Once, according to legend, a king named Epiphanes who ruled many galaxies took a dislike to one of them. Wishing to eradicate all life from that galaxy, he caused its stars go out for what he considered to be a sufficient period of time. When the stars gradually came back on, one by one as they seem to do in the evening, there was a single planet circling in its star's habitable zone which still bore the seeds of life. Life reappeared on that planet and spread from thence throughout the galaxy.
Laotzu was visiting friends on a planet where this tradition was celebrated by lighting candles for a week. Each evening they lit more and more candles until there were quite a lot. This reminded them of the stars turning back on, in the story. At supper on the first evening, when there was just one candle, they thought about what it means for your planet to be in the habitable zone: neither too near and hot nor too distant and cold.
So at supper on the first evening they talked about how much sunlight was in a cubic mile of space at any one moment at their distance from the star, measured in their customary energy units of talent (mile/minute)2. In conventional Earth terms we'd say such a unit was equal to 5 food Calories. It happens that at Earth's distance from the sun a cubic mile of space contains one such unit of sunlight! This is true to within about two percent, the actual figure is 0.98 instead of one. But where Laotzu was visiting the figure for sunlight was exactly one.
Having your planet orbit in a region where there is around one unit of sunlight energy in a cubic mile of space pretty much defines what it means to be in the habitable zone. With cooler stars it is closer in and with hotter ones it is further out—the main thing is the oceans shouldn't boil away or freeze solid.
All through the first day of Hanukkah the people would fast, and all through the day they would be especially mindful of the light from their star. At sunset they would break their fast by each having a little morsel of food with 5 Calories—one of their energy units. This was to remind them of the amount of energy in a cubic mile of space. And then they would have a big supper cooked by the Alpha Female.
Light is coming to their planet and to ours at ten million miles a minute. That means a square mile of surface facing the light receives ten million energy units in a minute—you can think of it as a tall stack of ten million cubic-mile blocks of space each holding one energy unit, and the whole stack arriving in the course of a minute. This tells us the maximum brightness of sunlight at our distance from the sun: ten million power units per square mile, or ten million energy units per minute on a square mile facing direct sunlight.
Two hands held palms up can make a squarish platform about one tenthousandth of a mile wide. In direct sunlight an area like that receives about a tenth of an energy unit (so about half a food Calorie) in a minute. So it takes ten minutes for it to receive sunlight equal to that morsel that breaks the fast on the first day---the energy of sunlight in a cubic mile of space.
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note: Using mile-minute-talent units means not only that the energy unit talent(mile/minute)2 works out to be 5 food Calories but also that the power unit — delivering one energy unit per minute — turns out to be about half a horsepower or, in metric terms, 360 watts. It is in terms of this halfhorsepower unit, talent mile2/minute3, that the brightness of direct sunlight works out to be 107 (ten million) power units per square mile. In "Laotzu's Creation Story" we encounter the fantastic Big Bang brightness, the Planck unit of illumination, which is 10126 power units per square mile. This is 119 orders of magnitude (a factor of 10119) greater than sunlight on earth could be even through perfectly clear air.
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