One measurement we make with very high accuracy and precision is the "total solar irradiance," TSI for short, which is the solar energy impinging on 1 meter-squared perpendicular to the Sun's rays at the mean Earth-Sun distance. The best current value is 1361 Watts per square-meter. This measurement is made using instruments called "active cavity radiometers," and examples are the ACRIM, TIM, and Virgo. The TIM (Total Irradiance Monitor), onboard the SORCE satellite, is the most precise of these. TIM is a very dark ice-cream-cone shaped cavity that soaks up almost every particle of sunlight that hits it. TIM measures the TSI to an accuracy of 0.035 percent, or 350 ppm (part-per-million). Knowing TSI and the mean Sun-Earth distance, we can calculate (for those of us who enjoy calculating!) the Sun's total energy output, or luminosity, which turns out to be a whopping 3.85 x 10^26 Watts, where 10^26 means a 1 followed by 26 zeroes. That's right, coincidentally that's another zero for each letter of the English alphabet. How many 100 Watt lightbulbs would you need to put together to generate that amount of brightness? 1,000,000,000,000,000,000,000,000 – Is there a word for that many lightbulbs? I don't know. But it's a LOTTA bulbs! ;-)

From basic physics, we know that this total solar power is coming from each square meter of the Sun's surface, by an amount proportional to the 4th power of the Sun's surface temperature. That T^4 law called the "black body law" and the proportionality constant is called the Stefan-Boltzmann constant, after two famous 19th century physicists who first discoverd it. So, as a first approximation, if we assume every square meter of the Sun's surface has the same temperature, we can adjust this temperature until we get the correct total energy. The answer is about 5780 degrees Kelvin. (You can work out what that is in Celcius, or Fahrenheit, if you're curious. If you're curious and also lazy, go to http://www.onlineconversion.com/temperature.htm ).

Earth's average temperature is more than 20x cooler, or less than 300 degrees Kelvin. That temperature allows Earth to shed about the same amount of energy that is absorbs from the Sun. For the more physics-minded of you, learn more at : http://climate.gsfc.nasa....c/cahalan/Radiation/