Friday, March 20, 2009

Tipping homeward

This evening as the Sun sets here on the island of Exuma, it will also cross the celestial equator, moving north. That is to say, the vernal equinox occurs at 7:45 PM Eastern Daylight Time, within an hour of sunset. Equi-nox. Night and day of equal length. Each day now for three months the Sun will track higher in the sky for us in the northern hemisphere. Sunlight will fall more directly onto the surface of the Earth, warming it more intensely. Seeds stir in the soil. Trees put out new leaves like solar sails to catch the energy of nuclear fusion. And we who fled the ice and snow for warmer climes begin to think about returning home.

Deep in the belly of the star, protons -- the nuclei of hydrogen -- fuse together to form the nuclei of helium. And here's the wonderful thing: The helium nucleus weighs about 1 percent less than the total weight of the four protons out of which it was made. Matter has vanished from the universe. And in its place -- energy. Every second at the Sun's core, 660 million tons of hydrogen is converted into 655 million tons of helium. The missing 5 million tons is turned into an amount of radiant energy equal to the missing mass times the speed of light squared -- Einstein's famed equation. Fuel for crocuses.

All of that energy produced deep in the Sun takes several million years to make its way to the surface, up through half-a-million miles of roiling plasma. At the surface, it is hurled into space as heat and light. Eight minutes later, a tiny fraction of this flux of energy bathes the Earth - to warm the planet, sustain photosynthesis, and draw snowbirds home from the south.

(BTW. You might think that at 5 million tons per second the Sun would disappear in no time. I just ran the numbers. In the 5 billion years or so of the Sun's life so far, it has radiated about a thousandth of its mass.)