The Planets Read Online Free

photosphere a grainy complexion, marred here and there by pairs of dark, irregularly shaped sunspots, with black centers and gray, graded shading around them like the penumbras of shadows. Sunspots designate areas of intense magnetic activity on the Sun, and their darkness bespeaks their relative coolness of about 4000 K, compared to neighboring areas at nearly 6000 K. * The level of solar activity rises and falls in cycles averaging eleven years, and sunspots mingle, morph, and multiply according to this same schedule. Their number and distribution vary like famine and plenty, from no spots at “solar minimum,” or just a few spots dotting the Sun’s high latitudes, to “solar maximum” five to six years later, when hundreds of them crowd closer to theequator. Although sunspots seem to gather and scud like clouds across the photosphere, really it is the Sun’s rotation that carries them around.
    The Sun rotates on its axis approximately once a month, in a continuation of the spinning motion it was born in. Being an enormous ball of gas, the Sun spins complexly, in layers of various speeds. The core and its immediate surroundings turn at one rate, as a solid body. The overlying zone spins faster, and, above that, the visible photosphere whirls around at several different rates, more quickly at the Sun’s equator than near its poles. These combined, contrary motions whip the Sun into a fury, with consequences felt clear across the Solar System.
    The “solar wind,” a hot exhalation of charged particles (reminiscent of the “wind from God”), blows out from the turbulent Sun and keeps up a constant barrage on the planets. Were it not for the protective envelope of Earth’s magnetic field, which deflects most of the solar wind, we could not withstand the onslaught. From time to time, especially during solar maximum, the steady solar wind is augmented by sudden blasts of higher-energy particles from solar flares on the Sun’s surface, or by gargantuan blobs of ejected solargas. Such outbursts can disable our communications satellites and disrupt power grids, causing blackouts. In milder doses, particles of solar wind trickle into the upper atmosphere near the North and South Poles, initiating cascades of electrical charge that draw curtains of colored lights across the sky—the so-called Northern and Southern Lights. Other planets also sprout colorful auroras in response to the solar wind, which billows on past Pluto all the way to the heliopause—the undiscovered boundary where the Sun’s influence ends.
    From Earth, we see the Sun as a blazing circle in the sky, brighter but no bigger than the circumference of the full Moon. The “two great lights,” as the Sun and Moon are described in Genesis, make a matched pair. For although the Moon measures only one four-hundredth the Sun’s million-mile diameter, it nevertheless lies four hundred times closer to Earth. This uncanny coincidence of size and distance enables the puny Moon to block out the Sun whenever the two bodies converge on their shared path across Earth’s sky.
    Approximately once every two years, some narrow swath of Earth—as often as not a godforsaken, all but inaccessible place—is blessedwith a total solar eclipse. There, dusk falls and dawn breaks twice on the same day, and the stars come out with the Sun still overhead. Temperatures may drop ten or fifteen degrees at a stroke, allowing even the most jaded observer to sense the bizarre disorientation that birds and animals share as they hasten to their nests or burrows through the sudden midday darkness.
    No total eclipse can last much longer than seven minutes, because of Earth’s persistent turning on its axis and the Moon’s unwavering march along its orbit. But totality of the briefest duration affords sufficient reason for scientific expeditions and curious individuals to travel halfway around the world, even if they have seen one or more eclipses before.
    At totality, when the Moon is