How to read a sun path diagram

A sun path diagram compresses the sun’s whole year onto one chart. It is used by architects to design windows and shading, by gardeners to plan beds, by solar installers to check for shade, and by photographers to see where the sun will be. Once you know how to read it, it answers a lot of questions at a glance.

• June solstice
• Equinox
• December solstice

The layout

Heliotempo draws sun path diagrams as if you were looking down on the sky dome from above:

• The outer circle is the horizon. Points on it have 0° elevation.
• The centre is the zenith, the point straight overhead (90° elevation).
• The dashed rings mark 30° and 60° elevation.
• Directions are around the circle: north at the top, east on the right, south at the bottom, west on the left, like a map.

A point on the diagram therefore tells you two things: its direction from the centre is the sun’s azimuth, and its distance from the rim is its elevation.

The paths

Each curve is the path of the sun on one day. Where a curve meets the rim, the sun rises (on the eastern side) or sets (on the western side). The part of the curve nearest the centre is solar noon, the sun’s highest point.

• The June solstice path (in the Northern Hemisphere) is the biggest: it rises furthest north-east, passes closest to the zenith and sets furthest north-west.
• The equinox path rises due east and sets due west.
• The December solstice path is the smallest: short and low in the south.

Every other day of the year lies between these curves. In the Southern Hemisphere everything is mirrored: the sun passes through the north at noon.

What you can read from it

• Sunrise and sunset directions in each season: where the paths meet the rim.
• Maximum sun height: how close each path comes to the centre. At 46° N, about 67° in June and 21° in December.
• Which obstacles matter: draw the outline of a building or tree onto the diagram as seen from your window. If the sun paths pass behind it, you know when it will shade you.
• Hours of sun on a facade: a wall facing south-east gets sun whenever the sun is in the half of the diagram in front of it.

Interactive versions

The interactive diagrams on Heliotempo add hour marks on the path for the selected day and a moving sun for the selected time. On the sun position map the same diagram is drawn directly onto the map around your chosen point, so you can see which streets, buildings and fields lie under the sun’s path, and the 3D sky view shows the same paths as arcs across the sky.