What is Sunlight?
The length of the day varies with latitude and the Sun will rise and set at different times of the year. In addition the Sun's energy is more intense nearer the equator as its rays are less spread out because of the curvature of the Earth. There may be cloud cover which lessens the amount of light reaching the Earth, and lots of the sunlight is scattered, reflected or absorbed by particles, by clouds and by the different surfaces of the Earth.

Why is it Important?
If we know how much daylight we have at a particular time of year, it helps us to decide on when streetlights need to be lit up or turned off. We know if it will be dark when we come home from school, and maybe too dark for us to play sports in the evening outside. Normal outdoor activities are restricted by the short hours of daylight in countries such as Britain. This also affects farmers who want to grow crops needing lots of warm sunlight.

We also need to know how intense the Sun will be in a particular place so that we can avoid getting burnt. A special sun index summarises this for us.