Sundial

How it works: the sun's position is worked out from the standard NOAA equations, including the way the atmosphere bends light and lifts a low sun slightly above where it geometrically ought to be. Shadow length is height divided by the tangent of the sun's height, which is why a six-foot fence throws barely a foot of shadow at midsummer noon and ten feet at midwinter — the same fence, the same yard, four months apart. That is the gap between what a plant tag calls full sun and what a bed actually receives. Searching for a place is the only part that leaves this page: the words you type go to OpenStreetMap and coordinates come back, and if you would rather not, type the numbers straight into the boxes. Two things it cannot know: hills or buildings on your horizon, which cut the real day shorter than the numbers here, and whether your clocks have gone forward, so check the UTC offset if the times look an hour out.