How to play
- Fill every cell with a sun or a moon. Tap once for a sun, twice for a moon, three times to clear. Drag across cells to fill several with the same symbol.
- Balance every line. Each row and each column holds the same number of suns and moons: three of each on a 6 by 6 board, four of each on 8 by 8.
- No three in a row. At most two suns or two moons may sit next to each other, across or down.
- Keep the signs. Two cells joined by = hold the same symbol. Two cells joined by × hold opposite symbols.
A cell that breaks a rule turns red right away, and the board is solved the moment the last cell fits.
Solving techniques
- Follow the signs. When one end of a sign is filled, the other end follows: the same symbol across =, the opposite across ×.
- Pairs need guards. Two suns side by side mean the cells at both ends of the pair are moons, and the other way round.
- Fill the sandwich. Sun, empty, sun means the middle must be a moon, or there would be three suns in a row.
- Finish full lines. Once a row has all of its suns, every empty cell left in it is a moon.
- Count the × pairs. A × sign inside a row always contributes one sun and one moon. Count it as such, and the rest of the row often has only one way to balance.
- Watch the long gaps. In a row that still needs two moons and one sun, look at where the sun could go. Often only one place avoids three moons side by side, and that settles the row.
- Try a what-if. On expert boards, place a symbol in a key cell and follow the simple rules for a step or two. If a rule breaks, the cell must hold the other symbol.
Levels and dailies
| Levels | Board | Difficulty |
|---|---|---|
| 1 to 20 | 6 by 6 | Gentle introduction |
| 21 to 150 | 6 by 6 | Gentle to medium |
| 151 to 300 | 6 by 6 | Easy to expert |
| 301 to 400 | 8 by 8 | Easy to hard |
The daily puzzle is a 6 by 6 board, the same for everyone. It is easiest on Monday, hardest on Saturday, and the result screen keeps your streak.
How the boards are made
Each board starts as a complete, valid grid of suns and moons. The generator then removes symbols one at a time and adds = and × signs taken from the answer, checking after every change that exactly one solution remains. A solver that uses only the techniques above grades the board, and the hints use the same solver, so every hint sentence describes a real step on your board.
Questions
- What kind of puzzle is Sun & Moon?
- It is a binary puzzle, a family also known as Takuzu or Binairo, with signs between some cells. If you know the daily sun and moon puzzles, the rules are the same.
- Do rows have to be different from each other?
- No. Some binary puzzles add a rule that no two rows may be identical. Sun & Moon does not use it; the balance rule, the no-three rule and the signs are enough for one solution.
- Is there always one answer?
- Yes. Every board is checked by a solver to have exactly one solution, and a second solver confirms it can be finished with reasoning alone, without trial and error beyond a short what-if on the hardest boards.
- What do the shaded cells mean?
- Shaded cells are part of the puzzle and cannot be changed. Everything else is yours to fill.
- How do hints work?
- A hint highlights the cells it is reasoning about, tells you the rule in a short sentence and lets you apply the move with one tap. Three hints per puzzle are free.
- Can I use the keyboard?
- Yes. Arrow keys move, S places a sun, M a moon, Backspace clears, U undoes and H asks for a hint.