Pairs and gaps
Two matching beacons side by side force the opposite symbol on both ends of the pair: two suns together must be fenced by moons, or there would be three in a row. Two matching beacons with one empty cell between them force the opposite symbol into the gap.
These two patterns alone fill much of an easy grid. Scan every row and column for them before anything else, and scan again after each placement.
“Scan for these first. They do half the work before you’ve had to think.”
Zhao
Count to three
Each line needs exactly three suns and three moons. As soon as a row has three suns, every empty cell in it is a moon. When a row has two suns and the remaining empty cells include a stretch where a sun would make three in a row, that narrows down where the last sun can go.
Use the links
An = link between two cells means they match; a ≠ link means they differ. A ≠ link places one sun and one moon in that pair, which counts toward the line’s three-and-three even before you know which is which. An = link next to a filled cell gives you its partner straight away.
Two cells joined by ≠ always contain one of each, so the other four cells of that row must hold two suns and two moons.
The unique-line rule
Near the end, a row with two empty cells can often be finished two ways. If one of them would copy a row that is already complete, the other is the answer. The same applies to columns. It is easy to forget this rule, and it is often the only way to settle the last cells.