Sunlight simulator for any address

Need to see how sunlight will fall on a street, a garden or a building on a particular day? Search the address in Sundial, set the date and time, and the real buildings around it cast their shadows in 3D, right in your browser.

Free, no sign-up, nothing to install. Works on phones and desktops.

What the simulator does

Sundial loads the buildings within 300 m of the address from OpenStreetMap and draws them in 3D. It then places the sun where it really is for that spot, date and minute, and renders the shadows. Move the time slider and the shadows sweep round. Move the date slider and you can watch them stretch towards winter.

Both sliders snap to useful moments: the equinoxes and solstices on the date, and sunrise, solar noon and sunset on the clock. Two play buttons run the simulation for you. One plays through a whole day in about sixteen seconds. The other keeps the clock time fixed and plays through the year, which is the quickest way to see that a patio sunny at 4 pm in June is in shade at 4 pm in November.

Over the map sits the sun's arc for the chosen day, with the June and December solstice arcs and a compass for comparison. The panel gives sunrise, solar noon, sunset, day length, and the sun's height and bearing, all in the location's own time zone.

How it calculates

The sun's position comes from SunCalc, an open-source library of standard astronomical formulas. For a given latitude, longitude and moment it returns the sun's height above the horizon and its compass bearing. Sundial lights the 3D model from exactly that direction and draws the shadows with WebGL on your own device, so nothing is rendered on a server.

The Sun hours view goes one step further. It repeats the shadow calculation every ten minutes from sunrise to sunset and adds up which surfaces were lit, then colours each patch of ground and roof by its total hours of direct sun for that day. Tap or hover a spot to read the number. You can open the same square in the sun hours view to see it.

To give a sense of how much the light changes, here is London across the year, from the same formulas:

A sun 15° up at its highest point throws shadows nearly four times the height of whatever casts them. That's why a simulation for a single summer afternoon tells you very little about December.

What people use it for

• Gardens. Find the beds that get six or more hours of sun, and see how far the house shadow reaches in spring when seeds go in. See what full sun means.
• Buying or renting. Check whether a flat's windows or a garden see any sun in winter, before you sign. The best direction for a house guide explains what to look for.
• Solar panels. A first look at whether neighbouring buildings shade a roof at the low winter sun. Read which way solar panels should face too.
• Extensions and new buildings. A rough sense of how a tall neighbour already shades your plot through the year.
• Events and photography. Pick the side of a square that will be in shade, or lit, at the hour you need.

Running your own simulation

1. Search an address or place, paste coordinates, or use your location.
2. Set the date, then drag the time, or press play to run the day or the year.
3. Switch to Sun hours for the day's total of direct sun on each surface.
4. Press Share to copy a link that reopens the same place, date and time.