How many hours of sun does a garden bed get?

Every plant label is an hour count

The labels are a measurement in disguise. In common garden use, full sun means roughly six hours or more of direct sun a day, part sun or part shade covers about four to six, and full shade is less than that. Tomatoes want the top of that range. Hostas do not. The label is asking you a question you can answer with a number.

Counting it honestly means either standing in the garden all day with a notebook, or working out where the sun goes and what is standing in its way. The second is faster and it works for a date that has not happened yet.

Why the bed you measured in June lies to you

Two things change between June and April, and both go the wrong way.

• The sun is lower. A lower sun means a longer shadow from the same fence. A hedge that clears the bed at midsummer can cover it completely in early spring, which is exactly when seedlings need the light.
• It rises and sets somewhere else. The sunrise azimuth moves a long way through the year. A bed that catches the early sun in June along the side of the house may be in the building's own shadow in April.

The useful test is not the best day of the year. It is the shoulder of the season, the weeks when you are actually asking the bed to produce something.

Getting the count for a spot

1. Stand on the bed and sweep the skyline

   Turn slowly on the spot with the camera along the horizon. Sunlit stores what it sees as 36 sectors of ten degrees: the fence, the hedge, the neighbour's garage, the slope of the hill. That profile is the thing standing between the bed and the sky.

2. Let it work out sunrise and sunset against that skyline

   Instead of the flat horizon time a weather app prints, you get the time the sun actually clears your obstruction, the time it goes back behind one, and the stretches in between when the sun is technically up and the bed is in shade. Subtract those and you have the hour count.

3. Repeat for a date in each season

   Late March, midsummer and late September will bracket the growing year for most gardens. Save the bed as a place first so you can come back to the same spot rather than re-measuring.

4. Use the shadow projection for the things you might move

   On the Map view, set the height of a shed, a fence or a tree and see where its shadow lands at any hour. It is the cheapest way to find out whether moving the water butt two metres buys the bed an hour.

What it does not know

The sun path is geometry and it is exact. The rest is not. Sunlit has no weather feed, so a count of direct sun hours is a clear sky count and your actual season will be cloudier than that. It also cannot tell a bare tree from a leafy one: sweep the skyline in winter and you have recorded the winter tree. The UV and irradiance figures in the app are clear sky models too, computed from geometry and an ozone climatology rather than measured, and they carry that label wherever they appear.

What this costs

Today at the spot you are standing on is free forever: the sun path, sunrise, sunset, solar noon, day length and shadow length. Checking a March date or a September one, saving beds as places, and the measured skyline with its obstruction periods are part of Sunlit Pro, one payment of 9.99 EUR with nothing renewing.