Why Do Painters Prefer Studios With North-Facing Windows?
― At Midday, the Sun Only Ever Crosses the Southern Sky
When people build a house, a sunny south-facing room is popular. Yet painters have long preferred north-facing windows for their workrooms. The reason is that a north window lets in no sunlight at all, only light from the sky. And that light hardly changes its look from morning to evening.
You are painting a piece of fruit on a table by a south-facing window. When you start in the morning, the apple's shadow stretches out sharply to the left.
Past noon, the shadow shortens and turns. By evening the light is yellowish, and even the apple's red looks different. Each time a cloud drifts by, the brightness of the room flickers.
Your half-finished painting and the fruit in front of you drift further and further apart. Painters chose north windows to avoid exactly this trouble.
Just two reasons
In the middle of the northern hemisphere, as in Japan, the midday sun is always in the southern sky. A north-facing window gets no direct sunlight all day.
What comes in is scattered light from the whole sky. Shadows are soft, and neither brightness nor colour shifts much as the hours pass.
In short, a north window shuts out the "wandering sun" and lets in only the "broad, calm sky". Let's look at each in turn.
Where does the sun travel?
The sun rises in the east and sets in the west. But look closely and it does not pass straight overhead from due east to due west. In Japan, the midday sun follows a tilted path through the southern sky.
Figure 1 is a map of the sky seen from directly above. The centre is the point over your head, and the edge is the horizon. In winter the sun traces only a small arc low in the southern sky. At the spring and autumn equinoxes it rises almost due east and sets almost due west. It never enters the northern sky.
Summer is a little different. The sunrise and sunset points shift north, so only in early morning and late afternoon does the sun pass through the northern sky. Around the summer solstice that adds up to about 7 hours in total. But that is only before about 8:30 a.m. and after about 3:30 p.m. During working hours in the middle of the day, no direct sun enters a north window on any day of the year.
Why is skylight easier to paint by?
What comes through a north window is the blue of the sky and the white of clouds. This is sunlight scattered by air molecules and clouds, so it reaches you from the whole sky. Because the source of light fills the whole window, the edges of shadows blur.
The sun is a very small point of light in the sky. That is why the shadow from direct sunlight has sharp edges. And as the sun moves, that shadow turns by 15 degrees every hour. With skylight filling the window, shadows are soft and hardly move.
The colour is calm too. Direct sunlight turns yellow or orange in the morning and evening, and white at midday. When a cloud covers the sun, the room suddenly darkens. The northern sky is the side away from the sun, so its brightness and colour are said to change less. For anyone who needs to judge colour accurately, this is a big advantage.
Factories with jagged "sawtooth roofs" were often built so that the vertical window faces pointed north. The reason is thought to be that spotting uneven colour in woven cloth needed light that stayed steady all day. Craftspeople who check the colour of gems and fabric are also said to have preferred north-window light.
In Australia and the southern parts of South America, the midday sun crosses the northern sky. So painters there choose south-facing windows. What matters is not the compass direction itself, but facing the "side of the sky the sun never visits".
Summary
The midday sun crosses the southern sky all year round. So a north window gets no direct sunlight, only soft light from the whole sky. That light has blurry shadows, and its brightness and colour change little over time, which suited work that copies colours and shapes over many hours.
A north window shuts out the sun and invites in only the sky.
What painters wanted was not brightness, but light that stays the same.
Why skylight is blue is explained in "Why is the sky blue, yet sunsets turn red?", and how differences in direction show up even in mountain trees is in "Why do trees differ between north-facing and south-facing slopes?".
- On a sunny day at noon, place an identical sheet of white paper near a south-facing window and near a north-facing window in your home.
- Stand a pencil upright on each sheet and compare the shadows. By the south window the shadow should have a sharp edge, and by the north window it should be vague.
- Look again about two hours later. Compare smartphone photos to see in which window the shadow's direction and length changed more.
In early morning or evening in summer, sun can reach a north window too. Do your comparison around noon. Never look directly at the sun through the window.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled by level, from junior-high science to university specialist courses
- Junior highCovered in junior-high science
- High schoolCovered in high-school earth science or physics
- High school+Advanced high-school material, or a textbook sidebar
- UniversityUniversity specialist subjects not taught in high school (positional astronomy, lighting engineering, colour science)
- ResearchTopics researchers are still investigating, not yet taught as settled fact even at university
Junior highTerms: this phenomenon has names
- Meridian altitude: the angle of the sun above the horizon when it is due south (at its highest point of the day). It is high in summer and low in winter.
- Skylight: sunlight scattered by air and clouds that reaches you from the whole sky. This is the light that enters a north window.
- Umbra and penumbra: the dark shadow where light is fully blocked, and the blurry fringe where it is only partly blocked. The larger the light source, the wider the penumbra.
Junior highHigh schoolChecking with a formula: how far north does the sun rise on a summer solstice morning?
