When they moved the grave and set the new stone, they had a practitioner fix the sitting and facing. Backing north, facing south. He levelled the compass and took the reading three times, and called the numbers out, and the son wrote them in a notebook.
At Qingming the next year the stone was off. He measured roughly with the compass app on his phone: three degrees. He reset it.
The third year, five. The fourth year, eight. The fifth, thirteen.
Always the same way round.
He bought a proper luopan, the kind the practitioner had used, two hundred and forty yuan, and started measuring properly once a year and writing it down.
The rate crept up a little each year but the direction never changed. He worked out how long a full revolution would take.
A hundred and twenty years.
He looked it up in the clan register. The grave was first marked with a stone a hundred and twenty years ago.
Which meant it had come the whole way round and was back where it started.
He set the luopan on top of the stone and waited for the needle to settle.
The reading matched the three the practitioner had called out, exactly.