Sunlight Compass for Houseplants
Point the compass at your window, set the distance and see how much direct sun actually reaches the plant.
Compass direction tells you when the sun can reach the glass; distance tells you how much of that light survives the trip into the room. Set both, then read the estimate below.
The sun path is modelled for a mid-latitude site at the equinox. Pick the hemisphere you garden in so the compass points at the right sky.
At the plant’s spot
- Hours of direct beam on a clear day
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- Relative brightness
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- Estimated light level
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- Rough illuminance
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Houseplants that suit this spot
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Estimates only. Glass type, overhangs, trees, curtains and wall colour all change real light; a phone light meter is a useful cross-check.
How it works
The diagram treats the sun as a geometric source. On an equinox at 45° latitude it rises due east, climbs to 45° at noon and sets due west. A window only catches direct beam while the sun’s azimuth is within about 80° of the direction the glass faces, so a north window sees almost none and a south window sees the middle of the day.
Moving the plant back from the glass changes two things. Diffuse sky light fades gradually with distance, and the direct beam has to travel further, so it fades faster. The “direct sun” figure counts the modelled hours when the beam can reach your chosen distance at all; the brightness figure weights those hours by how squarely the beam hits the glass.
| Window faces | Character | Direct beam |
|---|---|---|
| North | Even, cool, no sun | Almost none |
| East | Bright mornings, gentler afternoons | Moderate |
| South | Strongest and longest | Most |
| West | Hot afternoon sun | Moderate |
| NE / NW | Soft, short-lived | Little |
| SE / SW | Bright for half the day | Good |
The lux figure is a rough indoor estimate built from those geometry assumptions, rounded to the nearest 100. It is there to compare one spot with another, not to replace a meter. Seasonal sun height, buildings opposite, insect screens and dirty glass all shift the result.
Frequently asked questions
How are the direct-sun hours calculated?
The tool models a clear equinox day at 45° latitude, with a 1.6 m window and nothing outside blocking the sky. It counts the hours when the sun is inside the window’s field of view and high enough for the beam to reach your chosen distance. Cloud, season, trees and buildings will change the real number.
Why does moving the plant back change the result so much?
Light spreads out as it travels, and the beam also has to clear the window opening at an angle. A plant 3 m from the glass receives a small fraction of what sits on the sill.
Which window direction is brightest?
In the northern hemisphere a south-facing window gets the most sun; in the southern hemisphere it is north. East and west windows are bright for part of the day, while north and south respectively stay soft.
Does a north-facing window work for houseplants?
Yes, for species that evolved under canopy light: pothos, snake plants, ZZ plants, ferns and many philodendrons. Flowering and sun-loving plants will struggle there without a grow light.
Can I use this for a balcony or garden?
No. The model is built for light passing through a window into a room. Outdoor light is far stronger and arrives from the whole sky, not one opening.