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Sunlight is the general term for all of the electromagnetic radiation emitted by the Sun. Although we see the Sun with our eyes in the visible spectrum, the Sun is actually releasing everything from radio waves and infrared to ultraviolet and even X-ray radiation. Satellites in orbit estimate that the total amount of sunlight that reaches Earth is about 1.366 kilowatts per square meter.
The Earth is located at an average distance of about 150 million kilometers from the Sun. It takes sunlight about 8.3 minutes to cross this distance and reach the Earth. When the sunlight reaches Earth, it’s partly absorbed by the Earth’s atmosphere. Whatever reaches the ground helps to warm the Earth, and keep it hospitable for life.
The amount of sunlight that the Earth receives changes over the course of the year. This is because the Earth orbits the Sun in an elliptical orbit. At its closest point, the Earth actually receives 1.413 kW/m2, and then at the most distant point of its orbit, the Earth only receives 1.321 kW/m2.
The spectrum of the Sun’s radiation is close to the black body radiation of an object with a temperature of 5,800 kelvin. Half of that energy comes in the form of visible light radiation, and half is in the form of infrared radiation; we feel it as heat. A tiny portion also comes as ultraviolet radiation; this is how we get a sunburn if we’re not covered up in the sun.
Planets further away from the Sun receive less sunlight. Mars only gets about 550 kW/m2, while Neptune only receives 1.5 kW/m2.
We’ve written several articles about the Sun for Universe Today. Here’s an article about how long it takes sunlight to reach the Earth, and here’s an article about how there’s more sunlight hitting the Earth.
We’ve also recorded several episodes of Astronomy Cast about the Sun. Listen here, Episode 30: The Sun, Spots and All.