Direct sunlight enters a north window only when the sun is north of the line joining due east and due west. So we calculate the direction of sunrise on the summer solstice, when the sun rises furthest north, and the sun's height at noon.
| In symbols | cos A = sin δ ÷ cos φ (noon height is h = 90° − φ + δ) |
| In words | cosine of the sunrise direction = sine of the sun's declination ÷ cosine of the latitude |
| Where it comes from | It applies the spherical law of cosines to the spherical triangle on the celestial sphere formed by the celestial pole, the point overhead and the sun on the horizon. It is the relation at the instant the sun's height is zero. |
| A | Sunrise direction (angle measured from north toward east, in degrees) |
| δ | Sun's declination (the sun's tilt, set by the season: +23.4° at the summer solstice, −23.4° at the winter solstice) |
| φ | Latitude of the observation site (degrees) |
| h | Sun's height at noon (when it is due south, in degrees) |
| Tokyo's latitude φ | About 35.7° |
| Sun's declination δ at the summer solstice | About 23.4° |
| Sine of 23.4° | About 0.397 |
| Cosine of 35.7° | About 0.812 |
| Apparent size of the sun | About 0.0093 radians (about 0.5°) |
| Cosine of the sunrise direction | 0.397 ÷ 0.812 ≒ 0.489 |
| Angle whose cosine is 0.489 | About 60.7° (direction measured from north) |
| Offset from due east (90°) | 90 − 60.7 = 29.3 |
| Noon height at the equinoxes | 90 − 35.7 = 54.3 |
| Noon height at the summer solstice | 54.3 + 23.4 = 77.7 |
| Noon height at the winter solstice | 54.3 − 23.4 = 30.9 |
| Blur width of a direct-sunlight shadow (on a floor 1000 mm from the object) | 1000 × 0.0093 = 9.3 |
| For skylight from a window with an apparent width of 0.5 radians | 1000 × 0.5 = 500 |
The summer solstice sun rises about 29° north of due east. That is why a north window gets some sun in summer mornings and evenings. Even so, at noon it stands about 78° high in the southern sky, out of sight from a north window. The blur at a shadow's edge is about 9 mm for the sun but about 50 cm for a window full of sky, a difference of more than 50 times. The values in Figure 1 are calculated with the same formula.
High schoolHigh school+Why the sun's path changes with the seasons, and the colour of skylight
High schoolThe Earth's axis is tilted by about 23.4° relative to its orbital plane. Because of this, the sun's declination swings from +23.4° to −23.4° over a year, and the sunrise direction and noon height change with the seasons. At mid-latitudes in the northern hemisphere, only between the spring and autumn equinoxes, when the declination is positive, does the sun rise and set to the north of the east–west line.
High school+Clear sky is blue because of Rayleigh scattering, in which shorter wavelengths are scattered more strongly by air molecules. So north-window light is bluer than direct sunlight. In lighting, the tint of light is described by a number called "colour temperature", and the bluer the light, the higher the colour temperature. Painters are thought to have got used to this bluish light and put steadiness of colour first.
UniversityCalculating the sun's position, and judging the quality of light
The precise position of the sun is found from the altitude and azimuth formulas of positional astronomy, with corrections added for the equation of time and for atmospheric refraction (the bending of light near the horizon). The calculation in this article ignores refraction, so the real sunrise is slightly earlier and its direction slightly more to the north. In lighting engineering, the amount of light entering through a window is rated by the daylight factor, and the distribution of brightness across the sky is handled with "sky luminance distribution" models. How faithfully colours are reproduced is given by the colour rendering index.
📖 Derivations and further reading: Solar altitude (Wikipedia, in Japanese) ／ Colour temperature (Wikipedia, in Japanese)
ResearchWhat is still not clear
- How much does the brain correct for differences in the colour of light? The mechanism of "colour constancy", which makes objects look the same colour even when the lighting changes, is not yet fully understood. It may also play a part in why north-window light feels easy to paint by.
- How strictly did painters stick to north windows? When and where the habit of preferring north windows spread is explained differently depending on the source, even within art history. The Impressionists did the opposite: they went outdoors to paint the shifting light itself.
- How does natural light indoors affect body and mind? The effect of window light on how easy work is and on mood continues to be studied in architecture and medicine.
In other words, this article too is "an explanation within what is known today". The sun's path can be calculated exactly, but how people feel about that light still holds many mysteries.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: the sun's daily motion and seasonal change | Figure 1, meridian altitude calculation |
| High school | Basic earth science: Earth's rotation and orbit | Declination and sunrise direction |
| High school+ | Physics: scattering of light | Why skylight is blue, colour temperature |
| University | Positional astronomy, lighting engineering | Sun position corrections, daylight factor |
| Research | Colour vision research, architectural environment | Colour constancy, natural light and body and mind |
| ― | Links to daily life | Choosing a room, photography, lighting for close work |
- National Astronomical Observatory of Japan (国立天文台), ed., Rika Nenpyo (理科年表, Chronological Scientific Tables), Maruzen Publishing (sun's declination, sunrise and sunset directions)
- Color Science Association of Japan (日本色彩学会), ed., Shinpen Shikisai Kagaku Handbook (新編 色彩科学ハンドブック, New Handbook of Color Science), University of Tokyo Press
- Wikipedia (Japanese), "Solar altitude" (太陽高度)
- Wikipedia (Japanese), "Colour temperature" (色温度)
- Wikipedia (Japanese), "Rayleigh scattering" (レイリー散乱)
* This article is a general-audience science explanation. The figures given are rough guides to help you understand how things work. The sun's position is calculated for a site near Tokyo and ignores refraction by the atmosphere. Never look directly at the sun.