Ancient civilizations knew how important the Sun’s light was for growing crops, although they weren’t aware that the Sun is just an ordinary star like the ones they saw twinkling in the sky. Actually, they didn’t know what stars were either, but that’s a story for another book.
What they certainly didn’t know is that the Sun is a gigantic thermonuclear furnace that continually fuses hydrogen into helium deep in its core and releases huge amounts of energy in both visible and invisible wavelengths. The fusion process is very difficult to achieve here on Earth because great pressure and temperature are required, but it’s a snap for Old Sol since temperatures at the inner 25 percent of its core (where most of the fusion reactions occur) average around 27,000,000°F. In the heart of the Sun it’s not the humidity; it’s the heat.
Remember, the Sun is really only visible if you’re wearing suitable eye protection or are viewing it through an approved solar filter. Never, ever look at the Sun, even for a moment, without proper eye protection.
Just as with Earth’s atmosphere, scientists divide the Sun’s layers into zones. Energy generated in the Sun’s core gradually works its way toward the surface through convection, and on reaching the photosphere, which is the outer luminous layer that we can see from Earth, becomes visible as light.
The photosphere is the layer that contains sunspots, huge areas of cooler gases that were first discovered by Galileo back in 1610. In this case “cooler” is a relative term—sunspots are only about 3,000°F less than the photosphere’s average temperature of 10,000 to 12,000°F, so you’d still need a sunscreen with a rating of at least SPF one million to get anywhere near one. Sunspots are thought to be caused by magnetic fields deep in the Sun that break the surface, creating dark blotches that are often a sign of increased activity deep in the Sun’s interior. Sunspots range in size from roughly Earth-sized to more than twenty times the diameter of our planet.
The next layer out is the chromosphere, which is virtually invisible from Earth except during a total solar eclipse, when it can be seen as a narrow red or pink band around the Sun. The chromosphere’s temperature is around 14,400°F at the bottom and 36,000°F at the top, so it actually gets hotter as you move away from the Sun. Solar flares originate in the chromosphere, releasing as much energy as a million hydrogen bombs going off at the same time.
The next layer of the Sun’s atmosphere is the corona, which is thousands of times fainter than the photosphere and is invisible from Earth except, again, during a total solar eclipse. At that time the corona appears as an elongated, ragged halo around the Sun, with thin white filaments stretching out millions of miles into space like celestial cirrus clouds.
Consistent with the chromosphere, the corona continues to get hotter as distance from the core increases. The temperature of the corona varies from 2,000,000°F to nearly 4,000,000°F. How does the corona get so hot with a cooler layer below it? Astrophysicists think that huge magnetic bands in the photosphere generate massive amounts of electricity and carry it up into the corona. There are tens of thousands of these magnetic loops scattered around the surface of the Sun, and any one of them could satisfy the US’s electrical needs for a hundred years if it could somehow be harnessed.
The corona isn’t static; it continually blasts charged particles out into space, creating the heliosphere, an area of the Sun’s influence that actually extends out beyond the orbit of Pluto. These particles, collectively called the solar wind, can cause big problems for the earth when the Sun gets active. The Sun’s average distance from Earth is some 93,000,000 miles, but even small changes in its output can have major consequences here, as happens when huge bubbles of plasma erupt in the Sun’s outer layers. Such events blow billions of tons of particles from the Sun’s atmosphere in a blast called a coronal mass ejection (CME).
If a CME is directed toward Earth, it smashes into our atmosphere at a million miles an hour, sometimes damaging satellites and causing power and communications disruptions. That’s why NOAA and the US Air Force now jointly operate the Space Weather Prediction Center (SWPC), which provides warnings of impending solar explosions.
It’s a good thing Earth has a magnetosphere, which deflects most of the particles back into space. But some of the energy from a solar storm can still leak into the atmosphere near the poles where the magnetosphere is weaker, creating the aurora borealis, or northern lights. When appearing over the Southern Hemisphere, the lights are called the aurora australis.
Since the time of Copernicus in the sixteenth century, most people have understood that Earth revolves around the Sun. But the origin of the seasons has often been misunderstood, and even today there are misconceptions about it. When you put your hand close to a stove it gets hot, and when you move it away, it cools. So it would seem that when Earth is closer to the Sun it would be summertime, and when it is farther away, you’d have winter. Earth does indeed get closer and then farther from the Sun during the year because its orbit is slightly elliptical, or oval, but that fact has very little effect on the seasons.
In fact, Earth is closest to the Sun in January, when it’s winter in the Northern Hemisphere. In addition, if the seasons were caused by Earth’s proximity to the Sun, every country on the planet would experience winter and summer at the same time. But when it’s summer in the Southern Hemisphere, it’s winter in the Northern. What gives? It all has to do with Earth’s tilt and its orbit around the Sun.