Cool Sun Facts · Sunlight and shadows
How much sunlight falls on a surface?
An ideal model of the Sun's direct light on the ground, a roof, a wall or a panel, through any day, and the shadows it casts. Every number says what it is and what it leaves out; none of them is a forecast or a measurement.
Sunlight on a surface
A panel's idealised output
Transmission is there to show its effect, not to describe your sky: the share of sunlight that gets through cloud is not one minus the cloud cover, and this page knows nothing about the air where you are.
What these numbers are, and are not
| Quantity | Unit | Here |
|---|---|---|
| The Sun's height and direction | degrees | Calculated here, for your place and time. |
| Reference sunlight at Earth's distance | W/m² | Calculated: the IAU's nominal 1361 W/m² at 1 au, scaled by the inverse square of today's distance. A reference value for a model, not a live measurement. |
| Ideal sunlight on a surface | W/m² | Calculated: the reference sunlight times how squarely the Sun faces the surface, with no air in the way. |
| Energy over a day | Wh/m² | Calculated: the ideal sunlight added up over the day. Power is per instant; energy is power over time. |
| Sunlight actually reaching the ground | W/m² | Not calculated. It depends on the air, haze, smoke and clouds, and takes a measurement. |
| UV Index | an index | Not calculated. It also depends on ozone, clouds and altitude, which the Sun's position cannot tell (EPA). |
| X-ray flux from flares | W/m², 0.1–0.8 nm | Measured by GOES satellites and shown on Space weather. It is not how bright the Sun is. |
What if Earth were different?
Change Earth's tilt, the stretch of its orbit or its distance, and see the sunlight a whole year brings to your latitude. Earth is closest to the Sun in early January, yet the north has its winter then: the tilt matters more. This is a separate, pretend Earth; nothing else on the site changes.
Shadows
The shadow of an upright object at the time chosen above.
None of this needs you to look at the Sun. The experiments use shadows, and say how to stay safe.