Mount John is a thousand metres above Lake Tekapo, and there is a white dome on top of it with a shutter that opens on the sky. Tiare was ten. She had black hair in two plaits and a scarlet wool coat with a hood she never put up, and she was the night assistant. Her job was the log. The log was a hardback book with a green cover and ruled pages, and every page had the same headings along the top: date, observer, instrument, sky, times.

The rule was the first thing Tiare was told. Never guess. If you did not see it, write nothing. A blank in the book is a fact. A guess is a lie with tidy handwriting. Nobody looked at anything until the book said what time it was.

At the start of every night she set the observatory clock against the radio. The time signal comes at the top of each minute, ten short pips in the last ten seconds and one long one. She wrote how many seconds the wall clock was slow at the top of the page, because every time below counted from that number.

Then she wrote the sky in eighths of cloud and seeing from one to five, then the star the university had on the working list, and the steady star beside it that has never changed brightness. The steady one is known to a tenth, and the scale runs backwards, so a bigger number means a fainter star. So the book gets the difference between the two: one tenth at nine, one tenth at a quarter to ten, one tenth and a half at ten.

On the fourteenth of August the sky was the clearest of that winter. There was no cloud until nearly midnight, the seeing was four, which is steady, and she wrote her numbers with the red lamp turned low. At twenty to eleven the custodian came up the stairs: the water tank was filling again, the driver was in the car park with a radio on, and he had to be telephoned.

The telephone is in the gate house at the bottom of a dark gravel path, and it took forty minutes. When she came back the book was still open where she had left it, and five ruled lines lay empty between twenty-five to eleven and twenty to twelve. She wrote them in. Cloud two eighths. Seeing three. One tenth. One tenth. One tenth. And the time beside every line, in her best handwriting.

Dr Prowse came up on the Thursday and read the whole page before she said anything. Then she put one finger on the three numbers in the middle. “This star is never still,” she said. “Not three readings running. Not ever.” “I watched it.” “You were on the gravel with a telephone. You cannot know what the sky did. You cannot even know whether it was clear.” Tiare looked at the page. “Then I should not have written them,” she said.

“You should not have written them. And now nobody can tell which of my other numbers you watched and which one you made up, so we throw the whole night away.” They threw the night away. It had been the clearest night of the winter, and the curve they were building had a flat patch in the middle. The flat patch was not the star. The flat patch was her.

The rule changed after that, but not much. There is still no column for a gap. On the next page Tiare struck one line diagonally through the empty rows and wrote the two times and the reason beside it: no observation, assistant at the gate house, sky unseen. Dr Prowse read it twice and said it was the most useful line in the book, and that it had cost them a week.

The next clear night was the coldest of the winter. Tiare wrote every line herself, and the curve came out smooth, and the flat part of the old one was a hole, and the hole was honest. Then she copied the night into the big book and did the sums, and at the top of the page she wrote the line she wrote every night: the wall clock was four seconds slow at nine. The wall clock loses four seconds a day. She set it on Mondays.

So the Thursday page said four seconds, and four seconds was true on Monday night, and by Thursday it was twelve, and the twelve seconds went out of the door with the book. She never found out how far that clock drifted on a Sunday night. Nobody was at the observatory on a Sunday night to write it down.

She put the book on the shelf with the other forty years of them, and walked down the hill in the dark, and the dome sat above her with the shutter shut.