The rule, and what it actually says
Ask why painters want a north window and you will usually hear something about the light. The fuller version of the rule is narrower and more useful: a high window, on the side of the room, admitting no direct sun, so the light on the work does not move. North is how that rule is usually stated in the northern hemisphere. It is not the rule itself.
That distinction matters if you live south of the equator, or near it, or in a room whose only window faces the wrong way. The principle is steady, indirect, side light. The compass direction is a local solution to it. This is a field guide to testing which window in your own building actually delivers that, using a sun chart, a bearing, and one walk around the room.
Why north, in the northern hemisphere
In the northern hemisphere the Sun spends the day in the southern half of the sky. It rises in the southeast in winter and in the northeast in summer, and sets in the southwest or northwest accordingly; at the equinoxes it rises due east and sets due west. Those seasonal bearings are standard teaching material in solar geometry and are set out, for example, in the Lunar and Planetary Institute’s SunWatchers activity.
A north-facing window therefore avoids direct beam for most of the day and most of the year, at any latitude north of the tropics. What it receives instead is skylight: the diffuse glow of the whole dome of the sky, which is far steadier than a sunbeam and does not sweep across a canvas as the hours pass. The room stays lit; nothing in it is spotlit.
This is why the rule is often written as a prohibition rather than a preference. The seventeenth-century painter and art historian Joachim von Sandrart, writing in the 1600s, held that studios should be large and well lit but that direct rays of the sun should be avoided. The same source describes Dutch studios of the period as lit from the side, with top light not in use. The design rationale is older than the compass shorthand.
The geometry, in numbers you can hold
Two angles describe the Sun’s position at any moment. Altitude is its height above the horizon, from 0 degrees at the horizon to 90 degrees overhead. Azimuth is its bearing around the horizon. Official tables, such as the U.S. Naval Observatory’s altitude and azimuth service, tabulate both at intervals through a day for any location between 1700 and 2100, and compute azimuth with respect to true north, not magnetic north.
Sun charts use a different convention. In the northern hemisphere, azimuth on a stereographic sun chart is referenced to due south, and each latitude gets its own diagram. The chart plots the Sun’s track across the sky for dates through the year, so you can see at a glance which parts of the sky your window faces and when the Sun crosses them.
Body-scale anchors help you read the sky without instruments. A fist held at arm’s length spans about 10 degrees. Four upright fingers stacked against the horizon correspond roughly to an hour of remaining daylight. A noon shadow is best measured in yous: the length of your own shadow in your own foot-lengths, which changes with the season and tells you the Sun’s altitude without a single number.
Test your own window
You need three things: the true bearing of the window, the date, and a sun chart or altitude-azimuth table for your latitude.
Step one: find the bearing. Stand at the window and sight along the wall’s outward normal, or use a compass and correct for magnetic declination to get true north. Write the bearing down. A window facing 350 degrees true is not the same as one facing 10 degrees, even though both are called north.
Step two: plot the exposure. Take a stereographic sun chart for your latitude. Mark the window’s bearing on the perimeter and the arc of sky it sees, roughly 180 degrees wide for a full view. The Sun’s tracks that cross that arc are the days and hours when direct light can enter. If no track crosses it, the window is a true indirect window for the year.
Step three: check one day. Use the USNO altitude and azimuth table for a single date at your location. Note the hours when the Sun’s azimuth falls within the window’s arc and its altitude is above the horizon and below the top of the window. That is your direct-sun window, in clock time.
Step four: walk it. On a clear day, note the hour the first patch of sun reaches the easel or the work surface. Repeat at the solstices. The difference between the two dates is the seasonal swing your room actually experiences, which is more useful than any general rule.
When the window is wrong
Most rooms do not have the window the rule asks for. Two practitioner fixes appear repeatedly in studio accounts. The first is diffusion: cover the glass with tracing paper or a sheer neutral drape, which turns direct sun into an even, soft light and removes the moving patch. The second is orientation: turn the easel away from the window so the work is lit from the side and the window does not reflect in wet paint, then add artificial light above and to the side.
These are working methods, not measured results. They are worth trying because they address the actual problem, which is moving light and glare, rather than the compass label on the wall.
The southern hemisphere flip
South of the equator the geometry inverts. The Sun spends the day in the northern half of the sky, rising in the northeast in the southern summer and in the southeast in winter, and setting in the northwest or southwest. The steady, indirect window therefore faces south, not north. A north-facing window in, say, 35 degrees south will take direct sun for much of the day in summer, which is exactly what the rule is trying to avoid.
Near the equator the flip is less useful than the principle. The Sun passes close to overhead at the equinoxes and swings north and south through the year, so no single compass direction is reliably indirect. The practical answer is shade: a deep overhang, a high window, or a diffusing screen, oriented so that the beam never lands on the work. The sun chart for your latitude will show the band of sky the Sun occupies; design the window to miss it.
What the historical record does and does not say
It is often asserted that Vermeer painted in a north-facing studio. The retrieved source describes his workspace as lit from the left-hand window, with the scene staged near it, and describes Dutch studios generally as side-lit. It does not give a compass bearing for that window. The historical evidence supports the design rationale, not a specific orientation for a specific painter. Treat the north-window story as a useful tradition, not a documented fact about any individual studio.
One walk, one number
The next clear morning, note the window’s true bearing, the date, and the hour the first direct sun reaches the work surface. Do it again at the solstice. You will have a number for your own room, in degrees and in clock time, which is the only measurement that matters for the light you actually paint by.
FAQ
Does a north window really get no direct sun?
In the northern hemisphere, north of the tropics, a north-facing window avoids direct sun for most of the day and most of the year. Around the summer solstice, however, the Sun rises north of east and sets north of west, so even a due-north window can catch brief low-angle beam near sunrise or sunset. Check the bearing and plot the sun chart for your latitude to see whether your window’s arc crosses the Sun’s track.
What if I live in the southern hemisphere?
Use a south-facing window for the same steady, indirect light. Re-plot the sun chart for your latitude; do not copy the northern rule.
What if I live near the equator?
No compass direction is reliably indirect. Use shade or diffusion and check the sun chart for the band of sky the Sun crosses.
Can I fix a south-facing window?
Yes, with diffusion. Tracing paper or a sheer drape turns direct sun into even light. Turn the easel away from the glass and add artificial light from above and the side.
Where do I get the numbers?
The USNO altitude and azimuth service gives the Sun’s position at intervals through any day for your location, referenced to true north. A stereographic sun chart for your latitude shows the whole year at once.