weeks around the summer solstice, says Merrifield. Temporary parasols erected around this time should prevent direct sunlight ever hitting the mirror, he says.
Kapoor has also designed the shields, which will be placed around the square outside Nottingham's Playhouse theatre. He intends the mirror to create an illusion of the sky viewed through a hole. The parasols will be situated away from the mirror and will not be intrusive, Merrifield says.