How to check sunlight in a flat before you sign

The viewing is at three in the afternoon in May, and that tells you nothing

You get twenty minutes in an empty flat, usually at whatever hour the agent picked, usually not in winter. The room is bright, so you write down bright. Six months later the sun is 25 degrees lower, it rises far to the south of where it did in May, and the building across the courtyard is in the way from ten in the morning.

The fix is not to visit four times. It is to measure the geometry once, while you are standing in the room, and then let the arithmetic tell you about December.

Five minutes at the viewing

1. Stand at the window and read the azimuth

   Face the way the window faces and read the sun azimuth and the compass in Sunlit. Azimuth is degrees clockwise from north: 90 is due east, 180 due south, 270 due west. Note the number rather than the word. "South facing" covers everything from 135 to 225 degrees, and those two are very different flats.

2. Hold the phone up and look at the path

   The AR view puts today's sun path over the live image, and behind it the summer solstice, winter solstice and equinox curves. You are looking for one thing: where the winter curve sits relative to the roofline opposite. If the December track passes behind that roof, the room has no direct sun in December, whatever the agent says.

3. Sweep the skyline

   Turn slowly on the spot with the camera along the horizon. Sunlit records the real skyline as 36 sectors of ten degrees and then computes sunrise and sunset against that profile instead of a flat one, plus the stretches when the sun is up but behind the ridge or the building. The gap between the two is what you are actually buying.

4. Jump to a winter date and a summer date

   Set the date to around 21 December and read the day rail: sunrise, solar noon, sunset, day length, and the maximum solar altitude. Then set it to 21 June. Two numbers, ten seconds, and the seasonal swing stops being a guess.

5. Check the shadow, not just the sun

   On the Map view, drop the pin on the flat, set the height of the building opposite, and look at where its shadow falls at the hours you care about.

What the numbers mean once you are home

• Maximum solar altitude in December is the ceiling on everything. The lower it is, the further a shadow reaches and the more a modest building opposite costs you.
• Sunrise and sunset azimuth swing a long way through the year. A window at 90 degrees gets the sun early in June and much later, and further south, in December.
• Obstruction periods are the honest version of daylight hours. A published sunrise is the moment the sun clears an ocean horizon. Very few flats face an ocean.

What to be careful about

Sunlit computes geometry, not weather and not glass. It says where the sun will be and what is in the way along your measured skyline. It does not know about the tree that loses its leaves in November, the balcony above you, the net curtains, or how much light the window itself lets through. Use it for the part that is arithmetic and use your eyes for the rest.

What this costs

Today, at the place you are standing, is free forever, which covers the azimuth reading, the compass and today's sun path in AR. Jumping to a December date, saving the flat as a place, and the measured horizon profile with its obstruction periods are part of Sunlit Pro: one payment of 9.99 EUR, and nothing renews.