This is the coldest, darkest time of year for the northern hemisphere. The days are shortest, and the Sun scoots lowest across the sky. In ages gone by, many cultures tried to lure the Sun back with lights of their own — big bonfires outdoors, or big hearthfires indoors.

Some of this tradition of light has carried over into the modern celebration of Christmas, with Yule logs, candles, and in today’s world, twinkling electric lights as well.

And on this Christmas night, the Sun compensates a bit for its daytime absence by lighting up two brilliant night lights: the Moon and the planet Jupiter. Both shine not by producing their own light, but by reflecting sunlight.

The Moon is the same distance from the Sun as Earth is, so each square foot of the lunar surface receives the same amount of sunlight as the same area on Earth.

Jupiter is five times farther from the Sun, though, so the same area of its surface receives much less sunlight. Not one-fifth as much, as you might expect, but one twenty-fifth. That’s because at Jupiter’s distance, the sunlight has spread out. So the same amount of sunlight that falls on one square foot of Earth or the Moon spreads out to cover 25 square feet on Jupiter. And the light that Jupiter reflects also spreads out as it heads toward us.

Even so, there’s plenty of light to make Jupiter the third-brightest object in the night sky — and a beautiful decoration for Christmas night.

Script by Damond Benningfield, Copyright 2012