Sun & Moon has only three rules, and each one gives a solving technique. Most grids are solved with pairs and gaps; the larger ones also need counting and the unique-line rule.
Pairs
When two identical symbols sit next to each other, both neighbouring cells along that line must hold the other symbol. ☀☀ becomes ☾☀☀☾.
Gaps
When the same symbol appears with one empty cell between them, that cell must be the opposite symbol — otherwise there would be three in a row. ☀ _ ☀ becomes ☀☾☀.
Counting
Each line has exactly half suns and half moons. In a 6×6 grid, a row with three suns already has all its suns, so every empty cell in it is a moon. Counting is the key technique on 10×10 grids.
Unique rows and columns
Near the end, a row may have two empty cells that could be filled either way. If one of the two options would copy a finished row, the other option is the answer.
Work line by line
After each new symbol, check its row and its column again. One placed cell often creates a new pair or gap, and the grid fills in quickly from there.
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