‘Why, it enables me to calculate where I am, as I cross the turning globe of the earth. And, in a sense, when I am.’ He eyed her, and Jamil. ‘You understand that the world is a sphere.’

‘Actually a somewhat flattened sphere, according to Hatti astronomers,’ Uzzia snapped back. ‘Who in turn have built on studies by the Babylonians and others going back millennia. Please don’t condescend, old man.’

‘Very well. Then you’ll understand that as we travel north and south, the apparent position of the pole star in the sky will change. It would be directly overhead if we were at the north pole, whereas if we travel south—’

‘Yes, yes.’

‘So if I measure the star’s position—’

‘Or you get me to do it,’ murmured Avatak.

‘What’s that? Then I can determine the north-south arc of my position on the world’s spherical surface. Now, knowing the date and that single number, the northern arc, I can use my oracle to predict for me’ – he spun wheels and pressed levers, making the face of the oracle sparkle and shift – ‘the length of the day at this place, and the greatest height achieved by the sun in the sky. I have the boy check these independently with his sightings and his hourglass. The numbers are never identical, but the differences teach us about flaws in our methodology, and indeed the small digressions of the earth from its spherical state, to which you have alluded.’

Jamil studied the oracle longingly. ‘Must be useful at sea, that. And in some deserts I’ve crossed. So you have your position north to south. But what about east-west?’

‘Ah,’ Pyxeas said, enthused. ‘Excellent question, considering it’s you asking it. The oracle also contains, encoded into its dials and gears, a knowledge of eclipses of the sun and moon, both past and future.’ He tapped the face. ‘The little ivory moon slides across the golden sun . . . It’s really quite pretty to watch. And by matching the prediction with the reality of an eclipse, I can determine my distance from Northland, west to east. The procedure is a little tricky.’

Uzzia said, ‘Just tell me this one thing. You dream of saving the world. Is it through such means as this, the numbers of the sky?’

‘Yes! Yes, precisely. You see—’

But she held her forefinger to his lips. ‘Another time,’ she said gently. ‘For now, I understand enough. It is late. You two must finish your work, and come closer to the fire, and we will eat and sleep.’

Pyxeas seemed oddly charmed by her motherliness. ‘Another time, then,’ he agreed.

Heading ever east, following the vagaries of roads and passes, they moved out of the fertile plain into a land that was higher, dryer, much more forbidding. Avatak glimpsed mountains, streaked with ice.