The daily journey of the sun patch
Through one day the pool of direct sunlight sweeps round beneath the opening in an arc, starting on the western side of the room in the morning and finishing on the eastern side in the evening.
The reason is simple geometry. Light passes through the glass in a straight line, so the sun patch always lands on the side of the room facing away from the sun. At breakfast, with the sun in the east or south-east, a flat rooflight in a kitchen extension throws its light towards the west wall. Around midday the sun is due south and the patch lies to the north of the opening. By late afternoon the sun has moved into the west and the patch has swung across to the east side.
In Cambridge, solar noon (the moment the sun is due south and highest) falls near 12:00 GMT. Once the clocks go forward, that is about 13:00 BST, which catches people out: the lunchtime position of the patch in June is an hour later on the clock than they expect. Early and late on a clear day the patch may be gone altogether, because the sun is too low to see past the frame, the kerb and the sides of the light shaft.
The diffuse light from the sky does not move in this way. It is the soft, even brightness you get on a cloudy day, and it fills the room from the whole opening all day long. Only the direct beam travels.
Why the season changes where it lands
The height of the sun at noon sets how far from the glass the patch falls, and in Cambridge that height swings from about 14 degrees in midwinter to about 61 degrees in midsummer.
Cambridge sits at roughly 52 degrees north. On flat Cambridgeshire land there are no hills to cut off the sun, so these angles apply to almost every roof in the city and the villages around it. A useful way to picture them is the length of a shadow at noon.
| Time of year | Noon sun height | Noon shadow |
|---|---|---|
| Midwinter (Dec) | About 14° | About 4 times the object's height |
| Equinox (Mar, Sep) | About 38° | About 1.3 times |
| Midsummer (Jun) | About 61° | About half |
Light through a rooflight behaves like the reverse of a shadow. A post one metre tall throws a four-metre shadow at a December noon, and the sun patch from a ceiling opening lands a matching distance away from the point directly under the glass. The day is also longer in summer and the arc wider: in June the sun rises well north of east and sets well north of west, while in December it rises south of east and sets south of west after only a little under eight hours above the horizon.
- Winter solstice, about 14°. The lowest noon sun of the year travels furthest: it crosses the whole room and lands high on the back wall.
- Spring and autumn equinox, about 38°. The light lands on the floor, deep in the room.
- Summer solstice, about 61°. The highest, strongest sun drops steeply and lands close under the window.
- South-facing roof window. The same opening lets in a different patch of light every month.
- Deepest reach. In December the sun patch can be on the wall opposite, which is why we plan where it falls, not just how big the glass is.
Floor, walls or ceiling: a worked example
Take a flat rooflight set into a ceiling 2.5 metres above the floor, with no deep shaft, and follow the noon sun patch through the year.
- Midsummer: the patch lands about 1.4 metres north of the point below the glass, on the floor, and it is roughly the same size and shape as the opening. It is bright, compact and warm.
- Equinox: it falls about 3.2 metres away. In a narrow side-return extension on a Romsey or Mill Road terrace, that may already be at the foot of the far wall.
- Midwinter: the geometry puts it about 10 metres away, so in a real room it meets a wall and runs up it as a stretched, fainter band. Some of the time it lands on the inside of the shaft instead and never reaches the room below.
These are positions, not light levels, and they follow directly from the angles above. A deeper shaft or a tall upstand trims the low winter beam further, which is why a splayed light shaft can let more of that sun through. For what a room actually gets in December, see whether skylights give much light in winter.
Roof windows on a slope behave differently
A pitched roof window tilts its glass towards one part of the sky, so the slope it sits in decides when the patch appears and how long it stays.
On a south slope, the glass faces the low winter sun almost square on, so a loft room gets direct light through much of a December day and a steep patch across the floor. In summer the same window admits a high, strong beam for many hours. An east slope gives the patch in the morning and loses it by early afternoon; a west slope does the reverse, with a low evening beam that can reach deep into the room. A north slope sees direct sun only on summer mornings and evenings, when the sun is north of east and west, and gives steady sky light the rest of the time. The orientation guide compares the four slopes in full, and the roof window service covers fitting them.
Using the movement when you plan a rooflight
Once you know where the patch will travel, you can put the glass where the sunlight helps and keep it off the places where it annoys.
A breakfast table on the west side of a kitchen will catch morning sun from a rooflight above it. A desk or television on the north side of a south-lit room sits where the midday patch falls in spring and autumn, which is where glare starts; our page on stopping glare from a skylight covers the fixes, and whether a skylight needs a blind deals with shading. A bright sun patch on a sofa or an oak floor through a whole summer also fades fabric and finishes, so glass choice matters there.
This is the core of how we plan the light. At the survey we check which way the roof faces, the ceiling height and the shaft, and work out where the patch will land in June and in December before anything is cut. Every quote from Vision Skylights comes with that written light plan. You can read more about our skylight and rooflight installation in Cambridge, or book a light survey.
Questions people also ask
Does a north-facing skylight get any direct sun?
Only a little, and only in summer. From late spring to late summer the sun rises north of east and sets north of west, so a north-facing roof window can take a brief direct beam early and late in the day. The rest of the time it gives even sky light with no travelling patch.
Why is the sun patch from my rooflight on the wall in winter?
Because the December noon sun in Cambridge is only about 14 degrees above the horizon. At that angle the beam travels roughly four metres sideways for every metre it drops, so it reaches the wall before the floor.
Does the clock change affect where the light falls?
Not the sun's path, only the time on the clock. Solar noon is near 12:00 GMT all year, which reads as about 13:00 while British Summer Time is in force, so every position of the patch shifts an hour later on the clock from late March to late October.
Can I move the sun patch after a rooflight is fitted?
Not the patch itself, since the sun and the opening fix it. You can soften it with a blind or diffusing glass, or rearrange the room around it. Choosing the position of the skylight at the design stage is far easier than living with the wrong one.
