SunPetal: Sunlight Hours

Stand where you want to plant something. Hold your phone up and sweep it across the sky. SunPetal works out how many hours of direct sunlight that exact spot gets — not your garden, not your postcode, that spot.

It works entirely on your phone. No account, no network, no data leaves the device.

WHY A SWEEP, AND NOT AN ADDRESS

The difference between a bed that grows tomatoes and one that doesn't is often a single tree, a fence, or the corner of a house. None of that is in a map. The camera sweep measures the actual skyline above the actual spot, so the answer accounts for the things that are really in the way.

WHAT YOU GET

• Hours of direct sun a day, averaged across your growing season • Every month of the year, so you can see a bed that is bright in June and gone by September • Morning sun and afternoon sun counted separately — horticulturally they are not the same thing • Any single date, answered instantly: first sun, last sun, and the longest unbroken stretch • The sky as the scan saw it, with a slider through the year: watch the sun's path climb and fall, and the deciduous trees leaf out and go bare

A MAP OF YOUR GARDEN'S LIGHT

Walk from bed to bed, sweeping each one. The phone measures the steps between them with its own motion tracking — distances far too small for GPS — and SunPetal paints the sun on the ground between your spots. Tap anywhere for that patch's hours. Come back another day and add more spots to the same walk.

AT THE HEIGHT OF YOUR PLANTS

The phone measures how high you are holding it. Sweep once more with the phone down near the bed, and the answer can be read at soil, bed, waist or chest height — lettuce and a climbing bean do not see the same sky.

IT TELLS YOU HOW SURE IT IS

Every answer comes with a range, and with what that range is made of — how precisely north was fixed, how much of the sun's path the sweep actually covered, how much of the open sky reaches the spot. A number without an error bar is a guess wearing a suit.

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