The hammer rang once. A spark jumped off the anvil and did not go out. It waited. They followed.

It rolled into the little museum on the Green and settled on the loan desk, beside the purple date stamp.

Beside the stamp, the brass plate on the cracked desk still had its tiny hammer.

Ledger was at the desk in white cotton gloves, and lifted a flat bronze plate out of its box. A short bronze stick stood up from the middle of it.

"Gloves on, please," said Ledger, and handed Lan and Beto a pair each, and a paper slip.

Beto picked up the pencil and wrote on the first line straight away. A CLOCK.

"Write what you see first," said Ledger, gently. "What you think goes on the second line."

"But it is a clock," said Beto.

"Maybe," said Ledger. "But you haven't looked yet. The slip is for what is there."

Beto rubbed it out.

Lan looked for a long time.

She wrote: A flat bronze plate, a bit green at the edges. One short stick standing straight up in the middle. Thin lines scratched across the plate, like the spokes of a fan. One line is deeper than the others.

Then, on the second line: I think it is a sun clock. Maybe.

"Now you can open the note," said Ledger.

Inside the box was a folded card. A copy of a shadow stick, of the kind used in Alexandria, Egypt, in the time of Eratosthenes.

"Who was Eratosthenes?" asked Lan.

"The person who measured the whole Earth," said Ledger, "with a shadow."

A green chameleon with a protractor over one eye crept along the windowsill. This was Bevel.

"Long ago," said Bevel, "Eratosthenes heard about a town called Syene, far to the south. At noon on midsummer's day, the sun was straight overhead there. Sticks had no shadow."

"In Alexandria, on the same day, at the same time, sticks did have a shadow. A short one. So Eratosthenes measured how much of a turn the shadow made."

"How much?" asked Beto.

"Seven point two degrees," said Bevel. "Three hundred and sixty all the way round. And seven point two is a fiftieth of that."

Beto worked it out. Fifty times seven point two. Three hundred and sixty. "A fiftieth," he said.

A boy in a beanie striped like a tape measure was leaning on the door frame. This was Rod.

"So now turn the angle into a length," said Rod. "It was 5000 stadia from Syene to Alexandria. A stadion is an old Greek length, about the length of a running track."

"If the shadow was a fiftieth of a turn," said Rod, "then the walk from Syene to Alexandria is a fiftieth of the way round the Earth."

Lan's eyes went wide. "So the whole way round is fifty times that."

"Fifty times 5000," said Rod. "Measure twice."

"Two hundred and fifty thousand," said Beto. "Stadia."

"He never walked round the Earth," said Lan. "He just looked at a shadow and did a sum."

At noon, Ledger carried the plate out to the courtyard and set it on the stone table. The sky was clear. The stick's shadow was short and dark.

Lan held the protractor flat beside it. The shadow lay right on the deeper line.

She took the slip and finished it. Object returned, condition good. Learned with this object: you can measure something too big to walk round, using a shadow and a sum.

Ledger stamped it. It was the second slip in the children's folder.

Back on the Green

The spark dimmed to a warm dot and settled back on the anvil. The Green was just the Green again.

On the way home, Beto stopped by a lamp post and looked at its long shadow.

"I could measure that," he said. "Without a ladder."