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The solar constant is the amount of electromagnetic radiation that comes from the Sun at a distance of 1 astronomical unit (the average distance from the Earth to the Sun) and falls on a specific perpendicular area. Measured by satellites in space, the solar constant works out to be 1.366 kilowatts per square meter.
The Sun gives off electromagnetic radiation across the entire spectrum, from radio waves to infrared, from visible light through ultraviolet to X-rays. If you could add up the energy of all this radiation, you then get the total radiation. The solar constant is the amount of radiation that hits an area held perpendicular to the Sun. So imagine you held up a 1 meter square panel and faced it directly at the Sun. The solar constant would be the amount of radiation hitting the panel.
Of course, the amount of actual sunlight we see here on the surface of Earth is a fraction of the solar constant. That’s because the Earth’s atmosphere blocks out some of the Sun’s radiation. You also receive less sunlight depending on your point on the Earth, and your angle away from the Sun. The Sun is actually emitting about 2 billion times more energy than what’s received by the Earth.
The amount of radiation received by the Earth also changes depending on its point in orbit. Since the Earth follows an elliptical orbit around the Sun, at its closest point in orbit, the amount of energy it receives is actually 1.413 kW/m2. And then at its most distant point, it only receives 1.321 kW/m2.
We’ve also recorded several episodes of Astronomy Cast about the Sun. Listen here, Episode 30: The Sun, Spots and All.