Solsticio

How to solve sun & moon puzzles

Solsticio puzzles (a sun/moon take on Binairo, Takuzu and Tango) are always solvable by pure logic — you never have to guess. Here are the techniques that crack every board, from the first move to the last.

The three rules, quickly

New to it? Start with the rules explained. Below we turn those rules into moves.

Technique 1 — The pair forces a gap

Two of the same symbol side by side force their neighbours. If you see ☀☀, the cells on either side must be ☾ (a third sun would make three in a row):

☾ ☀ ☀ ☾

The same works with a gap: ☀ _ ☀ must be ☀ ☾ ☀, because a sun in the middle would make three:

☀ _ ☀ → the gap must be a moon.

Technique 2 — Finish the edge

A pair against the wall forces the third cell too. ☀☀ _ at the start of a line must continue ☀☀☾. Scan every row and column for two-in-a-rows touching an empty cell — each one is a free move.

Technique 3 — Balance the line

Every line holds the same number of suns and moons (three of each on a 6×6, two of each on a 4×4). So the moment a row or column reaches its quota of one symbol, every remaining cell is the other symbol. On a 6×6, once you place the third sun in a row, the rest are moons:

☀ ☾ ☀ ☾ ☀ ☾

Five cells filled with three suns → the last one must be a moon.

Technique 4 — Cross-reference rows and columns

A cell lives in a row and a column. If the row rule leaves two options but the column rule forbids one of them, the cell is decided. When you're stuck, pick an empty cell and ask: “would a sun here break its row or its column?” Often one symbol breaks a rule and the other doesn't — that's your move.

Technique 5 — Follow the connectors

On later worlds, = between two cells means they're equal and × means they're different. Treat them as extra constraints: if one side of a = is known, so is the other; a × next to a known cell fixes its neighbour. Connectors often start a chain that unlocks the rest of the board.

A fully solved board

Chain these ideas and there's never a dead end — only the next forced move, until the grid completes:

The golden rule: never guess

Every Solsticio board is verified to have exactly one solution reachable by deduction. If you can't find a move, you've missed one — re-scan for pairs and full lines rather than guessing. Guessing is never required, and it's usually slower.

Trying them on a bigger board

These five techniques scale, but they do not fire in the same order once the grid gets wide: on a large board the no-three rule does most of the work and the balance rule only pays off late. If you want to feel the difference, there is a full 14×14 takuzu to play, with its own deduction walkthrough.

Questions

What is the first move in a sun and moon puzzle? Look for two identical symbols side by side, or two alike with one empty cell between them. The cells next to a pair, and the gap between two alike, must take the opposite symbol: those forced moves are the quickest way into any board.
What if I get stuck? Scan the whole board again for forced moves: pairs, gaps between two alike, lines that already have their quota of one symbol, and cells where one symbol would break its row or its column. Don't guess — there is always a move you have missed.
Do I ever need to guess? No. Every Solsticio board is verified to have exactly one solution reachable by deduction alone, so there is always a logical next move. Guessing is never required, and it is usually slower.
How is difficulty measured? Mainly by the deduction techniques needed to finish the board, not by how many symbols are given at the start. Solsticio's solver tries them in order (no three in a row, line balance, connectors and, on the hardest boards, contradiction checks) and the hardest one it has to use sets the base of the score; the number of givens only fine-tunes it.

The difficulty rating is explained in full, with the solver's code, in Rating logic puzzles by the deductions they require. The blog has more on how the puzzles are generated, too.

Play today's puzzle The rules

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