# Lantern Valley's tide: a function of the hour you can learn

> The sea in Lantern Valley rises and falls by a simple cosine of the game hour, same every day, felt only far from the valley. How and why it is pure rules.

Canonical page: https://andersonmancini.dev/blog/lantern-valley-tides-as-rules

Published: 2026-09-28 · 1 min read · Tags: game-dev, javascript, design, lantern-valley, systems

The tide in [Lantern Valley](https://andersonmancini.dev/blog/lantern-valley.md) is a small file with no meshes and no three.js in it: a function of the game hour, the same every day, so that a player can **learn it**.

## The rule

Low water at 6h and 18h, high water at 0h and 12h:

```js
const tideAt = (hour) => -AMP * Math.cos((Math.PI * (hour - 6)) / 6)
```

The amplitude is 0.6 m either side of the mean. "High", "low" and "deep low" are thresholds on that level: `high` from 0.3 m, `low` below -0.3 m and `deep` below -0.5 m.

## Where you feel it

The valley itself does not move: its sea stays level. A weight says how much of the tide a place has, and it ramps smoothly with the distance from the middle of the valley, beginning around 210 m and whole at 300 m. So the tide shows up only far out, where the islands are.

That is a design decision as much as a technical one: the valley stays a calm, readable home, and the archipelago is where timing matters.

## Questions the rest of the game asks

Because it is pure rules, other systems ask it simple questions:

- `tideState(hour)`: level, rising or falling, and phase,
- `hoursUntil(hour, 'high' | 'low')`,
- `hoursUntilLevel(hour, level, above)`: the next hour the tide is above or below a level, which is the rule behind "wait for the tide".

## Why rules and not simulation

A tide that is a function can be tested without a renderer, shown in the journal as a clock, and used by quests, such as a puzzle that opens at low water. The fishing and the beacon systems read it the same way.

The rest of the world is data-driven in the same spirit: see [the quest condition language](https://andersonmancini.dev/blog/lantern-valley-quest-conditions.md) and [the save system](https://andersonmancini.dev/blog/lantern-valley-save-that-never-loses-progress.md). The game is on its [project page](https://andersonmancini.dev/projects/lantern-valley.md).

## Related projects

- [Lantern Valley](https://andersonmancini.dev/projects/lantern-valley.md) — Cozy adventure game for the browser