The tide on Areshnaat repeats itself every seven days, exactly, and has always done so. The tide-week is the oldest unit of reckoning on the water, and on most coasts it is older than any calendar laid over it.
Why seven
The Four Moons are not free to wander. Red, Green and Blue are locked to one another — Red circling four times for Green's two and Blue's one — and a locked system keeps time. Counted against the turning of the world rather than against the stars, in seven days:
| Moon | works the water |
|---|---|
| Red | three times |
| Green | five times |
| Blue | six times |
Three, five and six: whole numbers, every time. On the eighth day the three stand as they stood on the first, and the week begins again with nothing left over. White is too far out and too slow to disturb this; what White does is swell the whole week and then slacken it, on its own month.
How it stacks
- 7 days — one tide-week. The tide repeats.
- 4 tide-weeks — one month, which is also White full to full: 28 days.
- 13 months — 364 days, and fifty-two tide-weeks.
- One day over. The year is 365 and a quarter; the months account for 364. That day belongs to no month and no week.
What it is worth
The water here is fast and it is heavy — four moons pulling at once — but it is not unreadable. That combination is the whole of a pilot's trade, and it is why tide-knowledge on Areshnaat is not seamanship but property.
A passage that is safe on the third day of the week will drown a ship on the fifth, and will be safe again seven days later. Anyone who has computed the tables can sail it. Anyone who has not must wait, or guess, and the guess is usually fatal. So the tables are kept, copied under seal, and not sold.
The goblin Companhia is built on exactly this. Its fleets are ordinary; its almanacs are not. Charters, port fees, interest and wages are all reckoned by the tide-week rather than the month, because the week is what the sea actually obeys — and the house that issues the week issues the terms.
The day that is owed to nothing
The leftover day sits outside every month and every week. Contracts reckoned in tide-weeks do not run on it, debts reckoned in months do not accrue, and a charter that ends on the last day of the thirteenth month has not yet begun the next. What is done with it differs from port to port. Some keep it as a holiday, some treat it as a day on which anything may be agreed and nothing enforced, and in the Companhia's harbours it is not spoken of as free at all.