A sun path diagram shows where the sun is in the sky, for every hour and every day of the year, as seen from one place. Designers use it to plan shade, daylight, overheating protection and solar panels. This guide explains the two angles that fix the sun's position, the two common projections, how to read a solar chart and what an analemma shows.
Altitude and azimuth: the position of the sun
Two angles fix the position of the sun on the sky dome at a given moment:
- Altitude is the vertical angle of the sun above the horizon: 0° at sunrise and sunset, 90° when the sun is directly overhead.
- Azimuth is the horizontal angle of the sun, measured clockwise from true north: 90° is east, 180° is south, 270° is west.
Legend:
a = celestial vault
b = horizontal plane
c = parabolic projection surface
d = sun path on any given date
e = projection of the sun path on the parabolic surface
f = meridian
g = geographic north
h = projection of the parabolic surface onto a horizontal surface: sun chart
2D projections of the sky dome
A sun path diagram puts the sky dome on a flat drawing. Two projections are common.
Orthographic projection
The sky dome is projected straight down onto a horizontal plane. The drawing looks like the dome seen from above. It is common in architecture and daylight studies.
Stereographic projection
The sky dome is projected onto a plane from the point directly below the observer (the nadir). This projection keeps angles true (it is conformal). It is common in astronomy and in detailed solar geometry work.
| Feature | Orthographic projection | Stereographic projection |
|---|---|---|
| Projection | Straight down from the dome onto the ground plane | From the nadir onto a flat plane |
| Look | Like the dome seen from above | Flatter, more even spacing |
| Angles kept true | No | Yes (conformal) |
| Altitude rings near the horizon | Close together (compressed) | Wider apart (less compressed) |
| Horizon | Outer circle of the diagram | Outer circle of the diagram |
Use the orthographic view for a quick, intuitive picture. Use the stereographic view when you need to read low sun angles, for example winter sun or early morning sun on a facade.
Solar charts
A solar chart records the path of the sun over a full year for one latitude. It usually shows only some key dates, such as the equinoxes and the solstices. Because the chart depends on latitude, each place on Earth has its own chart.
A solar chart has these parts:
- Cardinal axes that show north, east, south and west.
- An outer circle divided into 360°, which is the horizon of the observer. Read the azimuth on it.
- Concentric rings that show altitude above the horizon, from 0° at the outer circle to 90° at the centre.
- Date curves that show the path of the sun on selected days.
- Hour curves that cross the date curves. Where an hour curve meets a date curve, you read the sun position at that hour on that date.
Analemmas
An analemma shows the position of the sun at the same clock hour (for example 9:00) on every day of the year, seen from one place. It has a figure-eight shape for two reasons:
- The tilt of the Earth's axis (about 23.5°) moves the sun north and south over the year.
- The elliptical orbit of the Earth makes the sun run a little ahead of or behind the clock during the year.
Many sun path diagrams draw one analemma for each hour. With them, you can read how the altitude and the azimuth of the sun at a given hour change over the year. This helps you check sun exposure, design shading and estimate solar energy at specific times and dates.
Solar radiation terms in a weather file
A sun path tells you where the sun is. A weather file (EPW) tells you how much energy arrives, hour by hour. Four radiation values appear in every EPW file and in most climate charts. Each is the energy in one hour, in Wh/m².
| Term | What it measures | Use it to |
|---|---|---|
| Global horizontal radiation (GHR) | All solar energy on a horizontal surface: direct sun plus diffuse sky light | Compare how sunny two places or two months are |
| Direct normal radiation (DNR) | Energy of the direct beam on a surface that faces the sun | Size shading devices and estimate glare and solar gain |
| Diffuse horizontal radiation (DHR) | Sky light on a horizontal surface, without the direct beam | Plan daylight on overcast days and in shade |
| Horizontal infrared radiation (HIR) | Longwave heat from the sky on a horizontal surface | Judge night cooling: a clear sky sends little back, so surfaces cool fast |
GHR is the sum of DHR and the direct beam on the horizontal plane (DNR times the sine of the sun altitude). Infrared Core reads all four from the weather file, hour by hour, for felt temperature (UTCI); solar radiation uses DNR and DHR.
From sun paths to results on your site
A sun path diagram shows where the sun is. It does not show what the sun reaches on your site, because buildings and trees block it. Infrared Core traces the sun and the sky for every point of the 3D scene, on the ground, on facades and on roofs. From that it computes:
- Direct sun hours: how many hours of direct sun each point gets over a chosen period.
- Solar radiation: the solar energy in kWh/m² on ground, facades and roofs.
- Sky view factor and daylight availability: how much sky each point sees and how much daylight reaches it.
- Felt heat (UTCI): sun and shade are the strongest drivers of how hot a street feels.
Read Simulation Engine Basics to see how the engine computes these results, or browse the simulation library.