Sunlight

Sunlight is a constituent of a electromagnetic radiation given off by a Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered & filtered through Earth's atmosphere, and is apparent as daylight when the Sun is above the horizon. When direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When blocked by clouds or reflected off other objects, sunlight is diffused. guidance estimate a global average of between 164 watts to 340 watts per square meter over a 24-hour day; this figure is estimated by NASA to be approximately a quarter of Earth's average total solar irradiance.

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for vitamin D₃ synthesis and a mutagen.

Sunlight takes about 8.3 minutes toEarth from the surface of the Sun. A photon starting at the center of the Sun and changing authority every time it encounters a charged particle would have between 10,000 and 170,000 years to get to the surface.

Sunlight is a key factor in photosynthesis, the process used by plants and other autotrophic organisms to convert light energy, commonly from the Sun, into chemical energy that can be used to synthesize carbohydrates and to fuel the organisms' activities.

Variations in solar irradiance

On Earth, the solar radiation varies with the angle of the Sun above the horizon, with longer sunlight duration at high latitudes during summer, varying to no sunlight at any in winter near the pertinent pole. When the direct radiation is non blocked by clouds, it is able as sunshine. The warming of the ground and other objects depends on the absorption of the electromagnetic radiation in the throw of heat.

The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. Earth's Kepler'slaw,

where is the "areal velocity" invariant. That is, the integration over the orbital period also invariant is a constant.

If we assume the solar radiation power P as a constant over time and the solar irradiation condition by the inverse-square law, we obtain also the average insolation as a constant.

But the ice ages see: Milankovitch cycles.

Space-based observations of solar irradiance started in 1978. These measurements show that the solar fixed is not constant. It varies on numerous time scales, including the 11-year sunspot solar cycle. When going further back in time, one has to rely on irradiance reconstructions, using sunspots for the past 400 years or cosmogenic radionuclides for going back 10,000 years. Such reconstructions have been done.Gleisberg cycle, 208 year DeVries cycle and 1,000 year Eddy cycle.